Picture of the Week

30 May 2011: The NASA/ESA Hubble Space Telescope is renowned for its breathtaking images and this snapshot of NGC 634 is definitely that — the fine detail and exceptionally perfect spiral structure of the galaxy make it hard to believe that this is a real observation and not an artist’s impression or a screenshot taken straight from Star Wars. This spiral galaxy was discovered back in the nineteenth century by French astronomer Édouard Jean-Marie Stephan, but in 2008 it became a prime target for observations thanks to the violent demise of a white dwarf star. The type Ia supernova known as SN2008a was spotted in the galaxy and briefly rivalled the brilliance of its entire host galaxy but, despite the energy of the explosion, it can no longer be seen this Hubble image, which was taken around a year and a half later. White dwarfs are thought to be the endpoint of evolution …

9 May 2011: This NASA/ESA Hubble Space Telescope image shows the edge-on profile of the slender spiral galaxy NGC 5775. Although the spiral is tilted away from us, with only a thin slither on view, such a perspective can be advantageous for astronomers because the regions above and below the galaxy’s disc can be seen much more clearly. For instance, astronomers have previously used the high inclination of this spiral to study the properties of the halo of hot gas that is visible when the galaxy is observed at X-ray wavelengths. The mechanism behind such haloes is unclear, but they are found around spirals that have a high star formation rate, like NGC 5775. Some astronomers think that hot gas from the disc is driven into the halo by supernova explosions, which is then returned to the disc as it cools — like a massive galactic fountain. Meanwhile, there is further disruption taking …

25 April 2011: Galaxies come in all sorts of shapes and sizes, with most being classed as either elliptical or spiral. However, some fall into the miscellaneous category known as irregulars, such as UGC 9128 shown here in this NASA/ESA Hubble Space Telescope image. UGC 9128 is a dwarf irregular galaxy, which means that in addition to not having a well-defined shape, it probably contains only around one hundred million stars — far fewer than are found in a large spiral such as the Milky Way. Dwarf galaxies are important in understanding how the Universe has evolved and they are often referred to as galactic building blocks, as galaxies are thought to grow as smaller ones merge. In recent years, astronomers have been trying to find out if dwarf galaxies contain a similar halo and disc structure to their much larger counterparts, whereby older stars are found in the extended spheroidal halo, with …

21 March 2011: This view from the NASA/ESA Hubble Space Telescope shows the beautiful spiral galaxy NGC 5584. This galaxy has played a key role in a new study that measures the expansion rate of the Universe to greater accuracy than ever before. NGC 5584 was first spotted as a faint glow in the constellation of Virgo by the great visual observer E. E. Barnard, back in 1881, using just a 12.5-cm telescope. But, by bringing the power of Hubble to bear, the galaxy can be resolved into thousands of separate stars. Some of these stars vary in brightness and are classified as Cepheids. These are brilliant pulsating stars with a remarkable property — once the time it takes a Cepheid to brighten and fade is known, then it is possible to find how bright it actually is. When this information is combined with a measurement of how bright the star appears it …

21 February 2011: The NASA/ESA Hubble Space Telescope has produced this finely detailed image of the beautiful spiral galaxy NGC 6384. This galaxy lies in the constellation of Ophiuchus (The Serpent Bearer), not far from the centre of the Milky Way on the sky. The positioning of NGC 6384 means that we have to peer at it past many dazzling foreground Milky Way stars that are scattered across this image. In 1971, one member of NGC 6384 stood out against these bright foreground stars when one of its stars exploded as a supernova. This was a Type Ia supernova, which occurs when a compact star that has ceased fusion in its core, called a white dwarf, increases its mass beyond a critical limit by gobbling up matter from a companion star. A runaway nuclear explosion then makes the star suddenly as bright as a whole galaxy. While many stars have already come to …

31 January 2011: Hubble's Advanced Camera for Surveys has captured this moment in the ever-changing life of a spiral galaxy called IC 391. Although these massive star cities appear static and unchanging, their stellar inhabitants are constantly moving and evolving, with new stars being born and old stars reaching the ends of their lives —often in spectacular fashion, with an immense supernova explosion that can be viewed from Earth. On 3 January 2001, members of the Beijing Astronomical Observatory discovered such an explosion within IC 391 and it was named SN 2001B. This was a Type Ib supernova, which occurs when a massive star runs out of fuel for nuclear fusion and collapses, emitting vast amounts of radiation and creating a powerful shock wave. Hubble has contributed much to our understanding of supernovae in recent years, and it has made an extensive study of supernova 1987A (heic0704), the brightest such stellar explosion to …

17 January 2011: The spiral galaxy NGC 1345 and its loose and ragged arms dominate this rich image from the NASA/ESA Hubble Space Telescope. It is a member of the Eridanus Galaxy Cluster — a group of about 70 galaxies that lies 85 million light-years away in the constellation of Eridanus (the River). This region of the night sky is well populated with bright galaxies, with the Fornax Cluster of galaxies also nearby on the celestial sphere, although the two clusters are actually separated by about 20 million light-years. Collectively, they are known as the Fornax Supercluster or the Southern Supercluster. John Herschel discovered NGC 1345 in 1835 from South Africa. He described it as small and very faint and it is still far from easy to see it even with quite a large amateur telescope, where it appears as a small, circular fuzz. Apart from the main galaxy that dominates the picture, …

20 December 2010: The galaxy captured in this image, called UGC 12158, certainly isn’t camera-shy: this spiral stunner is posing face-on to the NASA/ESA Hubble Space Telescope’s Advanced Camera for Surveys, revealing its structure in fine detail. UGC 12158 is an excellent example of a barred spiral galaxy in the Hubble sequence — a scheme used to categorise galaxies based on their shapes. Barred spirals, as the name suggest, feature spectacular swirling arms of stars that emanate from a bar-shaped centre. Such bar structures are common, being found in about two thirds of spiral galaxies, and are thought to act as funnels, guiding gas to their galactic centres where it accumulates to form newborn stars. These aren’t permanent structures: astronomers think that they slowly disperse over time, so that the galaxies eventually evolve into regular spirals. The appearance of a galaxy changes little over millions of years, but this image also contains a …

29 November 2010: Fresh starbirth infuses the galaxy NGC 6503 with a vital pink glow in this image from the NASA/ESA Hubble Space Telescope. This galaxy, a smaller version of the Milky Way, is perched near a great void in space where few other galaxies reside. This new image from Hubble’s Advanced Camera for Surveys displays, with particular clarity, the pink-coloured puffs marking where stars have recently formed in NGC 6503's swirling spiral arms. Although structurally similar to the Milky Way, the disc of NGC 6503 spans just 30 000 light-years, or just about a third of the size of the Milky Way, leading astronomers to classify NGC 6503 as a dwarf spiral galaxy. NGC 6503 lies approximately 17 million light-years away in the constellation of Draco (the Dragon). The German astronomer Arthur Auwers discovered this galaxy in July 1854 in a region of space where few other luminous bodies have been found. …

1 November 2010: At first glance NGC 3077 looks like a typical, relatively peaceful elliptical galaxy. However, as this NASA/ESA Hubble Space Telescope image dramatically reveals it is actually a hotbed of very energetic star formation and the whole galaxy is laced with dusty tendrils. It lies about 13 million light-years from Earth.NGC 3077 was first seen by William Herschel with his 47 cm telescope in England in 1801, when he was close to completing his sky surveys. It is located in the far northern sky in the constellation of Ursa Major (the Great Bear) and forms a triplet with two brighter nearby galaxies, the graceful spiral Messier 81 and the very peculiar and active starburst galaxy Messier 82.Although overshadowed by its brighter neighbours, NGC 3077 is also very active and resembles a less dramatic version of Messier 82. Interactions between the three galaxies have stoked the fires of star formation in the …

11 October 2010: The beautiful spiral galaxy NGC 406 was discovered in 1834 by John Herschel and is here imaged in great detail by the NASA/ESA Hubble Space Telescope. Located some 65 million light-years away, in the southern constellation of Tucana (the Toucan), NGC 406 is about 60 000 light-years across, roughly half the diameter of our galaxy, the Milky Way. It is a spiral galaxy quite similar to the well known Whirlpool galaxy (Messier 51, see http://www.spacetelescope.org/images/opo0110a/). In a moderate-sized amateur telescope NGC 406 would appear as a faint hazy blob, like thousands of others across the sky, and none of the spectacular fine detail in the Hubble picture could be made out. In this image the galaxy exhibits spiral arms that are mainly populated by young, massive, bluish stars and crossed by dark dust lanes. As is typically observed in this kind of spiral galaxy, the yellowish central bulge, dominated by …

26 July 2010: This NASA/ESA Hubble Space Telescope picture depicts the galaxy NGC 1533 in the southern constellation of Dorado (the Dolphin-fish). Around 62 million light-years from Earth, NGC 1533, which is classed as a lenticular galaxy, is a transitional type that shows characteristics of both spiral and elliptical galaxies. Like elliptical galaxies, NGC 1533 is largely made up of older and redder stars and vast numbers of them create the smooth glow across the whole picture. However, it also has a residual level of star formation and some young blue stars, which are revealed by its weak barred spiral structure that is faintly visible in this image. Astronomers studying star formation in this type of galaxy are able to subtract the bright light of the stars to reveal the details of a subtle spiral structure that cannot be well seen in less heavily processed images such as this one. John Herschel, son …

21 June 2010: The small spiral galaxy NGC 2758 is captured in great detail in this image from the Advanced Camera for Surveys (ACS) on the NASA/ESA Hubble Space Telescope. It was first seen by the American astronomer Frank Muller, at the McCormick Observatory in Virginia in the 1880s. This galaxy lies in the constellation of Hydra (the Sea Serpent), the largest and longest constellation in the sky. This very sharp Hubble image shows many of the bright blue stars within the galaxy’s loosely wound spiral arms as well as a bright companion dwarf galaxy to the upper left. Many more distant galaxies lie in the background. NGC 2758 is a fine example of a spiral galaxy that is close enough for the structure to be seen very clearly. In this case the Hubble images were obtained as part of a study of the central redder “bulge” component of galaxies and how they …

31 May 2010: The bright galaxy NGC 3810 demonstrates classical spiral structure in this very detailed image from Hubble. The bright central region is thought to be forming many new stars and is outshining the outer areas of the galaxy by some margin. Further out the galaxy displays strikingly rich dust clouds along its spiral arms. A close look shows that Hubble’s sharp vision also allows many individual stars to be seen. Hot young blue stars show up in giant clusters far from the centre and the arms are also littered with bright red giant stars. The original images were acquired by astronomers studying a supernova discovered late in the year 2000. It was the second supernova found in the galaxy in quick succession following another discovered in 1997. NGC 3810 is located about 50 million light-years from Earth in the constellation of Leo (the lion). It was discovered by William Herschel in …

17 May 2010: That galaxies come in very different shapes and sizes is dramatically demonstrated by this striking Hubble image of the Hickson Compact Group 59. Named by astronomer Paul Hickson in 1982, this is the 59th such collection of galaxies in his catalogue of unusually close groups. What makes this image interesting is the variety on display. There are two large spiral galaxies, one face-on with smooth arms and delicate dust tendrils, and one highly inclined, as well as a strangely disorderly galaxy featuring clumps of blue young stars. We can also see many apparently smaller, probably more distant, galaxies visible in the background. Hickson groups display many peculiarities, often emitting in the radio and infrared and featuring active star-forming regions. In addition their galaxies frequently contain Active Galactic Nuclei powered by supermassive black holes, as well large quantities of dark matter. The NASA/ESA Hubble Space Telescope's Advanced Camera for Surveys, using …

3 May 2010: The richly textured spiral galaxy NGC 2082 is found about 60 million light-years away in the constellation of Dorado (the Swordfish), deep in the southern sky. As seen here in a very detailed image from the Advanced Camera for Surveys on the NASA/ESA Hubble Space Telescope, filaments of dark dust splay across NGC 2082’s luminous curved arms and dense central bulge of stars. Hubble’s sharp vision also reveals many of the individual bright blue stars dotting the galaxy’s rather ragged spiral arms as well as many much more distant galaxies in the background. This galaxy is faintly visible in backyard telescopes and was first recorded by Sir John Herschel during his visit to the Cape of Good Hope in South Africa in the 1830s. NGC 2082 was also the host of a bright supernova that was spotted by the great visual supernova discoverer Rev R. Evans back in 1992. This …