Picture of the Week

Also see our pictures of the week on eso.org.
2 January 2012

Smoke without fire: a different view of the cigar galaxy

This image shows the most detailed view ever of the core of Messier 82 (M 82), also known as the Cigar Galaxy. Rich with dust, young stars and glowing gas, M 82 is both unusually bright and relatively close to Earth. The starburst galaxy is located around 12 million light-years away in the constellation of Ursa Major (The Great Bear).

This is not the first time Hubble has imaged the Cigar Galaxy. Previous images (for example heic0604) show a galaxy ablaze with stars. Yet this image looks quite unlike them, and is dominated instead by glowing gas and dust, with the stars almost invisible. Why such a difference?

The new image is more detailed than previous Hubble observations – in fact, it is the most detailed image ever made of this galaxy. But the reason it looks so dramatically different is down to the choices astronomers make when designing their observations. Hubble’s cameras do not see in colour: they are sensitive to a broad range of wavelengths which they image only in greyscale. Colour pictures can be constructed by passing the light through different coloured filters and combining the resulting images, but the choice of filters makes a big difference to the end result.

Using filters which allow through relatively broad bands of colours, similar to those our eyes see, results in natural-looking colours and bright stars, as starlight shines brightly across the spectrum.

Using filters transparent only to the wavelengths emitted by specific chemical elements, as in this image, isolates the light from glowing gas clouds, while blocking out much of the starlight. This explains why the stars appear faint in this image, and why the dust lanes are sharply silhouetted against the brightly glowing gas clouds.

The image shows the light emitted by sulphur (shown in red), visible and ultraviolet light from oxygen (shown green and blue, respectively), and light from hydrogen (cyan).

The field of view is approximately 2.7 by 2.7 arcminutes.


26 December 2011

Faint galaxy with popping pink features

The NASA/ESA Hubble Space Telescope has imaged a region of space containing the intriguing object IC 2574. Pink bubbles blown by supernova explosions abound in this faint galaxy. The colour of these shells comes from hydrogen gas irradiated by newborn stars. The formation of the stars was triggered by shock waves from earlier supernova detonations that compressed material together.

IC 2574 is commonly known as Coddington's Nebula after the American astronomer Edwin Coddington, who discovered it in 1898. Astronomers classify IC 2574 as a dwarf irregular galaxy due to its relatively small size and lack of organisation or structure. These galaxies are thought to resemble some of the earliest that formed in the Universe. Dwarf irregular galaxies thus serve as useful "living fossils" for studying the evolution of more complex galaxy types such as our home, the Milky Way, with its central bar and spiral arms. The expanding shells in IC 2574 are of particular interest to astronomers as they reveal how supernova-driven explosions ignite round after round of star formation.  

The constellation containing IC 2574 is Ursa Major (The Great Bear). IC 2574 is located about 12 million light-years away, belonging to the Messier 81 group of galaxies. This group is named after the most prominent galaxy in its midst, the big, bright and accordingly well-studied spiral galaxy Messier 81.

This picture was produced with Hubble’s Advanced Camera for Surveys, and covers a field of view of around 3.3 by 3.3 arcminutes.


19 December 2011

Hubble captures a “lucky” galaxy alignment

An interesting galaxy has been circled in this NASA/ESA Hubble Space Telescope image. The galaxy — one of a group of galaxies called Luminous Red Galaxies — has an unusually large mass, containing about ten times the mass of the Milky Way. However, it’s actually the blue horseshoe shape that circumscribes the red galaxy that is the real prize in this image.

This blue horseshoe is a distant galaxy that has been magnified and warped into a nearly complete ring by the strong gravitational pull of the massive foreground Luminous Red Galaxy. To see such a so-called Einstein Ring required the fortunate alignment of the foreground and background galaxies, making this object’s nickname “the Cosmic Horseshoe” particularly apt.

The Cosmic Horseshoe is one of the best examples of an Einstein Ring. It also gives us a tantalising view of the early Universe: the blue galaxy’s redshift — a measure of how the wavelength of its light has been stretched by the expansion of the cosmos — is approximately 2.4. This means we see it as it was about 3 billion years after the Big Bang. The Universe is now 13.7 billion years old.

Astronomers first discovered the Cosmic Horseshoe in 2007 using data from the Sloan Digital Sky Survey. But this Hubble image, taken with the Wide Field Camera 3, offers a much more detailed view of this fascinating object.

This picture was created from images taken in visible and infrared light on Hubble’s Wide Field Camera 3. The field of view is approximately 2.6 arcminutes wide.


12 December 2011

Standing out from the crowd

The compact nature of globular clusters is a double-edged sword. On the one hand, having so many stars of a similar age in one bundle gives astronomers insights into the chemical makeup of our galaxy in its early history. But, at the same time, the high density of stars in the cores of globulars also makes it difficult for astronomers to resolve individual stars.

The core of NGC 6642, shown here in this Hubble Space Telescope image, is particularly dense, making this globular a difficult observational target for most telescopes. Furthermore, it occupies a very central position in our galaxy, which means that images inadvertently capture many stars that don’t belong to the cluster — these “field stars” just get in the way.

However, using Hubble’s powerful Advanced Camera for Surveys (ACS), astronomers can identify and remove such distracting field stars, and resolve the cluster’s dense core in unprecedented detail. Using Hubble’s ACS, astronomers have already made many interesting finds about NGC 6642. For example, many “blue stragglers” (stars which seemingly lag behind in their rate of aging) have been spotted in this globular, and it is known to be lacking in low-mass stars.

This picture was created from visible and infrared images taken with the Wide Field Channel of the Advanced Camera for Surveys. The field of view is approximately 1.6 by 1.6 arcminutes.


5 December 2011

The Frosty Leo Nebula

Three thousand light-years from Earth lies the strange protoplanetary nebula IRAS 09371+1212, nicknamed the Frosty Leo Nebula. Despite their name, protoplanetary nebulae have nothing to do with planets: they are formed from material shed from their aging central star. The Frosty Leo Nebula has acquired its curious name as it has been found to be rich in water in the form of ice grains, and because it lies in the constellation of Leo.

This nebula is particularly noteworthy because it has formed far from the galactic plane, away from interstellar clouds that may block our view. The intricate shape comprises a spherical halo, a disc around the central star, lobes and gigantic loops. This complex structure strongly suggests that the formation processes are complex and it has been suggested that there could be a second star, currently unseen, contributing to the shaping of the nebula.

Protoplanetary nebulae like the Frosty Leo Nebula have brief lifespans by astronomical standards and are precursors to the planetary nebula phase, in which radiation from the star will make the nebula’s gas light up brightly. Their rarity makes studying them a priority for astronomers who seek to understand better the evolution of stars.

This picture was created from images taken with the High Resolution Channel of Hubble’s Advanced Camera for Surveys, which images a small area of sky (only 26 by 29 arcseconds) in high detail.


28 November 2011

The calm after the galactic storm

The NASA/ESA Hubble Space Telescope has caught sight of a soft, diffuse-looking galaxy that is probably the aftermath of a long-ago galactic collision. Two spiral galaxies, each perhaps much like the Milky Way, swirled together for millions of years.

In such mergers, the original galaxies are often stretched and pulled apart as they wrap around a common centre of gravity. After a few back-and-forths, this starry tempest settles down into a new, round object. The now subdued celestial body, catalogued as SDSS J162702.56+432833.9, is technically known as an elliptical galaxy.

When galaxies collide — a common event in the Universe — a fresh burst of star formation typically takes place as gas clouds mash together. At this point, the galaxy has a blue hue, but the colour does not mean it is cold: it is a result of the intense heat of newly formed blue–white stars. Those stars do not last long, and after a few billion years the reddish hues of aging smaller stars dominate an elliptical galaxy's spectrum. Hubble has helped astronomers learn of this sequence by observing galaxy mergers at all stages of the process.

In SDSS J162702.56+432833.9, some ribbons of dust notably obscure parts of the conglomerated galaxy's central, bluish region. Those dust lanes could be remnants of the spiral arms of the recently departed galaxies.

This picture was snapped by the Wide Field Camera of Hubble’s Advanced Camera for Surveys. The image was made through a red (F625W) and a blue (F438W) filter. The field of view is approximately 2.4 by 2.4 arcminutes.


21 November 2011

Probing a super-giant shell of gas and stars

In one of the largest known star formation regions in the Large Magellanic Cloud (LMC), a small satellite galaxy of the Milky Way, lie young and bright stellar groupings known as OB associations. One of these associations, called LH 72, was captured in this dramatic NASA/ESA Hubble Space Telescope image. It consists of a few high-mass, young stars embedded in a beautiful and dense nebula of hydrogen gas.

Much of the star formation in the LMC occurs in super-giant shells. These regions of interstellar gas are thought to have formed due to strong stellar winds and supernova explosions that cleared away much of the material around the stars creating wind-blown shells. The swept-up gas eventually cools down and fragments into smaller clouds that dot the edges of these regions and eventually collapse to form new stars.

The biggest of these shells, home to LH 72, is designated LMC4. With a diameter of about 6000 light-years, it is the largest in the Local Group of galaxies that is home to both the Milky Way and LMC. Studying gas-embedded young associations of stars like LH 72 is a way of probing the super-giant shells to understand how they formed and evolved.

This image was taken with Hubble’s Wide Field Planetary Camera 2 using five different filters in ultraviolet, visible and infrared light. The field of view is approximately 1.8 by 1.8 arcminutes.


14 November 2011

The belly of the cosmic whale

The NASA/ESA Hubble Space Telescope has peered deep into NGC 4631, better known as the Whale Galaxy. Here, a profusion of starbirth lights up the galactic centre, revealing bands of dark material between us and the starburst. The galaxy’s activity tapers off  in its outer regions where there are fewer stars and less dust, but these are still punctuated by pockets of star formation.

The Whale Galaxy is about 30 million light-years away from us in the constellation of Canes Venatici (The Hunting Dogs) and is a spiral galaxy much like the Milky Way. From our vantage point, however, we see the Whale Galaxy edge-on, seeing its glowing centre through dusty spiral arms. The galaxy's central bulge and asymmetric tapering disc have suggested the shape of a whale or a herring to past observers.

Many supernovae — the explosions of hot, blue, short-lived stars at least eight times the mass of the Sun — have gone off in the core of the Whale Galaxy. The stellar pyrotechnics have bathed the galaxy in hot gas, visible to X-ray telescopes like ESA’s XMM–Newton. Comparing the optical and near-infrared observations from Hubble with other telescopes sensitive to different wavelengths of light helps astronomers gather the full story behind celestial phenomena.

From such work, the triggers of the starburst in the Whale Galaxy and others can be elucidated. The gravitational "feeding" on intergalactic material, as well as clumping caused by the gravitational interactions with its galactic neighbours, creates the areas of greater density where stars start to coalesce. Just as blue whales, the biggest creatures on Earth, can gorge themselves on comparatively tiny bits of plankton, so the Whale Galaxy has become filled with the gas and dust that powers a high rate of star formation.


7 November 2011

First globular cluster outside the Milky Way

The object shown in this beautiful Hubble image, dubbed Messier 54, could be just another globular cluster, but this dense and faint group of stars was in fact the first globular cluster found that is outside our galaxy. Discovered by the famous astronomer Charles Messier in 1778, Messier 54 belongs to a satellite of the Milky Way called the Sagittarius Dwarf Elliptical Galaxy.

Messier had no idea of the significance of his discovery at the time, and it wasn’t until over two centuries later, in 1994, that astronomers found Messier 54 to be part of the miniature galaxy and not our own. Current estimates indicate that the Sagittarius dwarf, and hence the cluster, is situated almost 90 000 light-years away — more than three times as far from the centre of our galaxy than the Solar System.

Ironically, even though this globular cluster is now understood to lie outside the Milky Way, it will actually become part of it in the future. The strong gravitational pull of our galaxy is slowly engulfing the Sagittarius dwarf, which will eventually merge with the Milky Way creating one much larger galaxy.

This picture is a composite created by combining images taken with the Wide Field Channel of Hubble’s Advanced Camera for Surveys. Light that passed through a yellow-orange (F606W) was coloured blue and light passing through a near-infrared filter (F814W) was coloured red. The total exposure times were 3460 s and 3560 s, respectively and the field of view is approximately 3.4 by 3.4 arcminutes.


31 October 2011

Looking to the heavens

The pearly wisps surrounding the central star IRAS 10082-5647 in this Hubble image certainly draw the eye towards the heavens. The divine-looking cloud is a reflection nebula, made up of gas and dust glowing softly by the reflected light of nearby stars, in this case a young Herbig Ae/Be star.

The star, like others of this type, is still a relative youngster, only a few million years old. It has not yet reached the so-called main sequence phase, where it will spend around 80% of its life creating energy by burning hydrogen in its core. Until then the star heats itself by gravitational collapse, as the material in the star falls in on itself, becoming ever denser and creating immense pressure which in turn gives off copious amounts of heat.

Stars only spend around 1% of their lives in this pre-main sequence phase. Eventually, gravitational collapse will heat the star’s core enough for hydrogen fusion to begin, propelling the star into the main sequence phase, and adulthood.

The Advanced Camera for Surveys aboard the Hubble Space Telescope captured the whorls and arcs of this nebula, lit up with the light from IRAS 10082-5647. Visible (555 nm) and near-infrared (814 nm) filters were used, coloured blue and red respectively. The field of view is around 1.3 by 1.3 arcminutes.


24 October 2011

Hubble sizes up a dwarf galaxy

The NASA/ESA Hubble Space Telescope has taken this image of the Phoenix Dwarf Galaxy, which is located 1.4 million light-years away from Earth. It is located in the constellation of Phoenix in the southern sky. The object, a dwarf irregular galaxy, features younger stars in its inner regions and older ones at its outskirts. 

Dwarf galaxies are small galaxies composed of a few billion stars, compared to fully-fledged galaxies which can contain hundreds of billions of stars. In the Local Group, there are a number of such dwarf galaxies orbiting the larger galaxies such as the Milky Way or the Andromeda Galaxy. They are thought to have been created by tidal forces in the early stages of the creation of these galaxies, or as a result of collisions between galaxies, forming from ejected streams of material and dark matter from the parent galaxies. The Milky Way galaxy features at least 14 satellite dwarf galaxies orbiting it. 

Because of their shape, dwarf irregulars have often been mistaken for globular clusters: they do not feature a bulge or spiral arms like larger galaxies. However, their importance in terms of cosmology is in stark contrast to their unspectacular shapes, as their chemical makeup and high levels of gas are believed to be similar to those of the earliest galaxies that populated the Universe. They are thought to be contemporary versions of some of the remote galaxies observed in deep field galaxy surveys, and can thus help to understand the early stages of galaxy and star formation in the young Universe.

The galaxy was discovered in 1976 by Hans-Emil Schuster and Richard Martin West. Hans-Emil Schuster was acting director of ESO’s La Silla Observatory in Chile and was involved in the selection and testing of the sites for the observatories of both La Silla and Paranal. His great contribution to astronomy and to ESO was recognised by the Chilean government last week when he was awarded the medal of the Order of Bernardo O’Higgins


17 October 2011

Hubble revisits an old friend

Supernova SN 1987A, one of the brightest stellar explosions since the invention of the telescope more than 400 years ago, is no stranger to the NASA/ESA Hubble Space Telescope. The observatory has been on the frontline of studies into this brilliant dying star since its launch in 1990, three years after the supernova exploded on 23 February 1987. This image of Hubble’s old friend, retreived from the telescope’s data archive, may be the best ever of this object, and reminds us of the many mysteries still surrounding it.

Dominating this picture are two glowing loops of stellar material and a very bright ring surrounding the dying star at the centre of the frame. Although Hubble has provided important clues on the nature of these structures, their origin is still largely unknown.

Another mystery is that of the missing neutron star. The violent death of a high-mass star, such as SN 1987A, leaves behind a stellar remnant — a neutron star or a black hole. Astronomers expect to find a neutron star in the remnants of this supernova, but they have not yet been able to peer through the dense dust to confirm it is there.

The supernova belongs to the Large Magellanic Cloud, a nearby galaxy about 168 000 light-years away. Even though the stellar explosion took place around 166 000 BC, its light arrived here less than 25 years ago.

This picture is based on observations done with the High Resolution Channel of Hubble’s Advanced Camera for Surveys. The field of view is approximately 25 by 25 arcseconds.


10 October 2011

A snapshot of galactic evolution

Galaxies come in a variety of shapes and sizes, and these features change as they evolve. Some, like the galaxy in the centre of this NASA/ESA Hubble Space Telescope image, are beautiful spirals with graceful curved arms, while others are fuzzy oval-shaped blobs like the large object showing up near the bottom right of the frame. Others still are rather irregular in shape, such as the orange galaxy at the top of the image, which resembles a tiny wobbling string.

This picture is one of the few hundred exposures taken with Hubble’s Advanced Camera for Surveys to assemble the “Extended Groth Strip”. This strip, named after the Princeton University astronomer Edward Groth, is a composite picture of a rectangular region of the sky in the constellation of Ursa Major. It covers a relatively small area in the sky — equivalent to roughly the width of a finger stretched at arms’ length — but includes at least 50 000 galaxies.

The images that make up the Extended Groth Strip allow astronomers to peer into the last eight billion years of the Universe’s history and to see galaxies at various stages of their evolution. The large elliptical and spiral objects we see in the foreground of this image are fully-formed adult galaxies. But many of the ones in the background, fuzzier and more peculiar in shape, are representative of a time when galaxies were undergoing active formation.

Images like these help astronomers to understand how galaxies change in size and shape as they evolve, from their early formative years — punctuated by violent events such as the growth of the vast black holes at their centres and collisions with other galaxies — into their quieter adult lives.

This picture was created from visible and infrared exposures taken with the Wide Field Channel of Hubble’s Advanced Camera for Surveys


3 October 2011

Spot the difference — Hubble spies another globular cluster, but with a secret

Thousands and thousands of brilliant stars make up this globular cluster, Messier 53, captured with crystal clarity in this image from the NASA/ESA Hubble Space Telescope. Bound tightly by gravity, the cluster is roughly spherical and becomes denser towards its heart.

These enormous sparkling spheres are by no means rare, and over 150 exist in the Milky Way alone, including Messier 53. It lies on the outer edges of the galaxy, where many other globular clusters are found, almost equally distant from both the centre of our galaxy and the Sun. Although they are relatively common, the famous astronomer William Herschel, not at all known for his poetic nature, once described a globular cluster as “one of the most beautiful objects I remember to have seen in the heavens”, and it is clear to see why.

Globular clusters are much older and larger than open clusters, meaning they are generally expected to contain more old red stars and fewer massive blue stars. But Messier 53 has surprised astronomers with its unusual number of a type of star called blue stragglers.

These youngsters are rebelling against the theory of stellar evolution. All the stars in a globular cluster are expected to form around the same time, so they are expected follow a specific trend set by the age of the cluster and based on their mass. But blue stragglers don’t follow that rule; they appear to be brighter and more youthful than they have any right to be. Although their precise nature remains mysterious these unusual objects are probably formed by close encounters, possibly collisions, between stars in the crowded centres of globular clusters.

This picture was put together from visible and infrared exposures taken with the Wide Field Channel of Hubble's Advanced Camera for Surveys.The field of view is approximately 3.4 arcminutes across.


26 September 2011

An arc sculpted by gravity

This NASA/ESA Hubble Space Telescope image shows remarkable structures in a galaxy cluster around an object called LRG-4-606. LRG stands for Luminous Red Galaxy, and is the acronym given to a large collection of bright red galaxies found in the Sloan Digital Sky Survey (SDSS). These objects are mostly massive elliptical galaxies composed of huge numbers of old stars.

It is sobering to contemplate the sheer number of stars that this image must contain — hundreds of billions — but it also features one of the strangest phenomena known to astronomers. This particular red galaxy and its surrounding galaxies happen to be positioned so that their strong gravitational field has a dramatic effect.

Left of centre in the picture, blue galaxies in the background have been stretched and warped out of shape into narrow, pale blue arcs. This is because of an effect called gravitational lensing. The galaxy cluster has such a strong gravitational field that it is curving the fabric of space and amplifying the starlight from much more distant galaxies. Gravitational lensing normally creates elongated arcs and here, unusually, the alignment of the galaxies has made the separate arcs combine to form a half-circle.

This picture was assembled from a collection of exposures in visible and near infrared light taken with Hubble’s Wide Field Camera 3. The field of view is approximately 3 by 3 arcminutes.


19 September 2011

Galaxies in a swarm of star clusters

In this NASA/ESA Hubble Space Telescope image, NGC 4874 is the brightest object, located to the right of the frame and seen as a bright star-like core surrounded by a hazy halo. A few of the other galaxies of the cluster are also visible, looking like flying saucers dancing around NGC 4874. But the really remarkable feature of this image is the point-like objects around NGC 4874, revealed on a closer look: almost all of them are clusters of stars that belong to the galaxy. Each of these globular star clusters contains many hundreds of thousands of stars.

Recently, astronomers discovered that a few of these point-like objects are not star clusters but ultra-compact dwarf galaxies, also under the gravitational influence of NGC 4874. Being only about 200 light-years across and mostly made up of old stars, these galaxies resemble brighter and larger versions of globular clusters. They are thought to be the cores of small elliptical galaxies that, due to the violent interactions with other galaxies in the cluster, lost their gas and surrounding stars.

This Hubble image also shows many more distant galaxies that do not belong to the cluster, seen as small smudges in the background. While the galaxies in the Coma Cluster are located about 350 million light-years away, these other objects are much further out. Their light took several hundred million to billions of years to reach us.

Most unusually, the image also shows a very faint blue satellite trail, extending across the whole image, from the upper left corner of the frame to the lower right. Because Hubble’s cameras can only see a tiny part of the sky at one time, such trails are very rare.

This picture was created from optical and near-infrared exposures taken with the Wide Field Channel of Hubble’s Advanced Camera for Surveys. The field of view is 3.3 arcminutes across.


12 September 2011

A remote outpost of the Milky Way

This NASA/ESA Hubble Space Telescope image shows a compact and distant globular star cluster that lies in one of the smallest constellations in the night sky, Delphinus (The Dolphin). Due to its modest size, great distance and relatively low brightness, NGC 7006 is often ignored by amateur astronomers. But even remote globular clusters such as this one appear bright and clear when imaged by Hubble’s Advanced Camera for Surveys.

NGC 7006 resides in the outskirts of the Milky Way. It is about 135 000 light-years away, five times the distance between the Sun and the centre of the galaxy, and it is part of the galactic halo. This roughly spherical region of the Milky Way is made up of dark matter, gas and sparsely distributed stellar clusters.

Like other remote globular clusters, NGC 7006 provides important clues that help astronomers to understand how stars formed and assembled in the halo. The cluster now pictured by Hubble has a very eccentric orbit indicating that it may have formed independently, in a small galaxy outside our own that was then captured by the Milky Way.

Although NGC 7006 is very distant for a Milky Way globular cluster, it is much closer than the many faint galaxies that can be seen in the background of this image. Each of these faint smudges is probably accompanied by many globular clusters similar to NGC 7006 that are too faint to be seen even by Hubble.

This image was taken using the Wide Field Channel of the Advanced Camera for Surveys, in a combination of visible and near-infrared light. The field of view is a little over 3 by 3 arcminutes.


5 September 2011

A starfish in the sky

Protoplanetary nebulae offer glimpses of how stars similar to the Sun end their lives and how they make the transition to white dwarfs surrounded by planetary nebulae. As it ages, a Sun-like star eventually sheds its outer layers into space, creating a beautiful and often intricately shaped cloud of gas and dust around it. At first, still relatively cool, the star is unable to ionise this gas, which shines only by reflected and scattered stellar light. Only when the temperature of the star increases enough to ionise this protoplanetary nebula does the pattern of gas and dust become a fully fledged planetary nebula.

Protoplanetary nebulae are relatively rare and short-lived objects that provide astronomers with clues into how the often strangely asymmetric planetary nebulae are formed. Clearly visible in this image are five blue lobes that extend away from the central star and give the nebula its asymmetric starfish shape. While astronomers have come up with theories for the origin of these structures, such as direction-changing jets or explosive ejections of matter from the star, their formation is not entirely understood.

IRAS 19024+0044 is blue in colour as the blue component of the light coming from the star is more easily scattered by the gas and dust in the nebula, while the red and orange rays are relatively unaffected. This is similar to what happens to sunlight in the Earth’s atmosphere, giving the sky its distinctive shade of blue.

This picture was created from images taken with the High Resolution Channel of Hubble’s Advanced Camera for Surveys. It is a composite image created by the combination of exposures taken through a yellow–orange filter (F606W, coloured blue) and a near-infrared filter (F814W, coloured red). The total exposure times were 880 s and 140 s, respectively and the field of view is approximately 13 by 13 arcseconds.

Links


29 August 2011

Astronomical vision test

Peering into the depths of space, the sharp-eyed NASA/ESA Hubble Space Telescope has imaged the nearby but faint dwarf galaxy ESO 540-030. This object itself appears as a huge swarm of dim stars, but ESO 540-030 is actually just one point of interest in the picture.

ESO 540-030 is just over 11 million light-years distant, and is part of the Sculptor group of galaxies. This collection is the closest neighbour to our own Local Group of galaxies that includes the Milky Way. Due to its proximity the Sculptor group contains some of the brightest galaxies in the southern skies, although ESO 540-030 is not one of these; dwarf galaxies generally have low surface brightness, which make observations difficult.

Hubble has captured a snapshot of galaxy types in the background, with spirals, barred spirals, ellipticals and irregulars on display. Careful examination of this picture should allow examples of each galaxy type to be found. Some galaxies lie directly behind ESO 540-030, increasing the challenge. As well as the galaxies there are also five bright stars, which are much closer to us than the galaxies. The telltale diffraction spikes — four sharp lines of light emanating at 90 degree angles, caused by light diffracting in the telescope — are unmistakable signs of the stars in the picture.

Cataloguing galaxy types is an important task for scientists attempting to understand more about how our Universe evolved. Our own eyes are excellent tools for this, as participants of the Galaxy Zoo Hubble project will confirm [1].

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 4480 s and 3360 s, respectively and the field of view is about 3.1 arcminutes across.

Links

[1] http://www.galaxyzoo.org/


22 August 2011

Feeling the strain

NGC 2146 is classified as a barred spiral due to its shape, but the most distinctive feature is the dusty spiral arm that has looped in front of the galaxy's core as seen from our perspective. The forces required to pull this structure out of its natural shape and twist it up to 45 degrees are colossal. The most likely explanation is that a neighbouring galaxy is gravitationally perturbing it and distorting the orbits of many of NGC 2146’s stars. It is probable that we are currently witnessing the end stages of a process which has been occurring for tens of millions of years.

NCG 2146 is undergoing intense bouts of star formation, to such an extent that it is referred to as a starburst galaxy. This is a common state for barred spirals, but the extra gravitational disruption that NGC 2146 is enduring no doubt exacerbates the situation, compressing hydrogen-rich nebulae and triggering stellar birth.

Measuring about 80 000 light-years from end to end, NGC 2146 is slightly smaller than the Milky Way. It lies approximately 70 million light-years distant in the faint northern constellation of Camelopardalis (The Giraffe). Although it is fairly easy to see with a moderate-sized telescope as a faint elongated blur of light it was not spotted until 1876 when the German astronomer Friedrich Winnecke found it visually using just a 16 cm telescope.

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 (F814W, coloured blue and orange/brown) were combined with images taken in a filter that isolates the glow from hydrogen gas (F658N, coloured red). An additional green colour channel was also created by combining the two to help to create a realistic colour rendition for the final picture from this unusual filter combination. The total exposure times were 120 s and 700 s respectively and the field of view is covers 2.6 x 1.6 arcminutes.


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