Picture of the Week

Also see our pictures of the week on eso.org.
28 March 2011

Cluster’s deceptive serenity hides violent past

The high concentration of stars within globular clusters, like Messier 12, shown here in an image from the NASA/ESA Hubble Space Telescope, makes them beautiful photographic targets. But the cramped living quarters in these clusters also makes them home to exotic binary star systems where two stars are locked in tight orbits around each other and matter from one is gobbled up by its companion, releasing X-rays. It is thought that such X-ray binaries form from very close encounters between stars in crowded regions, such as globular clusters, and even though Messier 12 is fairly diffuse by globular cluster standards, such X-ray sources have been spotted there.

Astronomers have also discovered that Messier 12 is home to far fewer low-mass stars than was previously expected (eso0604). In a recent study, astronomers used the European Southern Observatory’s Very Large Telescope at Cerro Paranal, Chile, to measure the brightness and colours of more than 16 000 of the globular’s 200 000 stars. They speculate that nearly one million low-mass stars have been ripped away from Messier 12 as the globular has passed through the densest regions of the Milky Way during its orbit around the galactic centre.

It seems that the serenity of this view of Messier 12 is misleading and the object has had a violent and disturbed past.

Messier 12 lies about 23 000 light-years away in the constellation of Ophiuchus (The Serpent Bearer). This image was taken using the Wide Field Channel of Hubble’s Advanced Camera for Surveys. The colour image was created from exposures through a blue filter (F435W, coloured blue), a red filter (F625W, coloured green) and a filter that passes near-infrared light (F814W coloured red). The total exposure times were 1360 s, 200 s and 364 s, respectively. The field of view is about 3.2 x 3.1 arcminutes .

21 March 2011

Spiral key to Universe's expansion

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 is easy to work out how far away the star actually lies. This method is the most accurate and effective way to measure the distances to most nearby galaxies.

This trick has now been used as part of a major new study of the expansion rate of the Universe, led by Adam Riess at the Space Telescope Science Institute in Baltimore. By studying many Cepheids in several galaxies the team has been able to refine our knowledge of this expansion rate, expressed as a number known as Hubble’s constant, to an accuracy of 3.3 percent.

In addition to many Cepheids NGC 5584 was also recently the site of a type Ia supernova. These dramatic explosions of white dwarf stars are used as reference beacons for mapping the expansion, and acceleration, of the more remote Universe so this galaxy is a very valuable link between the two distance scales.

More details of this major study, and its significance for the understanding of dark energy, can be found in a press release from NASA: http://hubblesite.org/news/2011/08.

This picture was created from many exposures taken with Hubble’s Wide Field Channel 3. Images through three filters have been combined to create this composite picture. Light detected through a filter that transmits most visible light (F350LP) is coloured white, light coming through a yellow/green filter (F555W) is coloured blue and near infrared light (the F814W filter) is coloured red. The field of view 2.4 arcminutes across and the total exposure time was 20.8 hours.

14 March 2011

Family of stars breaking up

Most of the rich globular star clusters that orbit the Milky Way have cores that are tightly packed with stars, but NGC 288 is one of a minority of low-concentration globulars, with its stars more loosely bound together. This new image from the Advanced Camera for Surveys on the NASA/ESA Hubble Space Telescope completely resolves the old stars at the core of the cluster.

The colours and brightnesses of the stars in the picture tell the story of how the stars have evolved in the cluster. The many fainter points of light are normal low-mass stars that are still fusing hydrogen in the same way as the Sun. The brighter stars fall into two classes: the yellow ones are red giant stars that are at a later phase in their careers and are now bigger, cooler and brighter. The bright blue stars are even more massive stars that have left the red giant phase and are being powered by helium fusion in their cores.

The stars within globular clusters form at about the same time from the same cloud of gas, making these close families of stars. However, astronomers think that the stellar siblings in low-concentration globular clusters such as NGC 288, which are not so tightly bound together by gravity as richer and denser clusters, may eventually disperse and go their separate ways.

NGC 288 is found within the rather obscure southern constellation of Sculptor, at a distance of about 30 000 light-years. This constellation also contains NGC 253, more commonly called the Sculptor Galaxy due to its location, and these two deep sky objects are close enough together on the sky to be observed in the same binocular field of view. William Herschel first spotted NGC 288 in 1785 and also recognised that it was a globular cluster that could be resolved into stars in his telescope.

This picture was created from Hubble images taken using the Wide Field Channel of the Advanced Camera for Surveys through four different filters. Light recorded through a blue filter (F435W) is coloured blue, light through an orange filter (F606W) appears as green, light coming through a near-infrared filter (F814W) is red and finally the light from glowing hydrogen (F658N) is orange. The exposure times were 740 s, 530 s, 610 s and 1760 s respectively and the field of view is 3.2 arcminutes across.

7 March 2011

A dying star’s toxic legacy

The strange and irregular bundle of jets and clouds in this curious image from the NASA/ESA Hubble Space Telescope is the result of a burst of activity late in the life of a star. As its core runs out of nuclear fuel, the star’s unstable outer layers are puffing out a toxic concoction of gases including carbon monoxide and hydrogen cyanide.

The Westbrook Nebula — also known as PK166-06, CRL 618 and AFGL 618 — is a protoplanetary nebula, an opaque, dark and relatively short-lived cloud of gas that is ejected by a star as it runs out of nuclear fuel. As the star hidden deep in the centre of the nebula evolves further it will turn into a hot white dwarf and the gas around it will become a glowing planetary nebula, before eventually dispersing. Because this is a relatively brief stage in the evolution process of stars, only a few hundred protoplanetary nebulae are known in the Milky Way.

Protoplanetary nebulae are cool, and so emit little visible light. This makes them very faint, posing challenges to scientists who wish to study them. What this picture shows, therefore, is a composite image representing the different tricks that the astronomers used to unravel what is going on within this strange nebula. The picture includes exposures in visible light which shows light reflected from the cloud of gas, combined with other exposures in the near-infrared part of the spectrum, showing us the dim glow, invisible to human eyes, that is coming from different elements deep in the cloud itself.

One of the nebula’s names, AFGL 618, comes from its discovery by a precursor to the Hubble Space Telescope: the letters stand for Air Force Geophysics Laboratory. This US research organisation launched a series of suborbital rockets with infrared telescopes on board in the 1970s, cataloguing hundreds of objects that were impossible or difficult to observe from the ground. In some respects, these were a proof of concept for later orbital infrared astronomical facilities including Hubble and ESA’s Herschel Space Observatory.

This image was prepared from many separate exposures taken using Hubble’s newest camera, the Wide Field Camera 3. Exposures through a green filter (F547M) were coloured blue, those through a yellow/orange filter (F606W) were coloured green and exposures through a filter that isolates the glow from ionised nitrogen (F658N) have been coloured red. Images through filters that capture the glows from singly and doubly ionised sulphur (F673N and F953N) are also shown in red. The total exposure times were about nine minutes through each filter and the field of view is approximately 20 arcseconds across.


28 February 2011

Orion’s lesser-known nebula takes centre stage

The NASA/ESA Hubble Space Telescope has taken a close-up view of an outer part of the Orion Nebula’s little brother, Messier 43. This nebula, which is sometimes referred to as De Mairan’s Nebula after its discoverer, is separated from the famous Orion Nebula (Messier 42) by only a dark lane of dust. Both nebulae are part of the massive stellar nursery called the Orion molecular cloud complex, which includes several other nebulae, such as the Horsehead Nebula (Barnard 33) and the Flame Nebula (NGC 2024).

The Orion molecular cloud complex is about 1400 light-years away, making it one of the closest massive star formation regions to Earth. Hubble has therefore studied this extraordinary region extensively over the past two decades, monitoring how stellar winds sculpt the clouds of gas, studying young stars and their surroundings and discovering many elusive objects, such as brown dwarf stars.

This view shows several of the brilliant hot young stars in this less-studied region and it also reveals many of the curious features around even younger stars that are still cocooned by dust.

This picture was created from images taken using the Wide Field Channel of Hubble’s Advanced Camera for Surveys. Images through yellow (F555W, coloured blue) and near-infrared (F814W, coloured red) filters were combined. The exposure times were 1000 s per filter and the field of view is about 3.3 arcminutes across.

21 February 2011

Perfect spiral overlaid with Milky Way gems

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 the ends of their lives in NGC 6384, in the centre, star formation is being fuelled by the galaxy’s bar structure; astronomers think such galactic bars funnel gas inwards, where it accumulates to form new stars.

This picture was created from images take with the Wide Field Channel of Hubble’s Advanced Camera for Surveys. An image taken through a blue filter (F435W, coloured blue) was combined with an image taken through a near-infrared filter (F814W, coloured red). The total exposure times were 1050 s through each filter and the field of view is about 3 x 1.5 arcminutes.

14 February 2011

Old stars with a youthful glow

The dazzling stars in Messier 15 look fresh and new in this image from the NASA/Hubble Space Telescope, but they are actually all roughly 13 billion years old, making them some of the most ancient objects in the Universe. Unlike another recent Hubble Picture of the Week, which featured the unusually sparse cluster Palomar 1, Messier 15 is rich and bright despite its age.

Messier 15 is a globular cluster — a spherical conglomeration of old stars that formed together from the same cloud of gas, found in the outer reaches of the Milky Way in a region known as the halo and orbiting the Galactic Centre. This globular lies about 35 000 light-years from the Earth, in the constellation of Pegasus (The Flying Horse).

Messier 15 is one of the densest globulars known, with the vast majority of the cluster’s mass concentrated in the core. Astronomers think that particularly dense globulars, like this one, underwent a process called core collapse, in which gravitational interactions between stars led to many members of the cluster migrating towards the centre.

Messier 15 is also the first globular cluster known to harbour a planetary nebula, and it is still one of only four globulars known to do so. The planetary nebula, called Pease 1, can be seen in this image as a small blue blob to the lower left of the globular’s core.

This picture was put together from images taken with the Wide Field Channel of Hubble's Advanced Camera for Surveys. Images through yellow/orange (F606W, coloured blue) and near-infrared (F814W, coloured red) filters were combined. The total exposure times were 535 s and 615 s respectively and the field of view is 3.4 arcminutes across.

7 February 2011

Fiery young stars wreak havoc in stellar nursery

The NASA/ESA Hubble Space Telescope has imaged a violent stellar nursery called NGC 2174, in which stars are born in a first-come-first-served feeding frenzy for survival.

The problem is that star formation is a very inefficient process; most of the ingredients to make stars are wasted as the cloud of gas and dust, or nebula, gradually disperses. In NGC 2174, the rate at which the nebula disperses is further speeded up by the presence of hot young stars, which create high velocity winds that blow the gas outwards.

These fiery youngsters also bombard the surrounding gas with intense radiation, making it glow brightly, creating the brilliant scene captured here. The nebula is mostly composed of hydrogen gas, which is ionised by the ultraviolet radiation emitted by the hot stars, leading to the nebula’s alternative title as an HII region. This picture shows only part of the nebula, where dark dust clouds are strikingly silhouetted against the glowing gas.

NGC 2174 lies about 6400 light-years away in the constellation of Orion (The Hunter). It is not part of the much more familiar Orion Nebula, which lies much closer to us. Despite its prime position in a very familiar constellation this nebula is faint and had to wait until 1877 for its discovery by the French astronomer Jean Marie Edouard Stephan using an 80 cm reflecting telescope at the Observatoire de Marseille.

This picture was created from images from the Wide Field Planetary Camera 2 on Hubble. Images through four different filters were combined to make the view shown here. Images through a filter isolating the glow from ionised oxygen (F502N) were coloured blue and images through a filter showing glowing hydrogen (F656N) are green. Glowing ionised sulphur (F673N) and the view through a near-infrared filter (F814W) are both coloured red. The total exposure times per filter were 2600 s, 2600 s, 2600 s and 1000 s  respectively and the field of view is about 1.8 arcminutes across.

31 January 2011

The star city that never sleeps

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 be seen from Earth in over 400 years.

IC 391 lies about 80 million light-years away in the constellation of Camelopardalis (the Giraffe) in the far northern part of the sky. The British amateur observer William Denning discovered it in the late nineteenth century, and described it as faint, small and round.

This picture was assembled from images taken with Hubble’s Wide Field Channel on the Advanced Camera for Surveys. Images through a blue filter (F435W) were coloured blue, those through a green filter (F555W) are shown as green and those through a near-infrared filter (F814W) are shown in red. The exposure times were 800 s, 700 s and 700 s respectively and the field of view is 2.1 by 1.4 arcminutes.

24 January 2011

The secret of stellar youth

The NASA/ESA Hubble Space Telescope has captured a clear view of the unusual globular cluster Palomar 1, whose youthful beauty is a puzzle for astronomers. This faint and sparse object is very different from the more familiar brilliant and very rich globular clusters and had to wait until 1954 for its discovery by George Abell on photographs from the Palomar Schmidt telescope.

Globular clusters are tightly bound conglomerations of stars, which are found in the outer reaches of the Milky Way, in its so-called halo. They are amongst the oldest objects in a galaxy, containing very old stars and no gas, which means there is no possibility of newborn stars introducing some fresh blood into the cluster.

However, at 6.3 to 8 billion years old, Palomar 1 is a youngster in globular cluster terms — little more than half the age of most the other globulars in our Milky Way, which formed during our galaxy’s violent early history. However, astronomers suspect that globular youngsters, such as Palomar 1, formed in a more sedate manner. Possibly a gas cloud meandered around in the Milky Way’s halo until a trigger kick-started star formation. Another possibility is that the Milky Way captured the stellar group; perhaps it was adrift in the Universe before it was gravitationally attracted to our galaxy, or maybe it had a violent beginning after all and is the remnant of a dwarf galaxy that was devoured by the Milky Way.

Behind the sparsely populated Palomar 1 several background galaxies are seen and a few nearby bright foreground Milky Way stars are also visible. Together with Palomar 1 these objects make up an attractive “family portrait”.

This picture was created from images taken with the Wide Field Channel of the Advanced Camera for Surveys. Images through orange (F606W, coloured blue) and near-infrared (F814W, coloured red) filters were combined. The exposure times were 1965 s per filter and the field of view is 3.0 arcminutes across.

17 January 2011

Islands of stars in the river

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, lots more distant galaxies of many shapes and sizes can be seen in this image, some shining right through the foreground galaxy. NGC 1345 itself features an elongated bar extending from the nucleus and spiral arms that emanate outwards, making it a barred spiral type. Classifying galaxy shapes is an important part of astronomical research as it tells us much about how the Universe has evolved. But computers aren’t really ideal for the task; people are much better at recognising shapes, which is why a citizen-science project called Galaxy Zoo: Hubble is asking members of the public to help sift through the vast archive of images and classify galaxies by type. If you would like to join the cause, there’s a link to the project below.

This picture was created from images taken with the Wide Field Channel of Hubble’s Advanced Camera for Surveys. Images taken through a blue filter (F435W) were coloured blue and images through a near-infrared filter (F814W) were coloured red. The exposure times were 17.5 minutes per filter in total and the field of view is 3.2 by 1.6 arcminutes.


10 January 2011

Dying star cocooned within its own gases

Astronomers have used the NASA/ESA Hubble Space Telescope to image the tiny planetary nebula NGC 6886. These celestial objects signal the final death throes of mid-sized stars (up to about eight times the mass of the Sun); when such a star exhausts its supply of hydrogen fuel, the outer layers begin to expand and cool, which creates an envelope of gas and dust that shrouds the dying star. However, the star doesn't go down without a fight, finding alternative ways to prevent it from collapsing under its own gravity and emerging as a white dwarf. In the process, the star's surface temperature increases and it is eventually hot enough to emit strong ultraviolet radiation and make the cocoon of gas glow as a stunning planetary nebula.

Stellar death isn't quick and painless: the planetary nebula stage typically lasts several tens of thousands of years. By studying the elements that are present in the nebula today, astronomers can determine the original chemical make-up of the star. Studies suggest that the star belonging to NGC 6886 may have originally been similar to the Sun, containing similar quantities of carbon, nitrogen and neon, although heavier elements, such as sulphur, were less plentiful.

Keen amateur astronomers with mid-level telescopes will find it a rewarding challenge to track down NGC 6886 in the small constellation of Sagitta. It is tiny, but not particularly faint: high magnification, a good chart, a dark site and averted vision are needed to spot this elusive celestial jewel.

This picture was created by combining images taken using the Wide Field Planetary Camera 2 on Hubble. Filters that let through emission from ionised nitrogen gas (F658N, coloured red), ionised oxygen (F502N, coloured blue) and a broadband yellow filter (F555W, coloured green, and also contributing to the blue) were used. The exposure times were 700 s, 600 s and 320 s respectively. The field of view is merely 30 arcseconds across.

3 January 2011

Stellar powerhouses in the Eagle Nebula

A spectacular section of the well-known Eagle Nebula has been targeted by the NASA/ESA Hubble Space Telescope. This collection of dazzling stars is called NGC 6611, an open star cluster that formed about 5.5 million years ago and is found approximately 6500 light-years from the Earth. It is a very young cluster, containing many hot, blue stars, whose fierce ultraviolet glow make the surrounding Eagle Nebula glow brightly. The cluster and the associated nebula together are also known as Messier 16.

Astronomers refer to areas like the Eagle Nebula as HII regions. This is the scientific notation for ionised hydrogen from which the region is largely made. Extrapolating far into the future, this HII region will eventually disperse, helped along by shockwaves from supernova explosions as the more massive young stars end their brief but brilliant lives.

In this image, dark patches can also be spotted, punctuating the stellar landscape. These areas of apparent nothingness are actually very dense regions of gas and dust, which obstruct light from passing through. Many of these may be hiding the sites of the early stages of star formation, before the fledgling stars clear away their surroundings and burst into view. Dark nebulae, large and small, are dotted throughout the Universe. If you look up to the Milky Way with the naked eye from a dark, remote site, you can easily spot some huge dark nebulae blocking the background starlight.

This picture was created from images from Hubble’s Wide Field Channel of the Advanced Camera for Surveys through the unusual combination of two near-infrared filters (F775W, coloured blue, and F850LP, coloured red). The image has also been subtly colourised using a ground-based image taken through more conventional filters. The Hubble exposure times were 2000 s in both cases and the field of view is about 3.2 arcminutes across.

27 December 2010

Doing cartwheels to celebrate the end of an era

An image of the Cartwheel Galaxy taken with the NASA/ESA Hubble Space Telescope has been reprocessed using the latest techniques to mark the closure of the Space Telescope European Coordination Facility (ST-ECF), based near Munich in Germany, and to celebrate its achievements in supporting Hubble science in Europe over the past 26 years.

Astronomer Bob Fosbury, who is stepping down as Head of the ST-ECF, was responsible for much of the early research into the Cartwheel Galaxy along with the late Tim Hawarden — including giving the object its very apposite name — and so this image was selected as a fitting tribute. The object was first spotted on wide-field images from the UK Schmidt telescope and then studied in detail using the Anglo-Australian Telescope.

Lying about 500 million light-years away in the constellation of Sculptor, the cartwheel shape of this galaxy is the result of a violent galactic collision. A smaller galaxy has passed right through a large disc galaxy and produced shock waves that swept up gas and dust — much like the ripples produced when a stone is dropped into a lake — and sparked regions of intense star formation (appearing blue). The outermost ring of the galaxy, which is 1.5 times the size of our Milky Way, marks the shock wave’s leading edge. This object is one of the most dramatic examples of the small class of ring galaxies.

This image was produced after Hubble data was reprocessed using the free open source software FITS Liberator 3, which was developed at the ST-ECF. Careful use of this widely used state-of-the-art tool on the original Hubble observations of the Cartwheel Galaxy has brought out more detail in the image than ever before.

Although the ST-ECF is closing, ESA’s mission to bring amazing Hubble discoveries to the public will be unaffected, with Hubblecasts, press and photo releases, and Hubble Pictures of the Week continuing to be regularly posted on spacetelescope.org.


20 December 2010

Barred spiral bares all

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 short-lived and brilliant interloper — the bright blue star just to the lower left of the centre of the galaxy is very different from the several foreground stars seen in the image. It is in fact a supernova inside UGC 12158 and much further away than the Milky Way stars in the field — at a distance of about 400 million light-years! This stellar explosion, called SN 2004ef, was first spotted by two British amateur astronomers in September 2004 and the Hubble data shown here form part of the follow-up observations.

This picture was created from images taken with the Wide Field Channel of Hubble’s Advanced Camera for Surveys. Images through blue (F475W, coloured blue), yellow (F606W, coloured green) and red (F814W, coloured red) as well as a filter that isolates the light from glowing hydrogen (F658W, also coloured red) have been included. The exposure times were 1160 s, 700 s, 700 s and 1200 s respectively. The field of view is about 2.3 arcminutes across.

13 December 2010

A dazzling planetary nebula

The NASA/ESA Hubble Space Telescope has turned its eagle eye to the planetary nebula NGC 6572, a very bright example of these strange but beautiful objects. Planetary nebulae are created during the late stages of the evolution of certain stars that eject gas into space and emit intense ultraviolet radiation that makes the material glow. This picture of NGC 6572 shows the intricate shapes that can develop as stars exhale their last breaths. Hubble has even imaged the central white dwarf star, the origin of the dazzling nebula, but now a faint, but hot, vestige of its former glory.

NGC 6572 only began to shed its gases a few thousand years ago, so it is a fairly young planetary nebula. As a result the material is still quite concentrated, which explains why it is abnormally bright. The envelope of gas is currently racing out into space at a speed of around 15 kilometres every second and as it becomes more diffuse, it will dim.

NGC 6572 was discovered in 1825 by the German astronomer Friedrich Georg Wilhelm von Struve, who came from a family of distinguished stargazers. The name planetary nebula is left over from the time when the telescopes of early astronomers were not good enough to reveal the true nature of these objects. To many, the discs looked like the outer planets Uranus and Neptune. The application of spectral analysis, later in the 19th century, first revealed that they were glowing gas clouds.

NGC 6572 is magnitude 8.1, easily bright enough to make it an appealing target for amateur astronomers with telescopes. It is located within the large constellation of Ophiuchus (the Serpent Bearer) and at low magnification it will appear to be just a coloured star, but higher magnification will reveal its shape. Some observers report that NGC 6572 looks blue, while others state that it is green. Colour as seen through the eyepiece is often a matter of interpretation, so you may make your own decision!

This picture was created from images taken with Hubble’s Wide Field Camera 2. Images through a blue filter that isolates the glow from hydrogen gas (Hβ, F487N, coloured dark blue), a green filter that isolates emission from ionised oxygen (F502N, coloured blue), a yellow broadband filter (F555W, coloured green) and a red filter that passes emission from hydrogen (Hα, F656N) have been combined. The exposure times were 360 s, 240 s, 100 s and 180 s, respectively and the field of view is just 29 arcseconds across.

6 December 2010

Hubble peers deeply into the Eagle Nebula

The star cluster is very bright and was discovered in the mid-eighteenth century. The nebula, however, is much more elusive and it took almost a further two decades for it to be first noted by Charles Messier in 1764. Although it is commonly known as the Eagle Nebula, its official designation is Messier 16 and the cluster is also named NGC 6611. One spectacular area of the nebula (outside the field of view) has been nicknamed “The Pillars of Creation” ever since the Hubble Space Telescope captured an iconic image of dramatic pillars of star-forming gas and dust.

The cluster and nebula are fascinating targets for small and medium-sized telescopes, particularly from a dark site free from light pollution. Messier 16 can be found within the constellation of Serpens Cauda (the Tail of the Serpent), which is sandwiched between Aquila, Sagittarius, and Ophiuchus in the heart of one of the brightest parts of the Milky Way. Small telescopes with low power are useful for observing large, but faint, swathes of the nebula, whereas 30 cm telescopes and larger may reveal the dark pillars under good conditions. But a space telescope in orbit around the Earth, like Hubble — which boasts a 2.4-metre diameter mirror and state-of-the-art instruments — is required for an image as spectacular as this one.

This picture was created from images taken with the Wide Field Channel of Hubble’s Advanced Camera for Surveys. Images through a near-infrared filter (F775W) are coloured red and images through a blue filter (F475W) are blue. The exposures times were one hour and 54 minutes respectively and the field of view is about 3.3 arcminutes across.

29 November 2010

At the edge of the abyss

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.

NGC 6503 sits at the edge of a giant, hollowed-out region of space called the Local Void. The Hercules and Coma galaxy clusters, as well as our own Local Group of galaxies, circumscribe this vast, sparsely populated region. Estimates for the void’s diameter vary from 30 million to more than 150 million light-years — so NGC 6503 does not have a lot of galactic company in its immediate vicinity.

The isolation of NGC 6503 inspired the stargazer Stephen James O'Meara to name it the Lost-In-Space Galaxy in his book Hidden Treasures.

This Hubble image was created from exposures taken with the Wide Field Channel of the Advanced Camera for Surveys. The filters were unusual, which explains the peculiar colour balance of this picture. The red colouration derives from a 28-minute exposure through a filter that just allows the emission from hydrogen gas (F658N) to pass and which reveals the glowing clouds of gas associated with star-forming regions. This was combined with a 12-minute exposure through a near-infrared filter (F814W), which was coloured blue for contrast. The field of view is 3.3 by 1.8 arcminutes.

22 November 2010

Galactic moths drawn to a bright light

Smaller, dimmer galaxies appear to flit like moths around a radiant street light in this image captured by the NASA/ESA Hubble Space Telescope. The brilliant central object is a supergiant elliptical galaxy, the dominant member of a galaxy cluster with the mouthful of a name MACSJ1423.8+2404. This great swarm of galaxies is located about five billion light-years away in the constellation Boötes (the Herdsman). MACSJ1423.8+2404 and other distant galaxy clusters offer astronomers a peek into the earlier days of our Universe when these colossal groupings were still taking shape. Over the 13.7 billion-year history of the cosmos, such galaxy clusters have emerged as the largest observed gravitationally bound structures.

But there is much more than meets the eye when it comes to galaxy clusters — they also hint at the vast majority of the Universe’s substance that we have not yet directly detected. Astronomers study clusters such as MACSJ1423.8+2404 to better understand the influence of dark energy, a mysterious force credited with accelerating the expansion of the Universe and accounting for some 72 percent of the mass of the Universe.

The application of what we can see and detect to the study of what we cannot does not end there with MACSJ1423.8+2404 and its ilk. Dark matter, estimated to account for about 23 percent of the mass of the Universe, exists in great quantities in galaxy clusters. The “normal” matter that comprises stars, planets and us trickles in at less than 5 percent.

Astronomers observe clusters to study how this dark matter gravitationally gathers visible matter and underpins these vast cosmic metropolises. The galactic moths are drawn to the clusters not by their light, but by the vast unseen reservoir of dark matter.

This image was created from images taken using the Wide Field Channel of Hubble’s Advanced Camera for Surveys. The exposures were 75 and 76 minutes respectively, through yellow (F555W) and near-infrared (F814W) filters. The field of view is 3.2 arcminutes across.

15 November 2010

Hubble witnesses the crafting of a celestial masterpiece

The NASA/ESA Hubble Space Telescope has used its Advanced Camera for Surveys to peer closely at the strange cloud of gas and dust that envelops a star at a late stage in its life, a short-lived phenomenon known as a protoplanetary, or pre-planetary nebula. These fascinating celestial objects give astronomers an opportunity to watch the early stages of planetary nebula formation, as the gas and dust is moulded by high velocity winds — like watching a glassblower at work in his factory.

Despite their rather confusing names, these objects are unrelated to planets. The name arose because of the superficial visual similarity between planetary nebulae and the small discs of the outer planets in the Solar System when viewed through a telescope.

The protoplanetary nebula shown in this image is known as IRAS 20068+4051 and it is found in the constellation of Cygnus. The shell formed when its progenitor star exhausted its hydrogen fuel for nuclear fusion, causing the outer layers of the star to expand and cool, which created a spherical envelope of gas and dust around the star. The mechanism that drives high velocity winds to then shape this spherical envelope into the intricate structure that we see here is still unclear, which is why continued observation of protoplanetary nebulae is so important.

Meanwhile, as the central star continues to evolve, finding new ways to prevent itself from collapsing under its own gravity, it will eventually become hot enough to make the gas glow as a spectacular planetary nebula. These objects emit a broad spectrum of radiation, including visible light, making them great targets for both amateur and professional astronomers.

However, protoplanetary nebulae, which often appear smaller and are seen best in infrared light, are much trickier to observe, particularly since water vapour in the Earth’s atmosphere absorbs most infrared wavelengths. But Hubble has exceptionally sharp vision and an unobstructed vantage point in space, making it possible to capture stunning images of these peculiar objects.

This picture was created from images taken through yellow (F606W, coloured blue) and near-infrared (F814W, coloured red) filters using the High Resolution Channel of Hubble’s Advanced Camera for Surveys. The exposure times were 1280 s (F606W) and 200 s (F814W) and the field of view spans about 25 arcseconds.

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