Picture of the Week

The NASA/ESA Hubble Space Telescope has captured this image of nearby spiral galaxy Messier 61, also known as NGC 4303. The galaxy, located only 55 million light-years away from Earth, is roughly the size of the Milky Way, with a diameter of around 100 000 light-years. The galaxy is notable for one particular reason — six supernovae have been observed within Messier 61, a total that places it in the top handful of galaxies alongside Messier 83, also with six, and NGC 6946, with a grand total of nine observed supernovae.

In this Hubble image the galaxy is seen face-on as if posing for a photograph, allowing us to study its structure closely. The spiral arms can be seen in stunning detail, swirling inwards to the very centre of the galaxy, where they form a smaller, intensely bright spiral. In the outer regions, these vast arms are sprinkled with bright blue regions where new stars are being formed from hot, dense clouds of gas.

Messier 61 is part of the Virgo Galaxy Cluster, a massive group of galaxies in the constellation of Virgo (the Virgin). Galaxy clusters, or groups of galaxies, are among the biggest structures in the Universe to be held together by gravity alone. The Virgo Cluster contains more than 1300 galaxies and forms the central region of the Local Supercluster, an even bigger gathering of galaxies.

The image was taken using data from Hubble’s Wide Field Camera 2. Different versions of this image were submitted to the Hubble’s Hidden Treasures image processing competition by contestants Gilles Chapdelaine, Luca Limatola, and Robert Gendler.

Links

• Hubble’s Hidden Treasures

The contorted object captured by Hubble in this picture is IRAS 22491-1808, also known as the South America Galaxy. It is an ultraluminous infrared galaxy (ULIRG) that emits a huge amount of light at infrared wavelengths. The reason for this intense infrared emission lies in an episode of strong star formation activity, which was set off by a collision between two interacting galaxies. 

In this image the twisted shape hides a number of features. In the central region, which is very complex and disturbed, scientists have been able to distinguish two nuclei, remains of the two different galaxies that are currently colliding to form a new one. IRAS 22491-1808 is amongst the most luminous of these types of galaxies, and is considered to be mid-way through its merging stage.

The centre of this appealing object also shows several intense star-forming knots which, as seen in the picture, actually outshine the nuclei in optical wavelengths. To pick out the two merging nuclei in IRAS 22491-1808, scientists have had to observe it in infrared wavelengths, where they are more distinct.

Other traces of the galactic collision are the three very noticeable tails in the image — two linear and one circular. The tail extending towards the bottom of the image from the main body exhibits a red clump of star formation at its base.

Unlike the venomous fictional plants that share its name, the Trifid of the North, otherwise known as the Northern Trifid or NGC 1579, poses no threat to your vision. The nebula’s moniker is inspired by the better-known Messier 20, the Trifid Nebula, which lies very much further south in the sky and displays strikingly similar swirling clouds of gas and dust.

The Trifid of the North is a large, dusty region that is currently forming new stars. These stars are very hot and therefore appear to be very blue. During their short lives they radiate strongly into the gas surrounding them, causing it to glow brightly. Many regions like the Trifid of the North — named H II regions — are clumpy and strangely shaped due to the powerful winds emanating from the stars within them. H II regions also have relatively short lives, furiously forming baby stars until the immense winds from these bodies blow the gas and dust away, leaving just stars behind.

The image above, captured by the NASA/ESA Hubble Space Telescope, shows the bright body of the nebula, with dark dust lanes snaking across the frame. The Trifid of the North glows strongly due to the many stars within it, like young binary EM* LkHA 101. Visible to the bottom right of the image, this binary is thought to be surrounded by a hundred or so fainter and less massive stars, making up a recently formed cluster. It lies behind a cloud of dust so thick that it is almost invisible to astronomers at optical wavelengths. Infrared imaging has now penetrated this dusty veil and is uncovering the secrets of this binary star, which is about five thousand times brighter than our own Sun.

A version of this image by Bruno Conti was entered into the Hubble’s Hidden Treasures competition.

This new image from the NASA/ESA Hubble Space Telescope captures an ongoing cosmic collision between two galaxies — a spiral galaxy is in the process of colliding with a lenticular galaxy. The collision looks almost as if it is popping out of the screen in 3D, with parts of the spiral arms clearly embracing the lenticular galaxy’s bulge.

The image also reveals further evidence of the collision. There is a bright stream of stars coming out from the merging galaxies, extending out towards the right of the image. The bright spot in the middle of the plume, known as ESO 576-69, is what makes this image unique. This spot is believed to be the nucleus of the former spiral galaxy, which was ejected from the system during the collision and is now being shredded by tidal forces to produce the visible stellar stream.

A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Luca Limatola.

This beautiful, glittering swirl is named, rather unpoetically, J125013.50+073441.5. A glowing haze of material seems to engulf the galaxy, stretching out into space in different directions and forming a fuzzy streak in this image. It is a starburst galaxy — a name given to galaxies that show unusually high rates of star formation. The regions where new stars are being born are highlighted by sparkling bright blue regions along the galactic arms.

Studying starburst galaxies can tell us a lot about galactic evolution and star formation. These galaxies start off with huge amounts of gas, which is used to form new stars. This period of furious star formation is only a phase; once all the gas is used up, this starbirth slows down. Other famous starbursts captured by Hubble include the Antennae Galaxies and Messier 82, the latter of which is forming new stars ten times faster than our galaxy, the Milky Way.

The data for this image were collected as part of a study named LARS (Lyman Alpha Reference Sample) [1], which is investigating the interaction between radiation and matter in relatively nearby starburst galaxies. J125013.50+073441.5 is included as one of its fourteen targets. This study has characterised how a certain type of emission known as Lyman-alpha emission interacts with nearby gas, affecting how it travels out into space.

The data for this image were collected using Hubble’s Wide Field Camera 3.

More information

[1] Hayes, Östlin et al., The Lyman Alpha Reference Sample: extended Lyman alpha halos produced at low dust content, The Astrophysical Journal, 2013.

This Hubble image shows the galaxy cluster Abell S1077. Galaxy clusters are large groupings of galaxies, each of them including millions of stars. They are the largest existing structures in the Universe to be held together by their gravity.

The amount of matter condensed in such groupings is so high that their gravity is enough to warp the fabric of spacetime, distorting the path that light takes when it travels through the cluster. In some cases, this phenomenon produces an effect somewhat like a magnifying lens, allowing us to see objects that are aligned behind the cluster and which would otherwise be undetectable from Earth. In this image, you see stretched stripes that look like scratches on a lens but are, in fact, galaxies whose light is heavily distorted by the gravitational field of the cluster.

Astronomers use tools like the NASA/ESA Hubble Space Telescope and the effects of gravitational lensing to peer far back in time and space to see the furthest objects located in the early Universe. One of the record holders is MACS0647-JD, a galaxy seen by Hubble and the Spitzer Space Telescope with the help of a gravitational lens much like this one in the galaxy cluster MACS J0647.7+7015. Its light has taken 13.3 billion years to reach us.

This image is based in part on data spotted by Nick Rose in the Hubble’s Hidden Treasures image processing competition.

When we look into the distant cosmos, the great majority of the objects we see are galaxies: immense gatherings of stars, planets, gas, dust, and dark matter, showing up in all kind of shapes. This Hubble picture registers several, but the galaxy catalogued as 2MASX J05210136-2521450 stands out at a glance due to its interesting shape.

This object is an ultraluminous infrared galaxy which emits a tremendous amount of light at infrared wavelengths. Scientists connect this to intense star formation activity, triggered by a collision between two interacting galaxies.

The merging process has left its signs: 2MASX J05210136-2521450 presents a single, bright nucleus and a spectacular outer structure that consists of a one-sided extension of the inner arms, with a tidal tail heading in the opposite direction, formed from material ripped out from the merging galaxies by gravitational forces.

The image is a combination of exposures taken by Hubble’s Advanced Camera for Surveys, using near-infrared and visible light. A version of this image was submitted to the Hubble’s Hidden Treasures image processing competition by contestant Luca Limatola.

These delicate wisps of gas make up an object known as SNR B0519-69.0, or SNR 0519 for short. The thin, blood-red shells are actually the remnants from when an unstable progenitor star exploded violently as a supernova around 600 years ago. There are several types of supernova, but for SNR 0519 the star that exploded is known to have been a white dwarf star — a Sun-like star in the final stages of its life.

SNR 0519 is located over 150 000 light-years from Earth in the southern constellation of Dorado (The Dolphinfish), a constellation that also contains most of our neighbouring galaxy the Large Magellanic Cloud (LMC). Because of this, this region of the sky is full of intriguing and beautiful deep sky objects.

The LMC orbits the Milky Way galaxy as a satellite and is the fourth largest in our group of galaxies, the Local Group. SNR 0519 is not alone in the LMC; the NASA/ESA Hubble Space Telescope also came across a similar bauble a few years ago in SNR B0509-67.5, a supernova of the same type as SNR 0519 with a strikingly similar appearance.

A version of this image was submitted to the Hubble’s Hidden Treasures Image Processing Competition by Claude Cornen, and won sixth prize.

The Universe is rarely static, although the timescales involved can be very long. Since modern astronomical observations began we have been observing the birthplaces of new stars and planets, searching for and studying the subtle changes that help us to figure out what is happening within.

The bright spot located at the edge of the bluish fan-shaped structure in this Hubble image is a young star called V* PV Cephei, or PV Cep. It is a favourite target for amateur astronomers because the fan-shaped nebulosity, known as GM 1-29 or Gyulbudaghian’s Nebula, changes over a timescale of months. The brightness of the star has also varied over time.

Images of PV Cep taken in 1952 showed a nebulous streak, similar to a comet’s tail. However, these had vanished when new images of the star were obtained some twenty-five years later. Instead, the blue fan-shaped nebula had appeared. Twenty-five years is a very short period on cosmic timescales, so astronomers think that the mysterious streak may have been a temporary phenomenon, such as the remnants of a massive stellar flare — similar to the solar flares we are used to seeing in the Solar System.

At the same time as this was happening, the star itself was brightening. This provided the light to illuminate the newly formed fan-shaped nebula. This brightening might be related to the start of the hydrogen-burning phase of the star, which would mean that it was reaching maturity.

PV Cep is thought to be surrounded by a disc of gas and dust, which would stop light from escaping in all directions. The fan-like appearance is therefore probably a result of starlight escaping from the dust disc and projecting onto the nebula.

PV Cep is located in the northern constellation of Cepheus at a distance of over 1600 light-years from Earth.

A version of this image was entered into the Hubble’s Hidden Treasures competition by contestant Alexey Romashin.

Globular clusters are relatively common in our sky, and generally look similar. However, this image, taken using the NASA/ESA Hubble Space Telescope, shows a unique example of such a cluster — Palomar 2.

Palomar 2 is part of a group of 15 globulars known as the Palomar clusters. These clusters, as the name suggests, were discovered in survey plates from the first Palomar Observatory Sky Survey in the 1950s, a project that involved some of the most well-known astronomers of the day, including Edwin Hubble. They were discovered quite late because they are so faint — each is either extremely remote, very heavily hidden behind blankets of dust, or has a very small number of remaining stars.

This particular cluster is unique in more than one way. For one, it is the only globular cluster that we see in this part of the sky, the northern constellation of Auriga (The Charioteer). Globular clusters orbit the centre of a galaxy like the Milky Way in the same way that satellites circle around the Earth. This means that they normally lie closer in to the galactic centre than we do, and so we almost always see them in the same region of the sky. Palomar 2 is an exception to this, as it is around five times further away from the centre of the Milky Way than other clusters. It also lies in the opposite direction — further out than Earth — and so it is classed as an “outer halo” globular.

It is also unusual due to its brightness. The cluster is veiled by a mask of dust, dampening the apparent brightness of the stars within it and making it appear as a very faint burst of stars. The stunning NASA/ESA Hubble Space Telescope image above shows Palomar 2 in a way that could not be captured from smaller or ground-based telescopes —  some amateur astronomers with large telescopes attempt to observe all of the obscure and well-hidden Palomar 15 as a challenge, to see how many they can pick out from the starry sky.

The soft glow in the picture above is NGC 2768, an elliptical galaxy located in the northern constellation of Ursa Major (The Great Bear). It appears here as a bright oval on the sky, surrounded by a wide, fuzzy cloud of material. This image, taken by the NASA/ESA Hubble Space Telescope, shows the dusty structure encircling the centre of the galaxy, forming a knotted ring around the galaxy’s brightly glowing middle. Interestingly, this ring lies perpendicular to the plane of NGC 2768 itself, stretching up and out of the galaxy.

The dust in NGC 2768 forms an intricate network of knots and filaments. In the centre of the galaxy are two tiny, S-shaped symmetric jets. These two flows of material travel outwards from the galactic centre along curved paths, and are masked by the tangle of dark dust lanes that spans the body of the galaxy.

These jets are a sign of a very active centre. NGC 2768 is an example of a Seyfert galaxy, an object with a supermassive black hole at its centre. This speeds up and sucks in gas from the nearby space, creating a stream of material swirling inwards towards the black hole known as an accretion disc. This disk throws off material in very energetic outbursts, creating structures like the jets seen in the image above.

A version of this image was entered into the Hubble's Hidden Treasures image processing competition by contestant Judy Schmidt. One of her images, of newborn star XZ Tauri, was awarded third prize.

Visible as a small, sparkling hook in the dark sky, this beautiful object is known as J082354.96+280621.6, or J082354.96 for short. It is a starburst galaxy, so named because of the incredibly (and unusually) high rate of star formation occurring within it.

One way in which astronomers probe the nature and structure of galaxies like this is by observing the behaviour of their dust and gas components; in particular, the Lyman-alpha emission. This occurs when electrons within a hydrogen atom fall from a higher energy level to a lower one, emitting light as they do so. This emission is interesting because this light leaves its host galaxy only after extensive scattering in the nearby gas — meaning that this light can be used as a pretty direct probe of what a galaxy is made up of.

The study of this Lyman-alpha emission is common in very distant galaxies, but now a study named LARS (Lyman Alpha Reference Sample) [1] is investigating the same effect in galaxies that are closer by. Astronomers chose fourteen galaxies, including this one, and used spectroscopy and imaging to see what was happening within them. They found that these Lyman-alpha photons can travel much further if a galaxy has less dust — meaning that we can use this emission to infer how dusty the source galaxy is.

The LARS study relies heavily on the high resolving power of Hubble. When Hubble is decommissioned, no telescope will be able to make observations like this in the far ultraviolet part of the spectrum — meaning that small, glittering galaxies imaged and probed by studies like LARS may give us some of the most detailed data we have to work with for some time to come.

Credit: ESA/Hubble & NASA, M. Hayes

Notes

[1] Hayes, Östlin et al., The Lyman Alpha Reference Sample: extended Lyman alpha halos produced at low dust content, The Astrophysical Journal, 2013.

The object in this image is Jonckheere 900 or J 900, a planetary nebula — glowing shells of ionised gas pushed out by a dying star. Discovered in the early 1900s by astronomer Robert Jonckheere, the dusty nebula is small but fairly bright, with a relatively evenly spread central region surrounded by soft wispy edges.

Despite the clarity of this Hubble image, the two objects in the picture above can be confusing for observers. J 900’s nearby companion, a faint star in the constellation of Gemini, often causes problems for observers because it is so close to the nebula — when seeing conditions are bad, this star seems to merge into J 900, giving it an elongated appearance. Hubble’s position above the Earth’s atmosphere means that this is not an issue for the space telescope.

Astronomers have also mistakenly reported observations of a double star in place of these two objects, as the planetary nebula is quite small and compact.

J 900’s central star is only just visible in this image, and is very faint — fainter than the nebula’s neighbour. The nebula appears to display a bipolar structure, where there are two distinct lobes of material emanating from its centre, enclosed by a bright oval disc.

A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Josh Barrington.

This charming and bright galaxy, known as IRAS 23436+5257, was captured by the the NASA/ESA Hubble Space Telescope. It is located in the northern constellation of Cassiopeia, which is named after an arrogant, vain, and yet beautiful mythical queen.

The twisted, wormlike structure of this galaxy is most likely the result of a collision and subsequent merger of two galaxies. Such interactions are quite common in the Universe, and they can range from minor interactions involving a satellite galaxy being caught by a spiral arm, to major galactic crashes. Friction between the gas and dust during a collision can have a major effect on the galaxies involved, morphing the shape of the original galaxies and creating interesting new structures.

When you look up at the calm and quiet night sky it is not always easy to picture it as a dynamic and vibrant environment with entire galaxies in motion, spinning like children’s toys and crashing into whatever crosses their path. The motions are, of course, extremely slow, and occur over millions or even billions of years.

The aftermath of these galactic collisions helps scientists to understand how these movements occur and what may be in store for our own Milky Way, which is on a collision course with a neighbouring galaxy, Messier 31.

A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Judy Schmidt. Hidden Treasures was an initiative to invite astronomy enthusiasts to search the Hubble archive for stunning images that have never been seen by the general public. The competition has now closed and the results are published here.

Galaxies can take many forms — elliptical blobs, swirling spiral arms, bulges, and discs are all known components of the wide range of galaxies we have observed using telescopes like the NASA/ESA Hubble Space Telescope. However, some of the more intriguing objects in the sky around us include ring galaxies like the one pictured above — Zw II 28.

Ring galaxies are mysterious objects. They are thought to form when one galaxy slices through the disc of another, larger, one — as galaxies are mostly empty space, this collision is not as aggressive or as destructive as one might imagine. The likelihood of two stars physically colliding is minimal, and it is instead the gravitational effects of the two galaxies that causes the disruption.

This disruption upsets the material in both galaxies, causing it to redistribute to form a dense central core, encircled by bright stars. All this commotion causes clouds of gas and dust to collapse and triggers new periods of intense star formation in the outer ring, which is thus full of hot, young, blue stars and regions that are actively giving rise to new stars.

The sparkling pink and purple loop of Zw II 28 is not a typical ring galaxy due to its lack of a visible central companion. For many years it was thought to be a lone circle on the sky, but observations using Hubble have shown that there may be a possible companion lurking just inside the ring, where the loop appears to double back on itself. The galaxy has a knotty, swirling ring structure, with some areas appearing much brighter than others.

A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Judy Schmidt.

This image, speckled with blue, white, and yellow light, shows part of the spiral galaxy IC 5052. Surrounded by distant stars and galaxies, it emits a bright blue-white glow which highlights its narrow, intricate structure. It is viewed side-on in the constellation of Pavo (The Peacock), in the southern sky.

When spiral galaxies are viewed from this angle, it is very difficult to fully understand their properties and how they are arranged. IC 5052 is actually a barred spiral galaxy – its pinwheeling arms do not begin from the centre point but are instead attached to either end of a straight "bar" of stars that cuts through the galaxy's middle. Approximately two thirds of all spirals are barred, including the Milky Way.

Bursts of pale blue light are visible across the galaxy's length, partially blocked out by weaving lanes of darker gas and dust. These are pockets of extremely hot newborn stars. The bars present in spirals like IC 5052 are thought to help these formation processes by effectively funnelling material from the swirling arms inwards towards these hot stellar nurseries. 

A version of this image was submitted to the Hubble's Hidden Treasures image processing competition by contestant Serge Meunier.

It may look like something from The Lord of the Rings, but this fiery swirl is actually a planetary nebula known as ESO 456-67. Set against a backdrop of bright stars, the rust-coloured object lies in the constellation of Sagittarius (The Archer), in the southern sky.

Despite the name, these ethereal objects have nothing at all to do with planets; this misnomer came about over a century ago, when the first astronomers to observe them only had small, poor-quality telescopes. Through these, the nebulae looked small, compact, and planet-like — and so were labelled as such.

When a star like the Sun approaches the end of its life, it flings material out into space. Planetary nebulae are the intricate, glowing shells of dust and gas pushed outwards from such a star. At their centres lie the remnants of the original stars themselves — small, dense white dwarf stars.

In this image of ESO 456-67, it is possible to see the various layers of material expelled by the central star. Each appears in a different hue — red, orange, yellow, and green-tinted bands of gas are visible, with clear patches of space at the heart of the nebula. It is not fully understood how planetary nebulae form such a wide variety of shapes and structures; some appear to be spherical, some elliptical, others shoot material in waves from their polar regions, some look like hourglasses or figures of eight, and others resemble large, messy stellar explosions — to name but a few.

A version of this image was entered into the Hubble's Hidden Treasures image processing competition by contestant Jean-Christophe Lambry

This image shows an object known as HH 151, a bright jet of glowing material trailed by an intricate, orange-hued plume of gas and dust. It is located some 460 light-years away in the constellation of Taurus (The Bull), near to the young, tumultuous star HL Tau.

In the first few hundred thousand years of life, new stars like HL Tau pull in material that falls towards them from the surrounding space. This material forms a hot disc that swirls around the coalescing body, launching narrow streams of material from its poles. These jets are shot out at speeds of several hundred kilometres per second and collide violently with nearby clumps of dust and gas, creating wispy, billowing structures known as Herbig-Haro objects — like HH 151 seen in the image above.

Such objects are very common in star-forming regions. They are short-lived, and their motion and evolution can actually be seen over very short timescales, on the order of years. They quickly race away from the newly-forming star that emitted them, colliding with new clumps of material and glowing brightly before fading away.

A version of this image was entered into the Hidden Treasures image processing competition by Gilles Chapdelaine.

This large “flying V” is actually two distinct objects — a pair of interacting galaxies known as IC 2184. Both the galaxies are seen almost edge-on in the large, faint northern constellation of Camelopardalis (The Giraffe), and can be seen as bright streaks of light surrounded by the ghostly shapes of their tidal tails.

These tidal tails are thin, elongated streams of gas, dust and stars that extend away from a galaxy into space. They occur when galaxies gravitationally interact with one another, and material is sheared from the outer edges of each body and flung out into space in opposite directions, forming two tails. They almost always appear curved, so when they are seen to be relatively straight, as in this image, it is clear that we are viewing the galaxies side-on.

Also visible in this image are bursts of bright blue, pinpointing hot regions where the stars from both galaxies have begun to crash together during the merger.

The image consists of visible and infrared observations from Hubble’s Wide Field and Planetary Camera 2.

A version of this picture was entered into the Hubble’s Hidden Treasures image-processing competition by contestant Serge Meunier.

This thin, glittering streak of stars is the spiral galaxy ESO 121-6, which lies in the southern constellation of Pictor (The Painter's Easel). Viewed almost exactly side-on, the intricate structure of the swirling arms is hidden, but the full length of the galaxy can be seen — including the intense glow from the central bulge, a dense region of tightly packed young stars sitting at the centre of the spiral arms.

Tendrils of dark dust can be seen across the frame, partially obscuring the bright centre of the galaxy and continuing out towards the smattering of stars at its edges, where the dust lanes and shapes melt into the inky background. Numerous nearby stars and galaxies are visible as small smudges in the surrounding sky, and the brightest stars are dazzlingly prominent towards the bottom left of the image.

ESO 121-6 is a galaxy with patchy, loosely wound arms and a relatively faint central bulge. It actually belongs to a group of galaxies, a clump of no more than 50 similar structures all loosely bound to one another by gravity. The Milky Way is also a member of a galactic group, known as the Local Group. 

The further away you look, the further back in time you see. Astronomers use this fact to study the evolution of the Universe by looking at nearby and more distant galaxies and comparing their features. Hubble is particularly well suited for this type of work because of its extremely high resolution and its position above the atmosphere. This has allowed it to detect many of the most distant galaxies known, as well as making detailed images of faraway objects.

Comparing galaxies in the distant past with those around us today, astronomers have noticed that the nearby galaxies are far quieter and calmer than their distant brethren, seen earlier in their lives. Nearby galaxies (although not the Milky Way) are often large, elliptical galaxies with little or no ongoing star formation, and their stars tend to be elderly and red in colour. These galaxies, in astronomers' language, are "red and dead".

This is not so for galaxies further away, which typically show more vigorous star birth.

The reason for this appears to be that as the Universe has aged, galaxies have often collided and merged together, and these events disrupt gas clouds within them. A merger will usually be a trigger for such intense star formation that the supply of gas is used up, and no more star formation occurs afterwards. The merged elliptical galaxy then creeps into old age, getting redder as its stars get older. This is expected to happen to the Milky Way when it merges with the nearby Andromeda Galaxy, some four billion years from now.

The galaxy in this image, catalogued as 2MASX J09442693+0429569, marks a transitional phase in this process as young, star-forming galaxies settle to become massive, red and dead galaxies.

The galaxy has tail-like features extending from it, typical of a galaxy that has recently undergone a merger. Studying the properties of the light from this galaxy, astronomers see no sign of ongoing star formation; in other words, the merger triggered an event which has used up all the gas. However, the observations suggest that star formation was strong until the very recent past, and has ceased only within the last billion years. This image therefore shows a snapshot of the moment star formation stopped forever in a galaxy. 

A version of this image was entered into the Hidden Treasures image processing competition by contestant Nick Rose.

Globular clusters are roughly spherical collections of extremely old stars, and around 150 of them are scattered around our galaxy. Hubble is one of the best telescopes for studying these, as its extremely high resolution lets astronomers see individual stars, even in the crowded core. The clusters all look very similar, and in Hubble’s images it can be quite hard to tell them apart – and they all look much like NGC 411, pictured here.

And yet appearances can be deceptive: NGC 411 is in fact not a globular cluster, and its stars are not old. It isn’t even in the Milky Way.

NGC 411 is classified as an open cluster. Less tightly bound than a globular cluster, the stars in open clusters tend to drift apart over time as they age, whereas globulars have survived for well over 10 billion years of galactic history. NGC 411 is a relative youngster — not much more than a tenth of this age. Far from being a relic of the early years of the Universe, the stars in NGC 411 are in fact a fraction of the age of the Sun.

The stars in NGC 411 are all roughly the same age, having formed in one go from one cloud of gas. But they are not all the same size. Hubble’s image shows a wide range of colours and brightnesses in the cluster’s stars. These tell astronomers many facts about the stars, including their mass, temperature and evolutionary phase. Blue stars, for instance, have higher surface temperatures than red ones.

The image is a composite produced from ultraviolet, visible and infrared observations made by Hubble’s Wide Field Camera 3. This filter set lets the telescope “see” colours slightly further beyond red and the violet ends of the spectrum.

A busy patch of space has been captured in this image from the NASA/ESA Hubble Space Telescope. Scattered with many nearby stars, the field also has numerous galaxies in the background.

Located on the border of Triangulum Australe (The Southern Triangle) and Norma (The Carpenter’s Square), this field covers part of the Norma Cluster (Abell 3627) as well as a dense area of our own galaxy, the Milky Way.

The Norma Cluster is the closest massive galaxy cluster to the Milky Way, and lies about 220 million light-years away. The enormous mass concentrated here, and the consequent gravitational attraction, mean that this region of space is known to astronomers as the Great Attractor, and it dominates our region of the Universe.

The largest galaxy visible in this image is ESO 137-002, a spiral galaxy seen edge on. In this image from Hubble, we see large regions of dust across the galaxy’s bulge. What we do not see here is the tail of glowing X-rays that has been observed extending out of the galaxy — but which is invisible to an optical telescope like Hubble.

Observing the Great Attractor is difficult at optical wavelengths. The plane of the Milky Way — responsible for the numerous bright stars in this image — both outshines (with stars) and obscures (with dust) many of the objects behind it. There are some tricks for seeing through this — infrared or radio observations, for instance — but the region behind the centre of the Milky Way, where the dust is thickest, remains an almost complete mystery to astronomers.

This image consists of exposures in blue and infrared light taken by Hubble’s Advanced Camera for Surveys.

The constellation of Ursa Major (The Great Bear) is home to Messier 101, the Pinwheel Galaxy. One of the biggest and brightest spiral galaxies in the night sky, Messier 101 is also the subject of one of Hubble's most famous images (heic0602). Like the Milky Way, Messier 101 is not alone, with smaller dwarf galaxies in its neighbourhood.

NGC 5477, one of these dwarf galaxies in the Messier 101 group, is the subject of this image from the NASA/ESA Hubble Space Telescope. Without obvious structure, but with visible signs of ongoing starbirth, NGC 5477 looks much like an archetypal dwarf irregular galaxy. The bright nebulae that extend across much of the galaxy are clouds of glowing hydrogen gas in which new stars are forming. These glow pinkish red in real life, although the selection of green and infrared filters through which this image was taken makes them appear almost white.

The observations were taken as part of a project to measure accurate distances to a range of galaxies within about 30 million light-years from Earth, by studying the brightness of red giant stars.

In addition to NGC 5477, the image includes numerous galaxies in the background, including some that are visible right through NGC 5477. This serves as a reminder that galaxies, far from being solid, opaque objects, are actually largely made up of the empty space between their stars.

This image is a combination of exposures taken through green and infrared filters using Hubble's Advanced Camera for Surveys. The field of view is approximately 3.3 by 3.3 arcminutes. 

The Universe loves to fool our eyes, giving the impression that celestial objects are located at the same distance from Earth. A good example can be seen in this spectacular image produced by the NASA/ESA Hubble Space Telescope. The galaxies NGC 5011B and NGC 5011C are imaged against a starry background.

Located in the constellation of Centaurus, the nature of these galaxies has puzzled astronomers. NGC 5011B (on the right) is a spiral galaxy belonging to the Centaurus Cluster of galaxies lying 156 million light-years away from the Earth. Long considered part of the faraway cluster of galaxies as well, NGC 5011C (the bluish galaxy at the centre of the image) is a peculiar object, with the faintness typical of a nearby dwarf galaxy, alongside the size of an early-type spiral.

Astronomers were curious about the appearance of NGC 5011C. If the two galaxies were at roughly the same distance from Earth, they would expect the pair to show signs of interactions between them. However, there was no visual sign of interaction between the two. How could this be possible?

To solve this problem, astronomers studied the velocity at which these galaxies are receding from the Milky Way and found that NGC 5011C is moving away far more slowly than its apparent neighbour, and its motion is more consistent with that of the nearby Centaurus A group at a distance of 13 million light-years. Thus, NGC 5011C, with only about ten million times the mass of the Sun in its stars, must indeed be a nearby dwarf galaxy rather than member of the distant Centaurus Cluster as was believed for many years.

Problem solved.

This image was taken with Hubble’s Advanced Camera for Surveys using visual and infrared filters.

The NASA/ESA Hubble Space Telescope provides us this week with a spectacular image of the bright star-forming ring that surrounds the heart of the barred spiral galaxy NGC 1097. In this image, the larger-scale structure of the galaxy is barely visible: its comparatively dim spiral arms, which surround its heart in a loose embrace, reach out beyond the edges of this frame.

This face-on galaxy, lying 45 million light-years away from Earth in the southern constellation of Fornax (The Furnace), is particularly attractive for astronomers. NGC 1097 is a Seyfert galaxy. Lurking at the very centre of the galaxy, a supermassive black hole 100 million times the mass of our Sun is gradually sucking in the matter around it. The area immediately around the black hole shines powerfully with radiation coming from the material falling in.

The distinctive ring around the black hole is bursting with new star formation due to an inflow of material toward the central bar of the galaxy. These star-forming regions are glowing brightly thanks to emission from clouds of ionised hydrogen. The ring is around 5000 light-years across, although the spiral arms of the galaxy extend tens of thousands of light-years beyond it.

NGC 1097 is also pretty exciting for supernova hunters. The galaxy experienced three supernovae (the violent deaths of high-mass stars) in the 11-year span between 1992 and 2003. This is definitely a galaxy worth checking on a regular basis.

However, what it is really exciting about NGC 1097 is that it is not wandering alone through space. It has two small galaxy companions, which dance “the dance of stars and the dance of space” like the gracious dancer of the famous poem The Dancer by Khalil Gibran.

The satellite galaxies are NGC 1097A, an elliptical galaxy orbiting 42 000 light-years from the centre of NGC 1097 and a small dwarf galaxy named NGC 1097B. Both galaxies are located out beyond the frames of this image and they cannot be seen. Astronomers have indications that NGC 1097 and NGC 1097A have interacted in the past.

This picture was taken with Hubble’s Advanced Camera for Surveys using visual and infrared filters.

A version of this image was submitted to the Hubble’s Hidden Treasures image processing competition by contestant Eedresha Sturdivant.

Like finding a silver needle in the haystack of space, the NASA/ESA Hubble Space Telescope has produced this beautiful image of the spiral galaxy IC 2233, one of the flattest galaxies known.

Typical spiral galaxies like the Milky Way are usually made up of three principal visible components: the disc where the spiral arms and most of the gas and dust is concentrated; the halo, a rough and sparse sphere around the disc that contains little gas, dust or star formation; and the central bulge at the heart of the disc, which is formed by a large concentration of ancient stars surrounding the Galactic Centre.

However, IC 2233 is far from being typical. This object is a prime example of a super-thin galaxy, where the galaxy’s diameter is at least ten times larger than the thickness. These galaxies consist of a simple disc of stars when seen edge on. This orientation makes them fascinating to study, giving another perspective on spiral galaxies. An important characteristic of this type of objects is that they have a low brightness and almost all of them have no bulge at all.

The bluish colour that can be seen along the disc gives evidence of the spiral nature of the galaxy, indicating the presence of hot, luminous, young stars, born out of clouds of interstellar gas. In addition, unlike typical spirals, IC 2233 shows no well-defined dust lane. Only a few small patchy regions can be identified in the inner regions both above and below the galaxy’s mid-plane.

Lying in the constellation of Lynx, IC 2233 is located about 40 million light-years away from Earth. This galaxy was discovered by British astronomer Isaac Roberts in 1894.

This image was taken with the Hubble’s Advanced Camera for Surveys, combining visible and infrared exposures. The field of view in this image is approximately 3.4 by 3.4 arcminutes.

A version of this image was entered into the Hubble's Hidden Treasures image processing competition by contestant Luca Limatola.

Located in a relatively vacant region of space about 4200 light-years away and difficult to see using an amateur telescope, the lonesome planetary nebula NGC 7354 is often overlooked. However, thanks to this image captured by the NASA/ESA Hubble Space Telescope we are able to see this brilliant ball of smoky light in spectacular detail.

Just as shooting stars are not actually stars and lava lamps do not actually contain lava, planetary nebulae have nothing to do with planets. The name was coined by Sir William Herschel because when he first viewed a planetary nebula through a telescope, he could only identify a hazy smoky sphere, similar to gaseous planets such as Uranus. The name has stuck even though modern telescopes make it obvious that these objects are not planets at all, but the glowing gassy outer layers thrown off by a hot dying star.

It is believed that winds from the central star play an important role in determining the shape and morphology of planetary nebulae. The structure of NGC 7354 is relatively easy to distinguish. It consists of a circular outer shell, an elliptical inner shell, a collection of bright knots roughly concentrated in the middle and two symmetrical jets shooting out from either side. Research suggests that these features could be due to a companion central star, however the presence of a second star in NGC 7354 is yet to be confirmed.

NGC 7354 resides in Cepheus, a constellation named after the mythical King Cepheus of Aethiopia and is about half a light-year in diameter.

A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Bruno Conti.

The brilliant cascade of stars through the middle of this image is the galaxy ESO 318-13 as seen by the NASA/ESA Hubble Space Telescope. Despite being located millions of light-years from Earth, the stars captured in this image are so bright and clear you could almost attempt to count them.

Although ESO 318-13 is the main event in this image, it is sandwiched between a vast collection of bright celestial objects. Several stars near and far dazzle in comparison to the neat dusting contained within the galaxy. One that particularly stands out is located near the centre of the image, and looks like an extremely bright star located within the galaxy. This is, however, a trick of perspective. The star is located in the Milky Way, our own galaxy, and it shines so brightly because it is so much closer to us than ESO 318-13.

There are also a number of tiny glowing discs scattered throughout the frame that are more distant galaxies. In the top right corner, an elliptical galaxy can be clearly seen, a galaxy which is much larger but more distant than ESO 318-13. More interestingly, peeking through the ESO 318-13, near the right-hand edge of the image, is a distant spiral galaxy.

Galaxies are largely made up of empty space; the stars within them only take up a small volume, and providing a galaxy is not too dusty, it can be largely transparent to light coming from the background. This makes overlapping galaxies like these quite common. One particularly dramatic example of this phenomenon is the galaxy pair NGC 3314 (heic1208). 

The NASA/ESA Hubble Space Telescope provides us this week with an impressive image of the irregular galaxy NGC 5253.

NGC 5253 is one of the nearest of the known Blue Compact Dwarf (BCD) galaxies, and is located at a distance of about 12 million light-years from Earth in the southern constellation of Centaurus. The most characteristic signature of these galaxies is that they harbour very active star-formation regions. This is in spite of their low dust content and comparative lack of elements heavier than hydrogen and helium, which are usually the basic ingredients for star formation.

These galaxies contain molecular clouds that are quite similar to the pristine clouds that formed the first stars in the early Universe, which were devoid of dust and heavier elements. Hence, astronomers consider the BCD galaxies to be an ideal testbed for better understanding the primordial star-forming process.

NGC 5253 does contain some dust and heavier elements, but significantly less than the Milky Way galaxy. Its central regions are dominated by an intense star forming region that is embedded in an elliptical main body, which appears red in Hubble’s image. The central starburst zone consists of a rich environment of hot, young stars concentrated in star clusters, which glow in blue in the image. Traces of the starburst itself can be seen as a faint and diffuse glow produced by the ionised oxygen gas.

The true nature of BCD galaxies has puzzled astronomers for a long time. Numerical simulations following the current leading cosmological theory of galaxy formation, known as the Lambda Cold Dark Matter model, predict that there should be far more satellite dwarf galaxies orbiting big galaxies like the Milky Way. Astronomers refer to this discrepancy as the Dwarf Galaxy Problem.

This galaxy is considered part of the Centaurus A/Messier 83 group of galaxies, which includes the famous radio galaxy Centaurus A and the spiral galaxy Messier 83. Astronomers have pointed out the possibility that the peculiar nature of NGC 5253 could result from a close encounter with Messier 83, its closer neighbour.

This image was taken with the Hubble’s Advanced Camera for Surveys, combining visible and infrared exposures. The field of view in this image is approximately 3.4 by 3.4 arcminutes.

A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Nikolaus Sulzenauer.

The NASA/ESA Hubble Space Telescope has spotted the spiral galaxy ESO 499-G37, seen here against a backdrop of distant galaxies, scattered with nearby stars.

The galaxy is viewed from an angle, allowing Hubble to  reveal its spiral nature clearly. The faint, loose spiral arms can be distinguished as bluish features swirling around the galaxy’s nucleus. This blue tinge emanates from the hot, young stars located in the spiral arms. The arms of a spiral galaxy have large amounts of gas and dust, and are often areas where new stars are constantly forming.

The galaxy’s most characteristic feature is a bright elongated nucleus. The bulging central core usually contains the highest density of stars in the galaxy, where typically a large group of comparatively cool old stars are packed in this compact, spheroidal region.

One feature common to many spiral galaxies is the presence of a bar running across the centre of the galaxy. These bars are thought to act as a mechanism that channels gas from the spiral arms to the centre, enhancing the star formation.

Recent studies suggest that ESO 499-G37’s nucleus sits within a small bar up to a few hundreds of light-years along, about a tenth the size of a typical galactic bar. Astronomers think that such small bars could be important in the formation of galactic bulges since they might provide a mechanism for bringing material from the outer regions down to the inner ones. However, the connection between bars and bulge formation is still not clear since bars are not a universal feature in spiral galaxies.

Lying in the constellation of Hydra, ESO 499-G37 is located about 59 million light-years away from the Sun. The galaxy belongs to the NGC 3175 group.

ESO 499-G37 was first observed in the late seventies within the ESO/Uppsala Survey of the ESO (B) atlas. This was a joint project undertaken by the European Southern Observatory (ESO) and the Uppsala Observatory, which used the ESO 1-metre Schmidt telescope at La Silla Observatory, Chile, to map a large portion of the southern sky looking for stars, galaxies, clusters, and planetary nebulae.

This picture was created from visible and infrared exposures taken with the Wide Field Channel of the Advanced Camera for Surveys. The field of view is approximately 3.4 arcminutes wide.

Luminous galaxies glow like fireflies on a dark night in this image snapped by the NASA/ESA Hubble Space Telescope. The central galaxy in this image is a gigantic elliptical galaxy designated 4C 73.08. A prominent spiral galaxy seen from "above" shines in the lower part of the image, while examples of galaxies viewed edge-on also populate the cosmic landscape.

In the optical and near-infrared light captured to make this image, 4C 73.08 does not appear all that beastly. But when viewed in longer wavelengths the galaxy takes on a very different appearance. Dust-piercing radio waves reveal plumes emanating from the core, where a supermassive black hole spews out twin jets of material. 4C 73.08 is classified as a radio galaxy as a result of this characteristic activity in the radio part of the electromagnetic spectrum.

Astronomers must study objects such as 4C 73.08 in multiple wavelengths in order to learn their true natures, just as seeing a firefly’s glow would tell a scientist only so much about the insect. Observing 4C 73.08 in visible light with Hubble illuminates galactic structure as well as the ages of constituent stars, and therefore the age of the galaxy itself. 4C 73.08 is decidedly redder than the prominent, bluer spiral galaxy in this image. The elliptical galaxy’s redness comes from the presence of many older, crimson stars, which shows that 4C 73.08 is older than its spiral neighbour.

The image was taken using Hubble’s Wide Field Camera 3 through two filters: one which captures green light, and one which captures red and near-infrared light.

The NASA/ESA Hubble Space Telescope has captured a beautiful galaxy that, with its reddish and yellow central area, looks rather like an explosion from a Hollywood movie. The galaxy, called NGC 5010, is in a period of transition. The aging galaxy is moving on from life as a spiral galaxy, like our Milky Way, to an older, less defined type called an elliptical galaxy. In this in-between phase, astronomers refer to NGC 5010 as a lenticular galaxy, which has features of both spirals and ellipticals.

NGC 5010 is located around 140 million light-years away in the constellation of Virgo (The Virgin). The galaxy is oriented sideways to us, allowing Hubble to peer into it and show the dark, dusty, remnant bands of spiral arms. NGC 5010 has notably started to develop a big bulge in its disc as it takes on a more rounded shape.

Most of the stars in NGC 5010 are red and elderly. The galaxy no longer contains all that many of the fast-lived blue stars common in younger galaxies that still actively produce new populations of stars.

Much of the dusty and gaseous fuel needed to create fresh stars has already been used up in NGC 5010. Overt time, the galaxy will grow progressively more "red and dead”, as astronomers describe elliptical galaxies.

Hubble's Advanced Camera for Surveys (ACS) snapped this image in violet and infrared light.

The NASA/ESA Hubble Space Telescope offers an impressive view of the centre of globular cluster NGC 6362. The image of this spherical collection of stars takes a deeper look at the core of the globular cluster, which contains a high concentration of stars with different colours.

Tightly bound by gravity, globular clusters are composed of old stars, which, at around 10 billion years old, are much older than the Sun. These clusters are fairly common, with more than 150 currently known in our galaxy, the Milky Way, and more which have been spotted in other galaxies.

Globular clusters are among the oldest structures in the Universe that are accessible to direct observational investigation, making them living fossils from the early years of the cosmos.

Astronomers infer important properties of globular clusters by looking at the light from their constituent stars. For many years, they were regarded as ideal laboratories for testing the standard stellar evolution theory. Among other things, this theory suggests that most of the stars within a globular cluster should be of a similar age.

Recently, however, high precision measurements performed in numerous globular clusters, primarily with the Hubble Space Telescope, has led some to question this widely accepted theory. In particular, certain stars appear younger and bluer than their companions, and they have been dubbed blue stragglers. NGC 6362 contains many of these stars.

Since they are usually found in the core regions of clusters, where the concentration of stars is large, the most likely explanation for this unexpected population of objects seems to be that they could be either the result of stellar collisions or transfer of material between stars in binary systems. This influx of new material would heat up the star and make it appear younger than its neighbours.

NGC 6362 is located about 25 000 light-years from Earth in the constellation of Ara (The Altar). British astronomer James Dunlop first observed this globular cluster on 30 June 1826.

This image was created combining ultraviolet, visual and infrared images taken with the Wide Field Channel of the Advanced Camera for Surveys and the Wide Field Camera 3. An image image of NGC 6362 taken by the MPG/ESO 2.2-metre telescope will be published by the European Southern Observatory on Wednesday. See it on www.eso.org from 12:00 on 31 October.

The NASA/ESA Hubble Space Telescope has imaged the faint irregular galaxy NGC 3738, a starburst galaxy. The galaxy is in the midst of a violent episode of star formation, during which it is converting reservoirs of hydrogen gas harboured in the galaxy’s centre into stars. Hubble spots this gas glowing red around NGC 3738, one of the most distinctive signs of ongoing star formation.

Lying in the constellation of Ursa Major (The Great Bear), NGC 3738 is located about 12 million light-years from the Sun, and belongs to the Messier 81 group of galaxies. This galaxy — first observed by astronomer William Herschel  back in 1789 — is a nearby example of a blue compact dwarf, the faintest type of starburst galaxy. Blue compact dwarfs are small compared to large spiral galaxies — NGC 3738 is around 10 000 light-years across, just one tenth of the size of the Milky Way.

This type of galaxy is blue in appearance by virtue of containing large clusters of hot, massive stars, which ionise the surrounding interstellar gas with their intense ultraviolet radiation. They are relatively faint and appear to be irregular in shape. Unlike spirals or elliptical galaxies, irregular galaxies do not have any distinctive features, such as a nuclear bulge or spiral arms. Rather, they are extremely chaotic in appearance. These galaxies are thought to resemble some of the earliest that formed in the Universe and may provide clues as to how stars appeared shortly after the Big Bang. 

This image was created by combining visual and infrared images taken with the Wide Field Channel of the Advanced Camera for Surveys aboard the Hubble Space Telescope. The field of view of the Wide Field Channel is approximately 3.4 by 3.4 arcminutes wide.

NGC 3344 is a glorious spiral galaxy around half the size of the Milky Way, which lies 25 million light-years distant. We are fortunate enough to see NGC 3344 face-on, allowing us to study its structure in detail.

The galaxy features an outer ring swirling around an inner ring with a subtle bar structure in the centre. The central regions of the galaxy are predominately populated by young stars, with the galactic fringes also featuring areas of active star formation.

Central bars are found in around two thirds of spiral galaxies. NGC 3344’s is clearly visible here, although it is not as dramatic as some (see for example heic1202).

The high density of stars in galaxies’ central regions gives them enough gravitational influence to affect the movement of other stars in their galaxy. However, NGC 3344’s outer stars are moving in an unusual manner, although the presence of the bar cannot entirely account for this, leaving astronomers puzzled. It is possible that in its past NGC 3344 passed close by another galaxy and accreted stars from it, but more research is needed to state this with confidence.

The image is a combination of exposures taken in visible and near-infrared light, using Hubble’s Advanced Camera for Surveys. The field of view is around 3.4 by 3.4 arcminutes, or around a tenth of the diameter of the full Moon.

The Universe is filled with mysterious objects. Many of them are as strange as they are beautiful. Among these, planetary nebulae are probably one of the most fascinating objects to behold in the night sky. No other type of object has such a large variety of shapes and structures. The NASA/ESA Hubble Space Telescope provides us this week with a striking image of Hen 3-1475, a planetary nebula in the making.

Planetary nebulae — the name arises because most of these objects resembled a planet when they were first discovered through early telescopes — are expanding, glowing shells of gas coming from Sun-like stars at the ends of their lives. They glow brightly because of the radiation that comes from a hot, compact core, which remains after the outer envelope is ejected, and is powerful enough to make these gossamer shells shine.

Each planetary nebula is complex and unique. Hen 3-1475 is a great example of a planetary nebula in the making, a phase which is known to astronomers as a protoplanetary or preplanetary nebula.

Since the central star has not yet blown away its complete shell, the star is not hot enough to ionise the shell of gas and so the nebula does not shine. Rather, we see the expelled gas thanks to light reflected off it. When the star’s envelope is fully ejected, it will begin to glow and become a planetary nebula.

Hen 3-1475 is located in the constellation of Sagittarius around 18 000 light-years away from us. The central star is more than 12 000 times as luminous as our Sun. Its most characteristic feature is a thick torus of dust around the central star and two S-shaped jets that are emerging from the pole regions of the central star. These jets are long outflows of fast-moving gas travelling at hundreds of kilometres per second.

The formation of these bipolar jets has puzzled astronomers for a long time. How can a spherical star form these complex structures? Recent studies suggest that the object’s characteristic shape and the large velocity outflow is created by a central source that ejects streams of gas in opposite directions and precesses once every thousand years. It is like an enormous, slowly rotating garden sprinkler in the middle of the sky. No wonder astronomers also have nicknamed this object the “Garden-sprinkler Nebula”.

This picture was taken with Hubble’s Wide Field Camera 3, which provides significantly higher resolution than previous observations made with the Wide Field and Planetary Camera 2 (heic0308).

Links

• Hubblecast 52: The Death of Stars explains how Sun-like stars end their lives as planetary nebulae

This dazzling image shows the globular cluster Messier 69, or M 69 for short, as viewed through the NASA/ESA Hubble Space Telescope. Globular clusters are dense collections of old stars. In this picture, foreground stars look big and golden when set against the backdrop of the thousands of white, silvery stars that make up M 69.

Another aspect of M 69 lends itself to the bejewelled metaphor: As globular clusters go, M 69 is one of the most metal-rich on record. In astronomy, the term “metal” has a specialised meaning: it refers to any element heavier than the two most common elements in our Universe, hydrogen and helium. The nuclear fusion that powers stars created all of the metallic elements in nature, from the calcium in our bones to the carbon in diamonds. Successive generations of stars have built up the metallic abundances we see today.

Because the stars in globular clusters are ancient, their metallic abundances are much lower than more recently formed stars, such as the Sun. Studying the makeup of stars in globular clusters like M 69 has helped astronomers trace back the evolution of the cosmos.

M 69 is located 29 700 light-years away in the constellation Sagittarius (the Archer). The famed French comet hunter Charles Messier added M 69 to his catalogue in 1780. It is also known as NGC 6637.

The image is a combination of exposures taken in visible and near-infrared light by Hubble’s Advanced Camera for Surveys, and covers a field of view of approximately 3.4 by 3.4 arcminutes.

The NASA/ESA Hubble Space Telescope has provided us with another outstanding image of a nearby galaxy. This week, we highlight the galaxy NGC 4183, seen here with a beautiful backdrop of distant galaxies and nearby stars. Located about 55 million light-years from the Sun and spanning about eighty thousand light-years, NGC 4183 is a little smaller than the Milky Way. This galaxy, which belongs to the Ursa Major Group, lies in the northern constellation of Canes Venatici (The Hunting Dogs).

NGC 4183 is a spiral galaxy with a faint core and an open spiral structure. Unfortunately, this galaxy is viewed edge-on from the Earth, and we cannot fully appreciate its spiral arms. But we can admire its galactic disc.

The discs of galaxies are mainly composed of gas, dust and stars. There is evidence of dust over the galactic plane, visible as dark intricate filaments that block the visible light from the core of the galaxy. In addition, recent studies suggest that this galaxy may have a bar structure. Galactic bars are thought to act as a mechanism that channels gas from the spiral arms to the centre, enhancing star formation, which is typically more pronounced in the spiral arms than in the bulge of the galaxy.

British astronomer William Herschel first observed NGC 4183 on 14 January 1778.

This picture was created from visible and infrared images taken with the Wide Field Channel of the Advanced Camera for Surveys. The field of view is approximately 3.4 arcminutes wide.

This image uses data identified by Luca Limatola in the Hubble's Hidden Treasures image processing competition.

The NASA/ESA Hubble Space Telescope has produced a sharp image of NGC 4634, a spiral galaxy seen exactly side-on. Its disc is slightly warped by ongoing interactions with a nearby galaxy, and it is crisscrossed by clearly defined dust lanes and bright nebulae.

NGC 4634, which lies around 70 million light-years from Earth in the constellation of Coma Berenices, is one of a pair of interacting galaxies. Its neighbour, NGC 4633, lies just outside the upper right corner of the frame, and is visible in wide-field views of the galaxy. While it may be out of sight, it is not out of mind: its subtle effects on NGC 4634 are easy to see to a well-trained eye.

Gravitational interactions pull the neat spiral forms of galaxies out of shape as they get closer to each other, and the disruption to gas clouds triggers vigorous episodes of star formation. While this galaxy’s spiral pattern is not directly visible thanks to our side-on perspective, its disc is slightly warped, and there is clear evidence of star formation.

Along the full length of the galaxy, and scattered around parts of its halo, are bright pink nebulae. Similar to the Orion Nebula in the Milky Way, these are clouds of gas that are gradually coalescing into stars. The powerful radiation from the stars excites the gas and makes it light up, much like a fluorescent sign. The large number of these star formation regions is a telltale sign of gravitational interaction.

The dark filamentary structures that are scattered along the length of the galaxy are caused by cold interstellar dust blocking some of the starlight.

Hubble’s image is a combination of exposures in visible light produced by Hubble’s Advanced Camera for Surveys and the Wide Field and Planetary Camera 2.

This image portrays a beautiful view of the galaxy NGC 7090, as seen by the NASA/ESA Hubble Space Telescope. The galaxy is viewed edge-on from the Earth, meaning we cannot easily see the spiral arms, which are full of young, hot stars.

However, our side-on view shows the galaxy’s disc and the bulging central core, where typically a large group of cool old stars are packed in a compact, spheroidal region. In addition, there are two interesting features present in the image that are worth mentioning.

First, we are able to distinguish an intricate pattern of pinkish red regions over the whole galaxy. This indicates the presence of clouds of hydrogen gas. These structures trace the location of ongoing star formation, visual confirmation of recent studies that classify NGC 7090 as an actively star-forming galaxy.

Second, we observe dust lanes, depicted as dark regions inside the disc of the galaxy. In NGC 7090, these regions are mostly located in lower half of the galaxy, showing an intricate filamentary structure. Looking from the outside in through the whole disc, the light emitted from the bright centre of the galaxy is absorbed by the dust, silhouetting the dusty regions against the bright light in the background.

Dust in our galaxy, the Milky Way, has been one of the worst enemies of observational astronomers for decades. But this does not mean that these regions are quite blind spots in the sky. At near-infrared wavelengths — slightly longer wavelengths than visible light — this dust is largely transparent and astronomers are able to study what is really behind it. At still longer wavelengths, the realm of radio astronomy, the dust itself can actually be observed, letting astronomers study the structure and properties of dust clouds and their relationship with star formation.

Lying in the southern constellation of Indus (The Indian), NGC 7090 is located about thirty million light-years from the Sun. Astronomer John Herschel first observed this galaxy on 4 October, 1834.

The image was taken using the Wide Field Channel of the Advanced Camera for Surveys aboard the Hubble Space Telescope and combines orange light (coloured blue here), infrared (coloured red) and emissions from glowing hydrogen gas (also in red).

A version of this image of NGC 7090 was entered into the Hubble’s Hidden Treasures Image Processing Competition by contestant Rasid Tugral. Hidden Treasures is an initiative to invite astronomy enthusiasts to search the Hubble archive for stunning images that have never been seen by the general public. The competition is now closed and the list of winners is available here.

This sparkling picture taken by the NASA/ESA Hubble Space Telescope shows the centre of globular cluster M 4. The power of Hubble has resolved the cluster into a multitude of glowing orbs, each a colossal nuclear furnace.

M 4 is relatively close to us, lying 7200 light-years distant, making it a prime object for study. It contains several tens of thousand stars and is noteworthy in being home to many white dwarfs — the cores of ancient, dying stars whose outer layers have drifted away into space.

In July 2003, Hubble helped make the astounding discovery of a planet called PSR B1620-26 b, 2.5 times the mass of Jupiter, which is located in this cluster. Its age is estimated to be around 13 billion years — almost three times as old as the Solar System! It is also unusual in that it orbits a binary system of a white dwarf and a pulsar (a type of neutron star).

Amateur stargazers may like to track M 4 down in the night sky. Use binoculars or a small telescope to scan the skies near the orange-red star Antares in Scorpius. M 4 is bright for a globular cluster, but it won’t look anything like Hubble’s detailed image: it will appear as a fuzzy ball of light in your eyepiece.

On Wednesday 5 September, the European Southern Observatory (ESO) will publish a wide-field image of M 4, showing the full spheroidal shape of the globular cluster. See it at www.eso.org on Wednesday.

A new image from the NASA/ESA Hubble Space Telescope shows NGC 5806, a spiral galaxy in the constellation Virgo (the Virgin). It lies around 80 million light years from Earth. Also visible in this image is a supernova explosion called SN 2004dg.

The exposures that are combined into this image were carried out in early 2005 in order to help pinpoint the location of the supernova, which exploded in 2004. The afterglow from this outburst of light, caused by a giant star exploding at the end of its life, can be seen as a faint yellowish dot near the bottom of the galaxy.

NGC 5806 was chosen to be one of a number of galaxies in a study into supernovae because Hubble’s archive already contained high resolution imagery of the galaxy, collected before the star had exploded. Since supernovae are both relatively rare, and impossible to predict with any accuracy, the existence of such before-and-after images is precious for astronomers who study these violent events.

Aside from the supernova, NGC 5806 is a relatively unremarkable galaxy: it is neither particularly large or small, nor especially close or distant.

The galaxy’s bulge (the densest part in the centre of the spiral arms) is a so-called disk-type bulge, in which the spiral structure extends right to the centre of the galaxy, instead of there being a large elliptical bulge of stars present. It is also home to an active galaxy nucleus, a supermassive black hole which is pulling in large amounts of matter from its immediate surroundings. As the matter spirals around the black hole, it heats up and emits powerful radiation.

This image is produced from three exposures in visible and infrared light, observed by Hubble’s Advanced Camera for Surveys. The field of view is approximately 3.3 by 1.7 arcminutes.

A version of this image was entered into the Hubble’s Hidden Treasures Image Processing Competition by contestant Andre van der Hoeven (who won second prize in the competition for his image of Messier 77). Hidden Treasures is an initiative to invite astronomy enthusiasts to search the Hubble archive for stunning images that have never been seen by the general public. The competition has now closed.

The NASA/ESA Hubble Space Telescope has produced this beautiful image of the globular cluster Messier 56 (also known as M 56 or NGC 6779), which is located about 33 000 light years away from the Earth in the constellation of Lyra (The Lyre). The cluster is composed of a large number of stars, tightly bound to each other by gravity.

However, this was not known when Charles Messier first observed it in January 1779.  He described Messier 56 as “a nebula without stars”, like most globular clusters that he discovered — his telescope was not powerful enough to individually resolve any of the stars visible here, making it look like a fuzzy ball through his telescope’s eyepiece. We clearly see from Hubble’s image how the development of technology over the years has helped our understanding of astronomical objects.

Astronomers typically infer important properties of globular clusters by looking at the light of their constituent stars. But they have to be very careful when they observe objects like Messier 56, which is located close to the Galactic plane. This region is crowded by “field-stars”, in other words, stars in the Milky Way that happen to lie in the same direction but do not belong to the cluster. These objects can contaminate the light, and hence undermine the conclusions reached by astronomers.  

A tool often used by scientists for studying stellar clusters is the colour-magnitude (or Hertzsprung-Russell) diagram. This chart compares the brightness and colour of stars – which in turn, tells scientists what the surface temperature of a star is.

By comparing high quality observations taken with the Hubble Space Telescope with results from the standard theory of stellar evolution, astronomers can characterise the properties of a cluster. In the case of Messier 56, this includes its age, which at 13 billion years is approximately three times the age of the Sun. Furthermore, they have also been able to study the chemical composition of Messier 56. The cluster has relatively few elements heavier than hydrogen and helium, typically a sign of stars that were born early in the Universe’s history, before many of the elements in existence today were formed in significant quantities.

Astronomers have found that the majority of clusters with this type of chemical makeup lie along a plane in the Milky Way’s halo. This suggests that such clusters were captured from a satellite galaxy, rather than being the oldest members of the Milky Way's globular cluster system as had been previously thought.

This image consists of visible and near-infrared exposures from Hubble’s Advanced Camera for Surveys. The field of view is approximately 3.3 by 3.3 arcminutes.

A version of this image was entered into the Hubble’s Hidden Treasures Image Processing Competition by contestant Gilles Chapdelaine. Hidden Treasures is an initiative to invite astronomy enthusiasts to search the Hubble archive for stunning images that have never been seen by the general public. The competition has now closed and the results will be published soon.

In terms of intergalactic real estate, our Solar System has a plumb location as part of a big, spiral galaxy, the Milky Way. Numerous, less glamorous dwarf galaxies, keep the Milky Way company. Many galaxies, however, are comparatively isolated, without close neighbours. One such example is the small galaxy known as DDO 190, snapped here in a new image from the NASA/ESA Hubble Space Telescope.

DDO 190 is classified as a dwarf irregular galaxy as it is relatively small and lacks clear structure. Older, reddish stars mostly populate DDO 190’s outskirts, while some younger, bluish stars gleam in DDO 190’s more crowded interior. Some pockets of ionised gas heated up by stars appear here and there, with the most noticeable one shining towards the bottom of DDO 190 in this picture. Meanwhile, a great number of distant galaxies with evident spiral, elliptical and less-defined shapes glow in the background.

DDO 190 lies around nine million light-years away from our Solar System. It is considered part of the loosely associated Messier 94 group of galaxies, not far from the Local Group of galaxies that includes the Milky Way. Canadian astronomer Sidney van der Bergh was the first to record DDO 190 in 1959 as part of the DDO catalogue of dwarf galaxies. (“DDO” stands for the David Dunlap Observatory, now managed by the Royal Astronomical Society of Canada, where the catalogue was created).

Although within the Messier 94 group, DDO 190 is on its own. The galaxy’s nearest dwarf galaxy neighbour, DDO 187, is thought to be no closer than three million light-years away. In contrast, many of the Milky Way’s companion galaxies, such as the Large and Small Magellanic Clouds, reside within a fifth or so of that distance, and even the giant spiral of the Andromeda Galaxy is closer to the Milky Way than DDO 190 is to its nearest neighbour.

Hubble’s Advanced Camera for Surveys captured this image in visible and infrared light. The field of view is around 3.3 by 3.3 arcminutes

A version of this image was entered into the Hubble’s Hidden Treasures Image Processing Competition by contestant Claude Cornen. Hidden Treasures is an initiative to invite astronomy enthusiasts to search the Hubble archive for stunning images that have never been seen by the general public. The competition has now closed and the results will be published soon.

Turning its 2.4-metre eye to the Tarantula Nebula, the NASA/ESA Hubble Space Telescope has taken this close-up of the outskirts of the main cloud of the Nebula.

The bright wispy structures are the signature of an environment rich in ionised hydrogen gas, called H II by astronomers. In reality these appear red, but the choice of filters and colours of this image, which includes exposures both in visible and infrared light, make the gas appear green.

These regions contain recently formed stars, which emit powerful ultraviolet radiation that ionises the gas around them. These clouds are ephemeral as eventually the stellar winds from the newborn stars and the ionisation process will blow away the clouds, leaving stellar clusters like the Pleiades.

Located in the Large Magellanic Cloud, one of our neighbouring galaxies, and situated at a distance of 170 000 light-years away from Earth, the Tarantula Nebula is the brightest known nebula in the Local Group of galaxies. It is also the largest (around 650 light-years across) and most active star-forming region known in our group of galaxies, containing numerous clouds of dust and gas and two bright star clusters. A recent Hubble image shows a large part of the nebula immediately adjacent to this field of view.

The cluster at the Tarantula nebula’s centre is relatively young and very bright. While it is outside the field of view of this image, the energy from it is responsible for most of the brightness of the Nebula, including the part we see here. The nebula is in fact so luminous that if it were located within 1000 light-years from Earth, it would cast shadows on our planet.

The Tarantula Nebula was host to the closest supernova ever detected since the invention of the telescope, supernova 1987A, which was visible to the naked eye.

The image was produced by Hubble’s Advanced Camera for Surveys, and has a field of view of approximately 3.3 by 3.3 arcminutes.

A version of this image was entered into the Hubble’s Hidden Treasures Image Processing Competition by contestant Judy Schmidt. Hidden Treasures is an initiative to invite astronomy enthusiasts to search the Hubble archive for stunning images that have never been seen by the general public. The competition has now closed and the results will be published soon.

The NASA/ESA Hubble Space Telescope offers this delightful view of the crowded stellar encampment called Messier 68, a spherical, star-filled region of space known as a globular cluster. Mutual gravitational attraction amongst a cluster’s hundreds of thousands or even millions of stars keeps stellar members in check, allowing globular clusters to hang together for many billions of years.

Astronomers can measure the ages of globular clusters by looking at the light of their constituent stars. The chemical elements leave signatures in this light, and the starlight reveals that globular clusters' stars typically contain fewer heavy elements, such as carbon, oxygen and iron, than stars like the Sun. Since successive generations of stars gradually create these elements through nuclear fusion, stars having fewer of them are relics of earlier epochs in the Universe. Indeed, the stars in globular clusters rank among the oldest on record, dating back more than 10 billion years.

More than 150 of these objects surround our Milky Way galaxy. On a galactic scale, globular clusters are indeed not all that big. In Messier 68's case, its constituent stars span a volume of space with a diameter of little more than a hundred light-years. The disc of the Milky Way, on the other hand, extends over some 100 000 light-years or more.

Messier 68 is located about 33 000 light-years from Earth in the constellation Hydra (The Female Water Snake). French astronomer Charles Messier notched the object as the sixty-eighth entry in his famous catalogue in 1780.

Hubble added Messier 68 to its own impressive list of cosmic targets in this image using the Wide Field Camera of Hubble’s Advanced Camera for Surveys. The image, which combines visible and infrared light, has a field of view of approximately 3.4 by 3.4 arcminutes.

The galaxy NGC 4700 bears the signs of the vigorous birth of many new stars in this image captured by the NASA/ESA Hubble Space Telescope.

The many bright, pinkish clouds in NGC 4700 are known as H II regions, where intense ultraviolet light from hot young stars is causing nearby hydrogen gas to glow. H II regions often come part-and-parcel with the vast molecular clouds that spawn fresh stars, thus giving rise to the locally ionised gas.

In 1610, French astronomer Nicolas-Claude Fabri de Peiresc peered through a telescope and found what turned out to be the first H II region on record: the Orion Nebula, located relatively close to our Solar System here in the Milky Way. Astronomers study these regions throughout the Milky Way and those easily seen in other galaxies to gauge the chemical makeup of cosmic environments and their influence on the formation of stars.

NGC 4700 was discovered back in March 1786 by the British astronomer William Herschel who noted it as a “very faint nebula”. NGC 4700, along with many other relatively close galaxies, is found in the constellation of Virgo (The Virgin) and is classified as a barred spiral galaxy, similar in structure to the Milky Way. It lies about 50 million light-years from us and is moving away from us at about 1400 km/second due to the expansion of the Universe.

The NASA/ESA Hubble Space Telescope has captured a crowd of stars that looks rather like a stadium darkened before a show, lit only by the flashbulbs of the audience’s cameras. Yet the many stars of this object, known as Messier 107, are not a fleeting phenomenon, at least by human reckoning of time — these ancient stars have gleamed for many billions of years.

Messier 107 is one of more than 150 globular star clusters found around the disc of the Milky Way galaxy. These spherical collections each contain hundreds of thousands of extremely old stars and are among the oldest objects in the Milky Way. The origin of globular clusters and their impact on galactic evolution remains somewhat unclear, so astronomers continue to study them through pictures such as this one obtained by Hubble.

As globular clusters go, Messier 107 is not particularly dense. Visually comparing its appearance to other globular clusters, such as Messier 53 or Messier 54 reveals that the stars within Messier 107 are not packed as tightly, thereby making its members more distinct like individual fans in a stadium's stands.

Messier 107 can be found in the constellation of Ophiuchus (The Serpent Bearer) and is located about 20 000 light-years from the Solar System.

French astronomer Pierre Méchain first noted the object in 1782, and British astronomer William Herschel documented it independently a year later. A Canadian astronomer, Helen Sawyer Hogg, added Messier 107 to Charles Messier's famous astronomical catalogue in 1947.

This picture was obtained with the Wide Field Camera of Hubble’s Advanced Camera for Surveys. The field of view is approximately 3.4 by 3.4 arcminutes.

This image snapped by the NASA/ESA Hubble Space Telescope reveals an exquisitely detailed view of part of the disc of the spiral galaxy NGC 4565. This bright galaxy is one of the most famous examples of an edge-on spiral galaxy, oriented perpendicularly to our line of sight so that we see right into its luminous disc. NGC 4565 has been nicknamed the Needle Galaxy because, when seen in full, it appears as a very narrow streak of light on the sky.

The edgewise view into the Needle Galaxy shown here looks very similar to the view we have from our Solar System into the core of the Milky Way. In both cases ribbons of dust block some of the light coming from the galactic disc. To the lower right, the dust stands in even starker contrast against the copious yellow light from the star-filled central regions. NGC 4565’s core is off camera to the lower right. For a full view of NGC 4565 for comparison’s sake, see this wider field of view from ESO’s Very Large Telescope.

Studying galaxies like NGC 4565 helps astronomers learn more about our home, the Milky Way. At a distance of only about 40 million light-years, NGC 4565 is relatively close by, and being seen edge-on makes it a particularly useful object for comparative study. As spiral galaxies go, NGC 4565 is a whopper — about a third as big again as the Milky Way.

The image was taken with Hubble’s Advanced Camera for Surveys and has a field of view of approximately 3.4 by 3.4 arcminutes.

A version of this image was entered into the Hubble’s Hidden Treasures Image Processing Competition by contestant Josh Barrington. Hidden Treasures is an initiative to invite astronomy enthusiasts to search the Hubble archive for stunning images that have never been seen by the general public. The competition has now closed and the results will be published soon.