Picture of the Week

Also see our pictures of the week on eso.org.
25 November 2013

Blue and gold

This sprinkling of cosmic glitter makes up the galaxy known as ESO 149-3, located some 20 million light-years away from us. It is an example of an irregular galaxy, characterised by its amorphous, undefined shape — a property that sets it apart from its perhaps more photogenic spiral and elliptical relatives. Around one quarter of all galaxies are thought to be irregular-type galaxies.

In this image taken with the NASA/ESA Hubble Space Telescope ESO 149-3 can be seen as a smattering of golden and blue stars, with no apparent central nucleus or arm structure. The surrounding sky is rich in other more distant galaxies, visible as small, colourful streaks and dashes.

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


18 November 2013

Best image of bright quasar 3C 273

This image from Hubble’s Wide Field and Planetary Camera 2 (WFPC2) is likely the best of ancient and brilliant quasar 3C 273, which resides in a giant elliptical galaxy in the constellation of Virgo (The Virgin). Its light has taken some 2.5 billion years to reach us. Despite this great distance, it is still one of the closest quasars to our home. It was the first quasar ever to be identified, and was discovered in the early 1960s by astronomer Allan Sandage.

The term quasar is an abbreviation of the phrase “quasi-stellar radio source”, as they appear to be star-like on the sky. In fact, quasars are the intensely powerful centres of distant, active galaxies, powered by a huge disc of particles surrounding a supermassive black hole. As material from this disc falls inwards, some quasars — including 3C 273  — have been observed to fire off super-fast jets into the surrounding space. In this picture, one of these jets appears as a cloudy streak, measuring some 200 000 light-years in length.

Quasars are capable of emitting hundreds or even thousands of times the entire energy output of our galaxy, making them some of the most luminous and energetic objects in the entire Universe. Of these very bright objects, 3C 273 is the brightest in our skies. If it was located 30 light-years from our own planet — roughly seven times the distance between Earth and Proxima Centauri, the nearest star to us after the Sun — it would still appear as bright as the Sun in the sky.  

WFPC2 was installed on Hubble during shuttle mission STS-61. It is the size of a small piano and was capable of seeing images in the visible, near-ultraviolet, and near-infrared parts of the spectrum.


11 November 2013

Antennae Galaxies reloaded

The NASA/ESA Hubble Space Telescope has snapped the best ever image of the Antennae Galaxies. Hubble has released images of these stunning galaxies twice before, once using observations from its Wide Field and Planetary Camera 2 (WFPC2) in 1997, and again in 2006 from the Advanced Camera for Surveys (ACS). Each of Hubble’s images of the Antennae Galaxies has been better than the last, due to upgrades made during the famous servicing missions, the last of which took place in 2009.

The galaxies — also known as NGC 4038 and NGC 4039 — are locked in a deadly embrace. Once normal, sedate spiral galaxies like the Milky Way, the pair have spent the past few hundred million years sparring with one another. This clash is so violent that stars have been ripped from their host galaxies to form a streaming arc between the two. In wide-field images of the pair the reason for their name becomes clear — far-flung stars and streamers of gas stretch out into space, creating long tidal tails reminiscent of antennae.

This new image of the Antennae Galaxies shows obvious signs of chaos. Clouds of gas are seen in bright pink and red, surrounding the bright flashes of blue star-forming regions — some of which are partially obscured by dark patches of dust. The rate of star formation is so high that the Antennae Galaxies are said to be in a state of starburst, a period in which all of the gas within the galaxies is being used to form stars. This cannot last forever and neither can the separate galaxies; eventually the nuclei will coalesce, and the galaxies will begin their retirement together as one large elliptical galaxy.

This image uses visible and near-infrared observations from Hubble’s Wide Field Camera 3 (WFC3), along with some of the previously-released observations from Hubble’s Advanced Camera for Surveys (ACS).


4 November 2013

Stellar explosions in NGC 6984

Supernovae are intensely bright objects. They are formed when a star reaches the end of its life with a dramatic explosion, expelling most of its material out into space. The subject of this new Hubble image, spiral galaxy NGC 6984, played host to one of these explosions back in 2012, known as SN 2012im. Now, another star has exploded, forming supernova SN 2013ek — visible in this image as the prominent, star-like bright object just slightly above and to the right of the galaxy's centre.

SN 2012im is known as a Type Ic supernova, while the more recent SN 2013ek is a Type Ib. Both of these types are caused by the core collapse of massive stars that have shed — or lost — their outer layers of hydrogen. Type Ic supernovae are thought to have lost more of their outer envelope than Type Ib, including a layer of helium.

The observations that make up this new image were taken on 19 August 2013, and aimed to pinpoint the location of this new explosion more precisely. It is so close to where SN 2012im was spotted that the two events are thought to be linked; the chance of two completely independent supernovae so close together and of the same class exploding within one year of one another is a very unlikely event. It was initially suggested that SN 2013ek may in fact be SN 2012im flaring up again, but further observations support the idea that they are separate supernovae — although they may be closely related in some as-yet-unknown way.


28 October 2013

New shot of Proxima Centauri, our nearest neighbour

Shining brightly in this Hubble image is our closest stellar neighbour: Proxima Centauri.

Proxima Centauri lies in the constellation of Centaurus (The Centaur), just over four light-years from Earth. Although it looks bright through the eye of Hubble, as you might expect from the nearest star to the Solar System, Proxima Centauri is not visible to the naked eye. Its average luminosity is very low, and it is quite small compared to other stars, at only about an eighth of the mass of the Sun.

However, on occasion, its brightness increases. Proxima is what is known as a “flare star”, meaning that convection processes within the star’s body make it prone to random and dramatic changes in brightness. The convection processes not only trigger brilliant bursts of starlight but, combined with other factors, mean that Proxima Centauri is in for a very long life. Astronomers predict that this star will remain middle-aged — or a “main sequence” star in astronomical terms — for another four trillion years, some 300 times the age of the current Universe.

These observations were taken using Hubble’s Wide Field and Planetary Camera 2 (WFPC2). Proxima Centauri is actually part of a triple star system — its two companions, Alpha Centauri A and B, lie out of frame.

Although by cosmic standards it is a close neighbour, Proxima Centauri remains a point-like object even using Hubble’s eagle-eyed vision, hinting at the vast scale of the Universe around us.


21 October 2013

Hubble snaps icy Comet ISON

This new NASA/ESA Hubble Space Telescope picture shows C/2012 S1, better known as Comet ISON, a high-profile celestial visitor to the Solar System. Hubble has already snapped this comet twice this year (opo1314a, opo1331a), but for some time it was temporarily blocked from view by the Sun. It was spotted again in August 2013, and this new image shows the comet as it appeared in our skies in early October.

ISON will be brightest in our skies in late November, just before and after it hurtles past the Sun. As it gets brighter, it may even become visible as a naked eye object, before it fades throughout December — the month of its closest approach to Earth. Depending on its fate as it passes close to the Sun, it could become spectacular or, on the contrary, it could completely disintegrate. Many observatories, as well as several ESA and NASA missions, aim to observe this icy visitor over the coming months.

In this Hubble image, taken on 9 October 2013, the comet's solid nucleus is unresolved because it is so small. If it had broken apart — a possibility as the Sun slowly warms it up during its approach — Hubble would have likely seen evidence for multiple fragments instead.

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14 October 2013

A special spiral galaxy for over 200 000 Facebook fans

This Hubble Picture of the Week is dedicated to the over 200 000 Facebook fans of ESA/Hubble, who share our passion for the NASA/ESA Hubble Space Telescope and the discoveries it brings. We thank you for staying in touch with us! 

The image, captured by the NASA/ESA Hubble Space Telescope, shows part of NGC 3621, an unusual spiral galaxy located over 20 million light-years away in the constellation of Hydra (The Water Snake).

The small, bright nucleus on the right of the image does not have the significant bulge of older stars that is common in spiral galaxies, marking NGC 3621 as a "pure-disc" galaxy. Many luminous clumps of blue young stars are scattered along the loose spiral arms, which are partially obscured by the dark dust lanes snaking across the frame. This galaxy is very useful for astronomers; some of its brightest stars can be used to estimate extragalactic distances, allowing us to measure the vast scale of the Universe.

A version of this image was entered into the Hubble's Hidden Treasures image processing competition by contestant Claude Cornen. The Hubble Space Telescope is the astronomers' tele-lens; a wider view of this galaxy was obtained with ESO's Very Large Telescope (potw1148a), and a wide-angle image with the Wide-Field Imager (WFI) at ESO's La Silla Observatory (eso1104). Just as for normal landscape photography, different lenses – or telescopes, in this case! – are used depending on what the photographer wants to shoot.


7 October 2013

A monster in the Milky Way

This image, not unlike a pointillist painting, shows the star-studded centre of the Milky Way towards the constellation of Sagittarius. The crowded centre of our galaxy contains numerous complex and mysterious objects that are usually hidden at optical wavelengths by clouds of dust — but many are visible here in these infrared observations from Hubble.

However, the most famous cosmic object in this image still remains invisible: the monster at our galaxy’s heart called Sagittarius A*. Astronomers have observed stars spinning around this supermassive black hole (located right in the centre of the image), and the black hole consuming clouds of dust as it affects its environment with its enormous gravitational pull.

Infrared observations can pierce through thick obscuring material to reveal information that is usually hidden to the optical observer. This is the best infrared image of this region ever taken with Hubble, and uses infrared archive data from Hubble’s Wide Field Camera 3, taken in September 2011. It was posted to Flickr by Gabriel Brammer, a fellow at the European Southern Observatory based in Chile. He is also an ESO photo ambassador.


30 September 2013

A pulsating stellar relic

This NASA/ESA Hubble Space Telescope image shows the planetary nebula NGC 2452, located in the southern constellation of Puppis. The blue haze across the frame is what remains of a star like our Sun after it has depleted all its fuel. When this happens, the core of the star becomes unstable and releases huge numbers of incredibly energetic particles that blow the star's atmosphere away into space.

At the centre of this blue cloud lies what remains of the nebula's progenitor star. This cool, dim, and extremely dense star is actually a pulsating white dwarf, meaning that its brightness varies over time as gravity causes waves that pulse throughout the small star's body.

NGC 2452 was discovered by Sir John Herschel in 1847. He initially defined it as "an object whose nature I cannot make out. It is certainly not a star, nor a close double star [...] I should call it an oblong planetary nebula". To early observers like Herschel with their smaller telescopes, planetary nebulae resembled gaseous planets, and so were named accordingly. The name has stuck, although modern telescopes like Hubble have made it clear that these objects are not planets at all, but the outer layers of dying stars being thrown off into space.

A version of this image was entered into the Hubble's Hidden Treasures image processing competition by contestants Luca Limatola and Budeanu Cosmin Mirel.


23 September 2013

A scattering of spiral and elliptical galaxies

This image shows the massive galaxy cluster MACS J0152.5-2852, captured in detail by the NASA/ESA Hubble Space Telescope's Wide Field Camera 3. Almost every object seen here is a galaxy, each containing billions of stars. Galaxies are not usually randomly distributed in space, but instead appear in concentrations of hundreds, held together by their mutual gravity. Elliptical galaxies, like the yellow fuzzy objects seen in the image, are most often found close to the centres of galaxy clusters, while spirals, such as the bluish patches, are usually found to be further out and more isolated.

A version of this image obtained tenth prize in the Hubble's Hidden Treasures image processing competition, entered by contestant Judy Schmidt.


16 September 2013

A smouldering star

This new image, snapped by NASA/ESA Hubble Space Telescope, shows the star HD 184738, also known as Campbell’s hydrogen star. It is surrounded by plumes of reddish gas — the fiery red and orange hues are caused by glowing gases, including hydrogen and nitrogen.

HD 184738 is at the centre of a small planetary nebula. The star itself is known as a  [WC] star, a rare class resembling their much more massive counterparts — Wolf-Rayet stars. These stars are named after two French astronomers, Charles Wolf and Georges Rayet, who first identified them in the mid-nineteenth century.

Wolf-Rayet stars are hot stars, perhaps 20 times more massive than the Sun, that are rapidly blowing away material and losing mass. [WC] stars are rather different: they are low-mass Sun-like stars at the end of their lives. While these stars have recently ejected much of their original mass, the hot stellar core is still losing mass at a high rate, creating a hot wind. It is these winds that cause them to resemble Wolf-Rayet stars.

However, astronomers can look more closely at the composition of these winds to tell the stars apart; [WC] stars are identified by the carbon and oxygen in their winds. Some true Wolf-Rayet stars are rich in nitrogen instead, but this is very rare among their low-mass counterparts.

HD 184738 is also very bright in the infrared part of the spectrum, and is surrounded by dust very similar to the material that the Earth formed from. The origin of this dust is uncertain.

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

This caption was revised on 18/09/2013 to more accurately describe this image.


9 September 2013

A flock of stars

The glittering specks in this image, resembling a distant flock of flying birds, are the stars that make up the dwarf galaxy ESO 540-31. Captured in this new image from the NASA/ESA Hubble Space Telescope, the dwarf galaxy lies just over 11 million light-years from Earth, in the constellation of Cetus (The Whale). The background of this image is full of many other galaxies, all located at vast distances from us.

Dwarf galaxies are the among the smaller and dimmer members of the galactic family, typically only containing around a few hundred million stars. Although this sounds like a large number, it is small when compared to spiral galaxies like our Milky Way, which are made up of hundreds of billions of stars.

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


2 September 2013

A spiral in the Air Pump

Lying over 110 million light-years away from Earth in the constellation of Antlia (The Air Pump) is the spiral galaxy IC 2560, shown here in an image from NASA/ESA Hubble Space Telescope. At this distance it is a relatively nearby spiral galaxy, and is part of the Antlia cluster — a group of over 200 galaxies held together by gravity. This cluster is unusual; unlike most other galaxy clusters, it appears to have no dominant galaxy within it.

In this image, it is easy to spot IC 2560's spiral arms and barred structure. This spiral is what astronomers call a Seyfert-2 galaxy, a kind of spiral galaxy characterised by an extremely bright nucleus and very strong emission lines from certain elements — hydrogen, helium, nitrogen, and oxygen. The bright centre of the galaxy is thought to be caused by the ejection of huge amounts of super-hot gas from the region around a central black hole.

There is a story behind the naming of this quirky constellation — Antlia was originally named antlia pneumatica by French astronomer Abbé Nicolas Louis de Lacaille, in honour of the invention of the air pump in the 17th century.

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


26 August 2013

Dark, dusty shells

The NASA/ESA Hubble Space Telescope has captured this image of PGC 10922, an example of a lenticular galaxy — a galaxy type that lies on the border between ellipticals and spirals.

Seen face-on, the image shows the disc and tightly-wound spiral structures of dark dust encircling the bright centre of the galaxy. There is also a remarkable outer halo of faint wide arcs or shells extending outwards, covering much of the picture. These are likely to have been formed by a gravitational encounter or even a merger with another galaxy. Some dust also appears to have escaped from the central structure and has spread out across the inner shells.

An extraordinarily rich background of more remote galaxies can also be seen in the image.

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


19 August 2013

A cosmic optical illusion

At first glance, this Hubble picture appears to capture two space colossi entangled in a fierce celestial battle, with two galaxies entwined and merging to form one. But this shows just how easy it is to misinterpret the jumble of sparkling stars and get the wrong impression — as it’s all down to a trick of perspective.

By chance, these galaxies appear to be aligned from our point of view. In the foreground, the irregular dwarf galaxy PGC 16389 — seen here as a cloud of stars — covers its neighbouring galaxy APMBGC 252+125-117, which appears edge-on as a streak. This wide-field image also captures many other more distant galaxies, including a quite prominent face-on spiral towards the right of the picture.

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


12 August 2013

Stars fleeing a cosmic crash

Astronomical pictures sometimes deceive us with tricks of perspective. Right in the centre of this image, two spiral galaxies appear to be suffering a spectacular collision, with a host of stars appearing to flee the scene of the crash in a chaotic stampede.

However, this is just a trick of perspective. It is true that two spiral galaxies are colliding, but they are millions of light-years away, far beyond the cloud of blue and red stars near the merging spiral. This sprinkling of stars is actually an isolated, irregular dwarf galaxy named ESO 489-056. The dwarf galaxy is actually much more distant than many bright stars in the foreground of the image, which are located much closer to us, in the Milky Way.

ESO 489-056 is located 16 million light-years from Earth in the constellation of Canis Major (The Greater Dog), in our local Universe. It is composed of a few billion red and blue stars — a very small number when compared to galaxies like the Milky Way, which is estimated to contain around 200 to 400 billion stars, or the Andromeda Galaxy, which contains a mind-boggling one trillion.

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


5 August 2013

Sunset in Mordor

Don’t be fooled by the title; the mysterious, almost mystical bright light emerging from these thick, ominous clouds is actually a telltale sign of star formation. Here, a very young star is being born in the guts of the dark cloud LDN 43 — a massive blob of gas, dust, and ices, gathered 520 light-years from Earth in the constellation of Ophiuchus (The Serpent Bearer).

Stars are born from cosmic dust and gas, which floats freely in space until gravity forces it to bind together. The hidden newborn star in this image, revealed only by light reflected onto the plumes of the dark cloud, is named RNO 91. It is what astronomers call a pre-main sequence star, meaning that it has not yet started burning hydrogen in its core.

The energy that allows RNO 91 to shine comes from gravitational contraction. The star is being compressed by its own weight until, at some point, a critical mass will be reached and hydrogen, its main component, will begin to fuse together, releasing huge amounts of energy in the process. This will mark the beginning of adulthood for the star. But even before this happens the adolescent star is bright enough to shine and generate powerful stellar winds, emitting intense X-ray and radio emission.

RNO 91 is a variable star around half the mass of the Sun. Astronomers have been able to observe the existence of a dusty, icy disc surrounding it, stretching out to over 1700 times the distance from Earth to the Sun. It is believed that this disc may host protoplanets — planets in the process of being formed — and will eventually evolve into a fully-fledged planetary system.

This image is based on data gathered by the NASA/ESA Hubble Space Telescope. A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Judy Schmidt.


29 July 2013

A spiral galaxy crowned by a star

Another treasure unearthed from the Hubble archives, this beautiful image shows a spiral galaxy named NGC 4517. Slightly bigger than our Milky Way, it is seen edge-on, crowned by a very bright star. The star is actually much closer to us than the galaxy, explaining why it appears to be so big and bright in the picture. 

NGC 4517 is located approximately 40 million light-years away in the constellation of Virgo (The Virgin). It has a bright centre, but this is not visible in this Hubble image. Its orientation has led to it being included in many studies of globular clusters, clumps of stars that orbit the centres of galaxies like satellites.

The galaxy was discovered in 1784 by William Herschel, who described this region as having “a pretty bright star situated exactly north of the centre of an extended milky ray”. Of course the “milky ray” seen by Herschel is actually this spiral galaxy, but with his 17th century observing gear he could only tell that there a fuzzy, blurry structure below the much brighter star.

This image is composed from visible and infrared light gathered by NASA/ESA Hubble Space Telescope. A version of this image was entered into the Hubble’s Hidden treasures image processing competition by contestant Gilles Chapdelaine.


22 July 2013

A mysterious old spiral

This striking cosmic whirl is the centre of galaxy NGC 524, as seen with the NASA/ESA Hubble Space Telescope. This galaxy is located in the constellation of Pisces, some 90 million light-years from Earth.

NGC 524 is a lenticular galaxy. Lenticular galaxies are believed to be an intermediate state in galactic evolution — they are neither elliptical nor spiral. Spirals are middle-aged galaxies with vast, pinwheeling arms that contain millions of stars. Along with these stars are large clouds of gas and dust that, when dense enough, are the nurseries where new stars are born. When all the gas is either depleted or lost into space, the arms gradually fade away and the spiral shape begins to weaken. At the end of this process, what remains is a lenticular galaxy — a bright disc full of old, red stars surrounded by what little gas and dust the galaxy has managed to cling on to.

This image shows the shape of NGC 524 in detail, formed by the remaining gas surrounding the galaxy’s central bulge. Observations of this galaxy have revealed that it maintains some spiral-like motion, explaining its intricate structure.

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


15 July 2013

A stranger in the crowd

The constellation of Virgo (The Virgin) is the largest of the Zodiac constellations, and the second largest overall after Hydra (The Water Snake). Its most appealing feature, however, is the sheer number of galaxies that lie within it. In this picture, among a crowd of face- and edge-on spiral, elliptical, and irregular galaxies, lies NGC 4866, a lenticular galaxy situated about 80 million light-years from Earth.

Lenticular galaxies are somewhere between spirals and ellipticals in terms of shape and properties. From the picture, we can appreciate the bright central bulge of NGC 4886, which contains primarily old stars, but no spiral arms are visible. The galaxy is seen from Earth as almost edge-on, meaning that the disc structure — a feature not present in elliptical galaxies — is clearly visible. Faint dust lanes trace across NGC 4866 in this image, obscuring part of the galaxy’s light.

To the right of the galaxy is a very bright star that appears to lie within NGC 4886’s halo. However, this star actually lies much closer to us; in front of the galaxy, along our line of sight. These kinds of perspective tricks are common when observing, and can initially deceive astronomers as to the true nature and position of objects such as galaxies, stars, and clusters.

This sharp image of NGC 4866 was captured by the Advanced Camera for Surveys, an instrument on the NASA/ESA Hubble Space Telescope. A version was entered into the Hubble’s Hidden Treasures image processing competition by contestant Gilles Chapdelaine.


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