Picture of the Week

Also see our pictures of the week on eso.org.
16 May 2011

The metamorphosis of Messier 8

Like a Dali masterpiece, this image of Messier 8 from the NASA/ESA Hubble Space Telescope is both intensely colourful and distinctly surreal. Located in the constellation of Sagittarius (The Archer), this giant cloud of glowing interstellar gas is a stellar nursery that is also known as the Lagoon Nebula.

Although the name definitely suits the beauty of this object, “lagoon” does suggest tranquillity and there is nothing placid about the high-energy radiation causing these intricate clouds to glow. The massive stars hiding within the heart of the nebula give off enormous amounts of ultraviolet radiation, ionising the gas and causing it to shine colourfully, as well as sculpting the surrounding nebula into strange shapes. The result is an object around four to five thousand light-years away which, on a clear night, is faintly visible to the naked eye.

Since it was first recorded back in the 1747 this object has been photographed and analysed at many different wavelengths. By using infrared detectors it is possible to delve into the centre of these dusty regions to study the objects within. However, while this optical image, taken with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope, cannot pierce the obscuring matter it is undoubtedly one of the most visually impressive.

Messier 8 is an enormous structure — around 140 by 60 light-years in extent — to put this in perspective the orbit of Neptune stretches only about four light-hours from our own Sun. This image depicts a small region in the centre of the nebula, while the region adjacent to the Lagoon Nebula, from the same Hubble observations, can be seen here.

This picture was created from exposures taken with the Wide Field Channel of the Advanced Camera for Surveys on Hubble. Light from glowing hydrogen (through the F658N filter) is coloured red. Light from ionised nitrogen (through the F660N filter) is coloured green and light through a yellow filter (F550M) is coloured blue. The exposure times through each filter are 1560 s, 1600 s and 400 s respectively. The blue-white flare at the lower left of the image is scattered light from a bright star just outside the field of view. The field of view is about 3.3 by 1.7 arcminutes.


9 May 2011

Galactic fountain of youth

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 place in the disc of NGC 5775, as it is in the early stages of a galactic merger. Astronomers have observed bridges of hydrogen gas connecting this edge-on galaxy with a neighbouring face-on spiral (NGC 5774). But neither galaxy yet features a tidal tail — a disrupted stream of gas and stars that extends into space — which are commonplace in strongly interacting pairs, such as the Antennae Galaxies.

NGC 5775 and 5774 are members of the Virgo Cluster and lie at a distance of about 85 million light-years. This colour picture was created from images taken using the Wide Field Channel of Hubble’s Advanced Camera for Surveys. Images through a red filter (F625W) were coloured blue and images through a filter that isolates the glow from hydrogen gas (F658N) have been coloured red. The exposure times were 2292 s and 6848 s, respectively, and the field of view is 3.2 arcminutes across.


2 May 2011

Breathing new life into an old cluster

The globular cluster Messier 5, shown here in this NASA/ESA Hubble Space Telescope image, is one of the oldest belonging to the Milky Way. The majority of its stars formed more than 12 billion years ago, but there are some unexpected newcomers on the scene, adding some vitality to this aging population.

Stars in globular clusters form in the same stellar nursery and grow old together. The most massive stars age quickly, exhausting their fuel supply in less than a million years, and end their lives in spectacular supernovae explosions. This process should have left the ancient cluster Messier 5 with only old, low-mass stars, which, as they have aged and cooled, have become red giants, while the oldest stars have evolved even further into blue horizontal branch stars.

Yet astronomers have spotted many young, blue stars in this cluster, hiding amongst the much more luminous ancient stars. Astronomers think that these laggard youngsters, called blue stragglers, were created either by stellar collisions or by the transfer of mass between binary stars. Such events are easy to imagine in densely populated globular clusters, in which up to a few million stars are tightly packed together.

Messier 5 lies at a distance of about 25 000 light-years in the constellation of Serpens (The Snake). This image was taken with Wide Field Channel of Hubble’s Advanced Camera for Surveys. The picture was created from images taken through a blue filter (F435W, coloured blue), a red filter (F625W, coloured green) and a near-infrared filter (F814W, coloured red). The total exposure times per filter were 750 s, 400 s and 567 s, respectively. The field of view is about 2.6 arcminutes across.


25 April 2011

Dwarf galaxy: small but perfectly formed

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 the flat disc being home to younger stars. Observations of UGC 9128 indicate that it does indeed contain a similar halo and disc structure.

UGC 9128 lies about 8 million light-years away, which means that it is part of the Local Group of more than 30 nearby galaxies, and it is found in the constellation of Boötes (The Herdsman). Despite its relative closeness it is very faint and was only discovered in the twentieth century. The Hubble image clearly resolves the galaxy’s starry population and also shows many much more distant galaxies in the background.

This picture was created from images taken with the Wide Field Channel of Hubble’s Advanced Camera for Surveys. Images through a yellow-orange filter (F606W, coloured blue) were combined with images taken in the near-infrared (F814W, coloured red). The total exposure times were 985 s and 1174 s respectively and the field of view is 3.2 arcminutes across.


18 April 2011

Hubble peers through the looking glass

The NASA/ESA Hubble Space Telescope usually works as a solo artist to capture awe-inspiring images of the distant Universe. For this picture, though, Hubble had a helping hand from the subject of the image, a galaxy cluster called LCDCS-0829, as the huge mass of the galaxies in the cluster acted like a giant magnifying glass. This strange effect is called gravitational lensing.

The object was discovered during the Las Campanas Distant Clusters Survey, which explains the cluster's unusual name. This survey was carried out in March 1995 using a 1-metre telescope at the Las Campanas Observatory in Chile. More than one thousand clusters of galaxies, most of them previously unknown, were found in a dedicated survey of a long, but narrow, section of the southern sky.

The bizarre phenomenon of gravitational lensing is a consequence of Albert Einstein’s general theory of relativity, which says that the huge mass of the galaxy cluster bends the fabric of the Universe, and the light from one of the distant galaxies will then travel along this bend in the fabric. In addition to making some objects appear bigger and brighter, gravitational lensing can produce multiple images of distant galaxies and stretch them into strange arcs. Many such arcs can be seen in this image.

This deep image of the cluster was created from a total of 36 exposures taken using the Wide Field Channel of Hubble’s Advanced Camera for Surveys. Images through a blue filter (F475W) were coloured blue, images through a near-infrared filter (F814W) were coloured green and images through a filter that passes infrared light of even longer wavelengths (F850LP) were coloured red. The total exposure times were 5280 s per filter and the field of view is about 2.8 arcminutes across.


11 April 2011

Arcade adventure for young stars

Astronomers have used the NASA/ESA Hubble Space Telescope to study the young open star cluster IC 1590, which is found within the star formation region NGC 281 — nicknamed the Pacman Nebula due to its resemblance to the famous arcade game character. This image only shows the central part of the nebula, where the brightest stars at the core of the cluster are found, with part of the Pacman’s hungry mouth visible as the dark region below.

But Pacman isn’t gobbling up these stars. Instead, the nebula’s gas and dust are being used as raw ingredients to make new stars. However, the stars in IC 1590 are still plotting their escape from the Pacman Nebula, as open clusters are only loosely bound together and the grouping will eventually disperse within a few tens of millions of years.

IC 1590 lies about ten thousand light-years from Earth in the constellation of Cassiopeia (The Queen). Through small telescopes the core of the cluster that appears at the top of this picture shows up as a triple star, but the nebula that surrounds it is much fainter and very hard to see. The eagle-eyed American astronomer E. E. Barnard, using a 15 cm telescope, first recorded it in the late nineteenth century.

This picture was created from images taken using the Wide Field Channel of Hubble’s Advanced Camera for Surveys. Images though yellow (F550M, coloured blue), orange (F660N, coloured green) and red (F658N) filters were combined. The F658N filter isolates light from glowing hydrogen gas. The total exposure times per filter were 450 s, 1017 s and 678 s, respectively and the field of view is about 3.3 arcminutes across.


4 April 2011

The beauty of asymmetry

The NASA/ESA Hubble Space Telescope has captured a planetary nebula with unconventional good looks.

Planetary nebulae signal the demise of mid-sized stars (up to about eight times the mass of the Sun); when the star’s hydrogen fuel supply is exhausted, its outer layers expand and cool, creating a cocoon of gas and dust. This gas then glows as it is bathed in the strong ultraviolet radiation from the central star. NGC 5882 is a quite bright, but small, example of a planetary nebula that lies deep in the southern Milky Way in the constellation of Lupus (The Wolf).

Planetary nebulae sometimes have a perfectly symmetrical appearance, with gas being bellowed out from the dying star evenly in every direction. However, this isn’t the case for NGC 5882, as this Hubble image shows. It appears to have two distinct, but non-uniform regions: an elongated inner shell of gas and a fainter aspherical shell that surrounds it.

Hubble’s sharp view reveals the intricate knots, filaments and bubbles within these shells. But it’s the dying star at the heart of the planetary nebula that dominates the image, shining brightly with an incredible surface temperature of about 70 000 degrees Celsius. (For comparison, the surface temperature of the Sun is only about 5500 degrees Celsius.) The high surface temperature of this white dwarf is a result of the star’s struggle for survival, finding new ways to prevent itself from collapsing under its own gravity.

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This picture comes from images taken with Hubble’s Wide Field Planetary Camera 2. Light that comes from glowing ionised oxygen is coloured blue (through the F502N filter), yellow/green light (through the broad F555W filter) is shown as green, the light from glowing hydrogen (through the F656N filter) is shown as dark red and light from glowing nitrogen is shown as bright red (through the F658N filter). The exposure times were 320 s, 104 s, 140 s and 1200 s, respectively and the field of view is just 29 arcseconds across.


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.

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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.

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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.


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