Picture of the Week

25 June 2012: Relatively few galaxies possess the sweeping, luminous spiral arms or brightly glowing centre of our home galaxy the Milky Way. In fact, most of the Universe's galaxies look like small, amorphous clouds of vapour. One of these galaxies is DDO 82, captured here in an image from the NASA/ESA Hubble Space Telescope. Though tiny compared to the Milky Way, such dwarf galaxies still contain between a few million and a few billion stars. DDO 82, also known by the designation UGC 5692, is not without a hint of structure, however. Astronomers classify it as an Sm galaxy, or Magellanic spiral galaxy, named after the Large Magellanic Cloud, a dwarf galaxy that orbits the Milky Way. That galaxy, like DDO 82, is said to have one spiral arm. In the case of DDO 82, gravitational interactions over its history seem to have discombobulated it so that this structure is not as …

4 June 2012: This image, taken by the NASA/ESA Hubble Space Telescope, shows a detailed view of the spiral arms on one side of the galaxy Messier 99. Messier 99 is a so-called grand design spiral, with long, large and clearly defined spiral arms — giving it a structure somewhat similar to the Milky Way. Lying around 50 million light-years away, Messier 99 is one of over a thousand galaxies that make up the Virgo Cluster, the closest cluster of galaxies to us. Messier 99 itself is relatively bright and large, meaning it was one of the first galaxies to be discovered, way back in the 18th century. This earned it a place in Charles Messier’s famous catalogue of astronomical objects. In recent years, a number of unexplained phenomena in Messier 99 have been studied by astronomers. Among these is the nature of one of the brighter stars visible in this image. Catalogued …

21 May 2012: The NASA/ESA Hubble Space Telescope captured this image of the spiral galaxy known as ESO 498-G5. One interesting feature of this galaxy is that its spiral arms wind all the way into the centre, so that ESO 498-G5's core looks like a bit like a miniature spiral galaxy. This sort of structure is in contrast to the elliptical star-filled centres (or bulges) of many other spiral galaxies, which instead appear as glowing masses, as in the case of NGC 6384. Astronomers refer to the distinctive spiral-like bulge of galaxies such as ESO 498-G5 as disc-type bulges, or pseudobulges, while bright elliptical centres are called classical bulges. Observations from the Hubble Space Telescope, which does not have to contend with the distorting effects of Earth's atmosphere, have helped to reveal that these two different types of galactic centres exist. These observations have also shown that star formation is still going on …

14 May 2012: Visible in the constellation of Andromeda, NGC 891 is located approximately 30 million light-years away from Earth. The NASA/ESA Hubble Space Telescope turned its powerful wide field Advanced Camera for Surveys towards this spiral galaxy and took this close-up of its northern half. The galaxy's central bulge is just out of the image on the bottom left. The galaxy, spanning some 100 000 light-years, is seen exactly edge-on, and reveals its thick plane of dust and interstellar gas. While initially thought to look like our own Milky Way if seen from the side, more detailed surveys revealed the existence of filaments of dust and gas escaping the plane of the galaxy into the halo over hundreds of light-years. They can be clearly seen here against the bright background of the galaxy halo, expanding into space from the disc of the galaxy. Astronomers believe these filaments to be the result of …

2 April 2012: This image from the NASA/ESA Hubble Space Telescope shows NGC 4980, a spiral galaxy in the southern constellation of Hydra. The shape of NGC 4980 appears slightly deformed, something which is often a sign of recent tidal interactions with another galaxy. In this galaxy’s case, however, this appears not to be the case as there are no other galaxies in its immediate vicinity. The image was produced as part of a research program into the nature of galactic bulges, the bright, dense, elliptical centres of galaxies. Classical bulges are relatively disordered, with stars orbiting the galactic centre in all directions. In contrast, in galaxies with so-called pseudobulges, or disc-type bulges, the movement of the spiral arms is preserved right to the centre of the galaxy. Although the spiral structure is relatively subtle in this image, scientists have shown that NGC 4980 has a disc-type bulge, and its rotating spiral structure …

26 March 2012: The NASA/ESA Hubble Space Telescope has spotted a UFO — well, the UFO Galaxy, to be precise. NGC 2683 is a spiral galaxy seen almost edge-on, giving it the shape of a classic science fiction spaceship. This is why the astronomers at the Astronaut Memorial Planetarium and Observatory gave it this attention-grabbing nickname. While a bird’s eye view lets us see the detailed structure of a galaxy (such as this Hubble image of a barred spiral), a side-on view has its own perks. In particular, it gives astronomers a great opportunity to see the delicate dusty lanes of the spiral arms silhouetted against the golden haze of the galaxy’s core. In addition, brilliant clusters of young blue stars shine scattered throughout the disc, mapping the galaxy’s star-forming regions. Perhaps surprisingly, side-on views of galaxies like this one do not prevent astronomers from deducing their structures. Studies of the properties of …

5 March 2012: The NASA/ESA Hubble Space Telescope has produced this beautiful image of the galaxy NGC 1483. NGC 1483 is a barred spiral galaxy located in the southern constellation of Dorado — the dolphinfish in Spanish. The nebulous galaxy features a bright central bulge and diffuse arms with distinct star-forming regions. In the background, many other distant galaxies can be seen. The constellation Dorado is home to the Dorado Group of galaxies, a loose group comprising an estimated 70 galaxies and located some 62 million light-years away. The Dorado group is much larger than the Local Group that includes the Milky Way (and which contains around 30 galaxies) and approaches the size of a galaxy cluster. Galaxy clusters are the largest groupings of galaxies (and indeed the largest structures of any type) in the Universe to be held together by their gravity.   Barred spiral galaxies are so named because of the …

6 February 2012: Many of the Universe’s galaxies are like our own, displaying beautiful spiral arms wrapping around a bright nucleus. Examples in this stunning image, taken with the Wide Field Camera 3 on the NASA/ESA Hubble Space Telescope, include the tilted galaxy at the bottom of the frame, shining behind a Milky Way star, and the small spiral at the top centre. Other galaxies are even odder in shape. Markarian 779, the galaxy at the top of this image, has a distorted appearance because it is likely the product of a recent galactic merger between two spirals. This collision destroyed the spiral arms of the galaxies and scattered much of their gas and dust, transforming them into a single peculiar galaxy with a unique shape. This galaxy is part of the Markarian catalogue, a database of over 1500 galaxies named after B. E. Markarian, the Armenian astronomer who studied them in the …

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 …

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 …

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 …

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 …

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 …

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’ …

29 August 2011: Peering into the depths of space, the sharp-eyed NASA/ESA Hubble Space Telescope has imaged the nearby but faint dwarf galaxy ESO 540-030. This object itself appears as a huge swarm of dim stars, but ESO 540-030 is actually just one point of interest in the picture. ESO 540-030 is just over 11 million light-years distant, and is part of the Sculptor group of galaxies. This collection is the closest neighbour to our own Local Group of galaxies that includes the Milky Way. Due to its proximity the Sculptor group contains some of the brightest galaxies in the southern skies, although ESO 540-030 is not one of these; dwarf galaxies generally have low surface brightness, which make observations difficult. Hubble has captured a snapshot of galaxy types in the background, with spirals, barred spirals, ellipticals and irregulars on display. Careful examination of this picture should allow examples of each galaxy type …

22 August 2011: NGC 2146 is classified as a barred spiral due to its shape, but the most distinctive feature is the dusty spiral arm that has looped in front of the galaxy's core as seen from our perspective. The forces required to pull this structure out of its natural shape and twist it up to 45 degrees are colossal. The most likely explanation is that a neighbouring galaxy is gravitationally perturbing it and distorting the orbits of many of NGC 2146’s stars. It is probable that we are currently witnessing the end stages of a process which has been occurring for tens of millions of years. NCG 2146 is undergoing intense bouts of star formation, to such an extent that it is referred to as a starburst galaxy. This is a common state for barred spirals, but the extra gravitational disruption that NGC 2146 is enduring no doubt exacerbates the situation, compressing …

8 August 2011: The NASA/ESA Hubble Space Telescope has imaged part of the Hickson Compact Group 7, or HCG 7 for short. This grouping is composed of one lenticular (lens-shaped) and three spiral galaxies in close proximity. In this image, one of the spirals dominates the foreground, with many more distant galaxies peppering the background. Observing tightly-knit galaxy groups like HCG 7 is important because they evolve in a different way from their more spaced-out counterparts in less crowded regions of the Universe.  A recent study using Hubble data analysed the star clusters in HCG 7. Three hundred young clusters and 150 globular clusters were charted, and their ages and distributions measured. The results suggest that the rate of star formation has been fairly steady through time, although quite high in the central regions. Additional studies, including searches for material between the galaxies, hint that the stars in the HCG 7 galaxies formed …

18 July 2011: The NASA/ESA Hubble Space Telescope has imaged an elongated stream of stars, gas and dust called IC 755, which is actually a spiral galaxy that we are seeing edge-on. In 1999 a star within IC 755 was seen to explode as a supernova and named SN 1999an. The supernova was discovered by the Beijing Astronomical Observatory Supernova Survey and three years later Hubble was used to study the environment in which the explosion took place. The inclination of the galaxy made the supernova a challenging target as many other intervening objects obscured the view. Valuable data were obtained and suggest that before detonation the star may have been around 20 times more massive than our Sun, and that it was likely to have been in the region of 14 million years old. Supernovae like SN 1999an are classified as Type IIs and they are dramatic events that mark the end …

20 June 2011: In this NASA/ESA Hubble Space Telescope image of NGC 7479 — created from observations at visible and near-infrared wavelengths — the tightly wound arms of the spiral galaxy create an inverted ‘S’, as they spin in an anticlockwise direction. However, at radio wavelengths, this galaxy, sometimes nicknamed the Propeller Galaxy, spins the other way, with a jet of radiation that bends in the opposite direction to the stars and dust in the arms of the galaxy. Astronomers think that the radio jet in NGC 7479 was put into its bizarre backwards spin following a merger with another galaxy. Star formation is reignited by galactic collisions, and indeed NGC 7479 is undergoing starburst activity, with many bright, young stars visible in the spiral arms and disc. The three brightest stars in this image, however, are foreground stars — caught on camera because they lie between the galaxy and Hubble. This striking …