Picture of the Week

23 January 2012: This NASA/ESA Hubble Space Telescope picture may trick you into thinking that the galaxy in it — known as UZC J224030.2+032131 — has not one but five different nuclei. In fact, the core of the galaxy is only the faint and diffuse object seen at the centre of the cross-like structure formed by the other four dots, which are images of a distant quasar located in the background of the galaxy. The picture shows a famous cosmic mirage known as the Einstein Cross, and is a direct visual confirmation of the theory of general relativity. It is one of the best examples of the phenomenon of gravitational lensing — the bending of light by gravity as predicted by Einstein in the early 20th century. In this case, the galaxy’s powerful gravity acts as a lens that bends and amplifies the light from the quasar behind it, producing four images of …

16 January 2012: Messier 100 is a perfect example of a grand design spiral galaxy, a type of galaxy with prominent and very well-defined spiral arms. These dusty structures swirl around the galaxy’s nucleus, and are marked by a flurry of star formation activity that dots Messier 100 with bright blue, high-mass stars. This image from the NASA/ESA Hubble Space Telescope, the most detailed made to date, shows the bright core of the galaxy and the innermost parts of its spiral arms. Messier 100 has an active galactic nucleus — a bright region at the galaxy’s core caused by a supermassive black hole that is actively swallowing material, which radiates brightly as it falls inwards. The galaxy’s spiral arms also host smaller black holes, including the youngest ever observed in our cosmic neighbourhood, the result of a supernova observed in 1979. Messier 100 is located in the direction of the constellation of Coma …

9 January 2012: This classic shot of a galaxy in the constellation of Ursa Major was taken by the NASA/ESA Hubble Space Telescope. NGC 3259 is a bright barred spiral galaxy located approximately 110 million light-years from Earth. Being a fully-formed active galaxy, its bright central bulge hosts a supermassive black hole, whose huge appetite for matter explains the high luminosity of the galaxy’s core: as it devours its surroundings, the black hole emits intense radiation across the whole electromagnetic spectrum, including in visible light. The beautiful spiral arms of the galaxy are not left out either as they contain dark lanes of dust and gas, ideal spawning grounds for stars. These bright, young, hot stars appear in rich clusters in the galaxy’s arms and are what gives the galaxy its blueish hue. Interestingly, the galaxy has a small companion (visible to the left of the image), a much smaller galaxy that may …

2 January 2012: This image shows the most detailed view ever of the core of Messier 82 (M 82), also known as the Cigar Galaxy. Rich with dust, young stars and glowing gas, M 82 is both unusually bright and relatively close to Earth. The starburst galaxy is located around 12 million light-years away in the constellation of Ursa Major (The Great Bear). This is not the first time Hubble has imaged the Cigar Galaxy. Previous images (for example heic0604) show a galaxy ablaze with stars. Yet this image looks quite unlike them, and is dominated instead by glowing gas and dust, with the stars almost invisible. Why such a difference? The new image is more detailed than previous Hubble observations – in fact, it is the most detailed image ever made of this galaxy. But the reason it looks so dramatically different is down to the choices astronomers make when designing their …

26 December 2011: The NASA/ESA Hubble Space Telescope has imaged a region of space containing the intriguing object IC 2574. Pink bubbles blown by supernova explosions abound in this faint galaxy. The colour of these shells comes from hydrogen gas irradiated by newborn stars. The formation of the stars was triggered by shock waves from earlier supernova detonations that compressed material together. IC 2574 is commonly known as Coddington's Nebula after the American astronomer Edwin Coddington, who discovered it in 1898. Astronomers classify IC 2574 as a dwarf irregular galaxy due to its relatively small size and lack of organisation or structure. These galaxies are thought to resemble some of the earliest that formed in the Universe. Dwarf irregular galaxies thus serve as useful "living fossils" for studying the evolution of more complex galaxy types such as our home, the Milky Way, with its central bar and spiral arms. The expanding shells in …

19 December 2011: An interesting galaxy has been circled in this NASA/ESA Hubble Space Telescope image. The galaxy — one of a group of galaxies called Luminous Red Galaxies — has an unusually large mass, containing about ten times the mass of the Milky Way. However, it’s actually the blue horseshoe shape that circumscribes the red galaxy that is the real prize in this image. This blue horseshoe is a distant galaxy that has been magnified and warped into a nearly complete ring by the strong gravitational pull of the massive foreground Luminous Red Galaxy. To see such a so-called Einstein Ring required the fortunate alignment of the foreground and background galaxies, making this object’s nickname “the Cosmic Horseshoe” particularly apt. The Cosmic Horseshoe is one of the best examples of an Einstein Ring. It also gives us a tantalising view of the early Universe: the blue galaxy’s redshift — a measure of …

12 December 2011: The compact nature of globular clusters is a double-edged sword. On the one hand, having so many stars of a similar age in one bundle gives astronomers insights into the chemical makeup of our galaxy in its early history. But, at the same time, the high density of stars in the cores of globulars also makes it difficult for astronomers to resolve individual stars. The core of NGC 6642, shown here in this Hubble Space Telescope image, is particularly dense, making this globular a difficult observational target for most telescopes. Furthermore, it occupies a very central position in our galaxy, which means that images inadvertently capture many stars that don’t belong to the cluster — these “field stars” just get in the way. However, using Hubble’s powerful Advanced Camera for Surveys (ACS), astronomers can identify and remove such distracting field stars, and resolve the cluster’s dense core in unprecedented detail. …

5 December 2011: Three thousand light-years from Earth lies the strange protoplanetary nebula IRAS 09371+1212, nicknamed the Frosty Leo Nebula. Despite their name, protoplanetary nebulae have nothing to do with planets: they are formed from material shed from their aging central star. The Frosty Leo Nebula has acquired its curious name as it has been found to be rich in water in the form of ice grains, and because it lies in the constellation of Leo. This nebula is particularly noteworthy because it has formed far from the galactic plane, away from interstellar clouds that may block our view. The intricate shape comprises a spherical halo, a disc around the central star, lobes and gigantic loops. This complex structure strongly suggests that the formation processes are complex and it has been suggested that there could be a second star, currently unseen, contributing to the shaping of the nebula. Protoplanetary nebulae like the Frosty …

28 November 2011: The NASA/ESA Hubble Space Telescope has caught sight of a soft, diffuse-looking galaxy that is probably the aftermath of a long-ago galactic collision. Two spiral galaxies, each perhaps much like the Milky Way, swirled together for millions of years. In such mergers, the original galaxies are often stretched and pulled apart as they wrap around a common centre of gravity. After a few back-and-forths, this starry tempest settles down into a new, round object. The now subdued celestial body, catalogued as SDSS J162702.56+432833.9, is technically known as an elliptical galaxy. When galaxies collide — a common event in the Universe — a fresh burst of star formation typically takes place as gas clouds mash together. At this point, the galaxy has a blue hue, but the colour does not mean it is cold: it is a result of the intense heat of newly formed blue–white stars. Those stars do not …

21 November 2011: In one of the largest known star formation regions in the Large Magellanic Cloud (LMC), a small satellite galaxy of the Milky Way, lie young and bright stellar groupings known as OB associations. One of these associations, called LH 72, was captured in this dramatic NASA/ESA Hubble Space Telescope image. It consists of a few high-mass, young stars embedded in a beautiful and dense nebula of hydrogen gas. Much of the star formation in the LMC occurs in super-giant shells. These regions of interstellar gas are thought to have formed due to strong stellar winds and supernova explosions that cleared away much of the material around the stars creating wind-blown shells. The swept-up gas eventually cools down and fragments into smaller clouds that dot the edges of these regions and eventually collapse to form new stars. The biggest of these shells, home to LH 72, is designated LMC4. With a …

14 November 2011: The NASA/ESA Hubble Space Telescope has peered deep into NGC 4631, better known as the Whale Galaxy. Here, a profusion of starbirth lights up the galactic centre, revealing bands of dark material between us and the starburst. The galaxy’s activity tapers off  in its outer regions where there are fewer stars and less dust, but these are still punctuated by pockets of star formation. The Whale Galaxy is about 30 million light-years away from us in the constellation of Canes Venatici (The Hunting Dogs) and is a spiral galaxy much like the Milky Way. From our vantage point, however, we see the Whale Galaxy edge-on, seeing its glowing centre through dusty spiral arms. The galaxy's central bulge and asymmetric tapering disc have suggested the shape of a whale or a herring to past observers. Many supernovae — the explosions of hot, blue, short-lived stars at least eight times the mass …

7 November 2011: The object shown in this beautiful Hubble image, dubbed Messier 54, could be just another globular cluster, but this dense and faint group of stars was in fact the first globular cluster found that is outside our galaxy. Discovered by the famous astronomer Charles Messier in 1778, Messier 54 belongs to a satellite of the Milky Way called the Sagittarius Dwarf Elliptical Galaxy. Messier had no idea of the significance of his discovery at the time, and it wasn’t until over two centuries later, in 1994, that astronomers found Messier 54 to be part of the miniature galaxy and not our own. Current estimates indicate that the Sagittarius dwarf, and hence the cluster, is situated almost 90 000 light-years away — more than three times as far from the centre of our galaxy than the Solar System. Ironically, even though this globular cluster is now understood to lie outside the …

31 October 2011: The pearly wisps surrounding the central star IRAS 10082-5647 in this Hubble image certainly draw the eye towards the heavens. The divine-looking cloud is a reflection nebula, made up of gas and dust glowing softly by the reflected light of nearby stars, in this case a young Herbig Ae/Be star. The star, like others of this type, is still a relative youngster, only a few million years old. It has not yet reached the so-called main sequence phase, where it will spend around 80% of its life creating energy by burning hydrogen in its core. Until then the star heats itself by gravitational collapse, as the material in the star falls in on itself, becoming ever denser and creating immense pressure which in turn gives off copious amounts of heat. Stars only spend around 1% of their lives in this pre-main sequence phase. Eventually, gravitational collapse will heat the star’s …

24 October 2011: The NASA/ESA Hubble Space Telescope has taken this image of the Phoenix Dwarf Galaxy, which is located 1.4 million light-years away from Earth. It is located in the constellation of Phoenix in the southern sky. The object, a dwarf irregular galaxy, features younger stars in its inner regions and older ones at its outskirts. Dwarf galaxies are small galaxies composed of a few billion stars, compared to fully-fledged galaxies which can contain hundreds of billions of stars. In the Local Group, there are a number of such dwarf galaxies orbiting the larger galaxies such as the Milky Way or the Andromeda Galaxy. They are thought to have been created by tidal forces in the early stages of the creation of these galaxies, or as a result of collisions between galaxies, forming from ejected streams of material and dark matter from the parent galaxies. The Milky Way galaxy features at least …

17 October 2011: Supernova SN 1987A, one of the brightest stellar explosions since the invention of the telescope more than 400 years ago, is no stranger to the NASA/ESA Hubble Space Telescope. The observatory has been on the frontline of studies into this brilliant dying star since its launch in 1990, three years after the supernova exploded on 23 February 1987. This image of Hubble’s old friend, retreived from the telescope’s data archive, may be the best ever of this object, and reminds us of the many mysteries still surrounding it. Dominating this picture are two glowing loops of stellar material and a very bright ring surrounding the dying star at the centre of the frame. Although Hubble has provided important clues on the nature of these structures, their origin is still largely unknown. Another mystery is that of the missing neutron star. The violent death of a high-mass star, such as SN …

10 October 2011: Galaxies come in a variety of shapes and sizes, and these features change as they evolve. Some, like the galaxy in the centre of this NASA/ESA Hubble Space Telescope image, are beautiful spirals with graceful curved arms, while others are fuzzy oval-shaped blobs like the large object showing up near the bottom right of the frame. Others still are rather irregular in shape, such as the orange galaxy at the top of the image, which resembles a tiny wobbling string. This picture is one of the few hundred exposures taken with Hubble’s Advanced Camera for Surveys to assemble the “Extended Groth Strip”. This strip, named after the Princeton University astronomer Edward Groth, is a composite picture of a rectangular region of the sky in the constellation of Ursa Major. It covers a relatively small area in the sky — equivalent to roughly the width of a finger stretched at arms’ …

3 October 2011: Thousands and thousands of brilliant stars make up this globular cluster, Messier 53, captured with crystal clarity in this image from the NASA/ESA Hubble Space Telescope. Bound tightly by gravity, the cluster is roughly spherical and becomes denser towards its heart. These enormous sparkling spheres are by no means rare, and over 150 exist in the Milky Way alone, including Messier 53. It lies on the outer edges of the galaxy, where many other globular clusters are found, almost equally distant from both the centre of our galaxy and the Sun. Although they are relatively common, the famous astronomer William Herschel, not at all known for his poetic nature, once described a globular cluster as “one of the most beautiful objects I remember to have seen in the heavens”, and it is clear to see why. Globular clusters are much older and larger than open clusters, meaning they are generally …

26 September 2011: This NASA/ESA Hubble Space Telescope image shows remarkable structures in a galaxy cluster around an object called LRG-4-606. LRG stands for Luminous Red Galaxy, and is the acronym given to a large collection of bright red galaxies found in the Sloan Digital Sky Survey (SDSS). These objects are mostly massive elliptical galaxies composed of huge numbers of old stars. It is sobering to contemplate the sheer number of stars that this image must contain — hundreds of billions — but it also features one of the strangest phenomena known to astronomers. This particular red galaxy and its surrounding galaxies happen to be positioned so that their strong gravitational field has a dramatic effect. Left of centre in the picture, blue galaxies in the background have been stretched and warped out of shape into narrow, pale blue arcs. This is because of an effect called gravitational lensing. The galaxy cluster has …

19 September 2011: In this NASA/ESA Hubble Space Telescope image, NGC 4874 is the brightest object, located to the right of the frame and seen as a bright star-like core surrounded by a hazy halo. A few of the other galaxies of the cluster are also visible, looking like flying saucers dancing around NGC 4874. But the really remarkable feature of this image is the point-like objects around NGC 4874, revealed on a closer look: almost all of them are clusters of stars that belong to the galaxy. Each of these globular star clusters contains many hundreds of thousands of stars. Recently, astronomers discovered that a few of these point-like objects are not star clusters but ultra-compact dwarf galaxies, also under the gravitational influence of NGC 4874. Being only about 200 light-years across and mostly made up of old stars, these galaxies resemble brighter and larger versions of globular clusters. They are thought …

12 September 2011: This NASA/ESA Hubble Space Telescope image shows a compact and distant globular star cluster that lies in one of the smallest constellations in the night sky, Delphinus (The Dolphin). Due to its modest size, great distance and relatively low brightness, NGC 7006 is often ignored by amateur astronomers. But even remote globular clusters such as this one appear bright and clear when imaged by Hubble’s Advanced Camera for Surveys. NGC 7006 resides in the outskirts of the Milky Way. It is about 135 000 light-years away, five times the distance between the Sun and the centre of the galaxy, and it is part of the galactic halo. This roughly spherical region of the Milky Way is made up of dark matter, gas and sparsely distributed stellar clusters. Like other remote globular clusters, NGC 7006 provides important clues that help astronomers to understand how stars formed and assembled in the halo. …