Astronomers using the NASA/ESA Hubble Space Telescope have made images of several galaxies containing quasars, which act as gravitational lenses to amplify and distort images of the galaxies aligned behind them.
Quasars are among the brightest objects in the Universe, far outshining the total output of their host galaxies. They are powered by supermassive black holes, which pull in surrounding material that then heats up as it falls towards the black hole. The path that the light from even more distant galaxies takes on its journey towards us is bent by the enormous masses at the centre of these galaxies. Gravitational lensing is a subtle effect which requires extremely high resolution observations, something for which Hubble is extremely well suited.
To find these rare cases of galaxy–quasar combinations acting as lenses, a team of astronomers led by Frederic Courbin at the Ecole Polytechnique Federale de Lausanne (EPFL, Switzerland) selected 23 000 quasar spectra in the Sloan Digital Sky Survey (SDSS). They looked for the spectral imprint of galaxies at much greater distances that happened to align with foreground galaxies. Once candidates were identified, Hubble’s sharp vision was used to look for the characteristic gravitational arcs and rings that would be produced by gravitational lensing.
In Hubble’s images, the quasars are the bright spots visible at the centre of the galaxies, while the lensed images of distant galaxies are visible as fainter arc-shaped forms that surround them. From left to right, the galaxies are: SDSS J0919+2720, with two bluish lensed images clearly visible above and below the galaxy’s centre; SDSS J1005+4016, with one yellowish arc visible to the right of the galaxy’s centre; and SDSS J0827+5224, with two lensed images very faintly visible, one above and to the right, and one below and to the left of the galaxy’s centre.
Quasar host galaxies are hard or sometimes even impossible to see because the central quasar far outshines the galaxy. Therefore, it is difficult to estimate the mass of a host galaxy based on the collective brightness of its stars. However, gravitational lensing candidates are invaluable for estimating the mass of a quasar’s host galaxy because the amount of distortion in the lens can be used to estimate a galaxy’s mass.
The breathtaking butterfly-like planetary nebula NGC 6881 is visible here in an image taken by the NASA/ESA Hubble Space Telescope. Located in the constellation of Cygnus, it is formed of an inner nebula, estimated to be about one fifth of a light-year across, and symmetrical “wings” that spread out about one light-year from one tip to the other. The symmetry could be due to a binary star at the nebula’s centre.
NGC 6881 has a dying star at its core which is about 60% of the mass of the Sun. It is an example of a quadrupolar planetary nebula, made from two pairs of bipolar lobes pointing in different directions, and consisting of four pairs of flat rings. There are also three rings in the centre.
A planetary nebula is a cloud of ionised gas, emitting light of various colours. It typically forms when a dying star — a red giant — throws off its outer layers, because of pulsations and strong stellar winds.
The star’s exposed hot, luminous core starts emitting ultraviolet radiation, exciting the outer layers of the star, which then become a newly born planetary nebula. At some point, the nebula is bound to dissolve in space, leaving the central star as a white dwarf — the final evolutionary state of stars.
The name “planetary” dates back to the 18th century, when such nebulae were first discovered — and when viewed through small optical telescopes, they looked a lot like giant planets.
Planetary nebulae usually live for a few tens of thousands of years, a short phase in the lifetime of a star.
The image was taken through three filters which isolate the specific wavelength of light emitted by nitrogen (shown in red), hydrogen (shown in green) and oxygen (shown in blue).
The NASA/ESA Hubble Space Telescope has produced this beautiful image of the galaxy NGC 1483. NGC 1483 is a barred spiral galaxy located in the southern constellation of Dorado — the dolphinfish in Spanish. The nebulous galaxy features a bright central bulge and diffuse arms with distinct star-forming regions. In the background, many other distant galaxies can be seen.
The constellation Dorado is home to the Dorado Group of galaxies, a loose group comprising an estimated 70 galaxies and located some 62 million light-years away. The Dorado group is much larger than the Local Group that includes the Milky Way (and which contains around 30 galaxies) and approaches the size of a galaxy cluster. Galaxy clusters are the largest groupings of galaxies (and indeed the largest structures of any type) in the Universe to be held together by their gravity.
Barred spiral galaxies are so named because of the prominent bar-shaped structures found in their centre. They form about two thirds of all spiral galaxies, including the Milky Way. Recent studies suggest that bars may be a common stage in the formation of spiral galaxies, and may indicate that a galaxy has reached full maturity.
The myriad faint stars that comprise the Antlia Dwarf galaxy are more than four million light-years from Earth, but this NASA/ESA Hubble Space Telescope image offers such clarity that they could be mistaken for much closer stars in our own Milky Way. This very faint and sparsely populated small galaxy was only discovered in 1997.
Although small, the Antlia Dwarf is a dynamic site featuring stars at many different stages of evolution, from young to old. The freshest stars are only found in the central regions where there is significant ongoing star formation. Older stars and globular clusters are found in the outer areas.
It is not entirely clear whether the Antlia Dwarf is a member our galactic neighbourhood, called the Local Group. It probably lies just beyond the normally accepted outer limits of the group. Although it is fairly isolated, some believe it has interacted with other star groups. Evidence comes from galaxy NGC 3109, close to the Antlia Dwarf (but not visible in this image). Both galaxies feature rifts of stars moving at comparable velocities; a telltale sign that they were gravitationally linked at some point in the past.
This picture was created from observations in visible and infrared light taken with the Wide Field Channel of Hubble’s Advanced Camera for Surveys. The field of view is approximately 3.2 by 1.5 arcminutes.
At the turn of the 19th century, the binary star system Eta Carinae was faint and undistinguished. In the first decades of the century, it became brighter and brighter, until, by April 1843, it was the second brightest star in the sky, outshone only by Sirius (which is almost a thousand times closer to Earth). In the years that followed, it gradually dimmed again and by the 20th century was totally invisible to the naked eye.
The star has continued to vary in brightness ever since, and while it is once again visible to the naked eye on a dark night, it has never again come close to its peak of 1843.
The larger of the two stars in the Eta Carinae system is a huge and unstable star that is nearing the end of its life, and the event that the 19th century astronomers observed was a stellar near-death experience. Scientists call these outbursts supernova impostor events, because they appear similar to supernovae but stop just short of destroying their star.
Although 19th century astronomers did not have telescopes powerful enough to see the 1843 outburst in detail, its effects can be studied today. The huge clouds of matter thrown out a century and a half ago, known as the Homunculus Nebula, have been a regular target for Hubble since its launch in 1990. This image, taken with the Advanced Camera for Surveys High Resolution Channel is the most detailed yet, and shows how the material from the star was not thrown out in a uniform manner, but forms a huge dumbbell shape.
Eta Carinae is not only interesting because of its past, but also because of its future. It is one of the closest stars to Earth that is likely to explode in a supernova in the relatively near future (though in astronomical timescales the “near future” could still be a million years away). When it does, expect an impressive view from Earth, far brighter still than its last outburst: SN 2006gy, the brightest supernova ever observed, came from a star of the same type.
This image consists of ultraviolet and visible light images from the High Resolution Channel of Hubble’s Advanced Camera for Surveys. The field of view is approximately 30 arcseconds across.
- Previous images of Eta Carinae from the Hubble Space Telescope:
It’s well known that the Universe is changeable: even the stars that appear static and predictable every night are subject to change.
This image from the NASA/ESA Hubble Space Telescope shows planetary nebula Hen 3-1333. Planetary nebulae are nothing to do with planets — they actually represent the death throes of mid-sized stars like the Sun. As they puff out their outer layers, large, irregular globes of glowing gas expand around them, which appeared planet-like through the small telescopes that were used by their first discoverers.
The star at the heart of Hen 3-1333 is thought to have a mass of around 60% that of the Sun, but unlike the Sun, its apparent brightness varies substantially over time. Astronomers believe this variability is caused by a disc of dust which lies almost edge-on when viewed from Earth, which periodically obscures the star.
It is a Wolf–Rayet type star — a late stage in the evolution of Sun-sized stars. These are named after (and share many observational characteristics with) Wolf–Rayet stars, which are much larger. Why the similarity? Both Wolf–Rayet and Wolf–Rayet type stars are hot and bright because their helium cores are exposed: the former because of the strong stellar winds characteristic of these stars; the latter because the outer layers of the stars have been puffed away as the star runs low on fuel.
The exposed helium core, rich with heavier elements, means that the surfaces of these stars are far hotter than the Sun, typically 25 000 to 50 000 degrees Celsius (the Sun has a comparatively chilly surface temperature of just 5500 degrees Celsius).
So while they are dramatically smaller in size, the Wolf–Rayet type stars such as the one at the core of Hen 3-1333 effectively mimic the appearance of their much bigger and more energetic namesakes: they are sheep in Wolf–Rayet clothing.
This visible-light image was taken by the high resolution channel of Hubble’s Advanced Camera for Surveys. The field of view is approximately 26 by 26 arcseconds.
Many of the Universe’s galaxies are like our own, displaying beautiful spiral arms wrapping around a bright nucleus. Examples in this stunning image, taken with the Wide Field Camera 3 on the NASA/ESA Hubble Space Telescope, include the tilted galaxy at the bottom of the frame, shining behind a Milky Way star, and the small spiral at the top centre.
Other galaxies are even odder in shape. Markarian 779, the galaxy at the top of this image, has a distorted appearance because it is likely the product of a recent galactic merger between two spirals. This collision destroyed the spiral arms of the galaxies and scattered much of their gas and dust, transforming them into a single peculiar galaxy with a unique shape.
This galaxy is part of the Markarian catalogue, a database of over 1500 galaxies named after B. E. Markarian, the Armenian astronomer who studied them in the 1960s. He surveyed the sky for bright objects with unusually strong emission in the ultraviolet.
Ultraviolet radiation can come from a range of sources, so the Markarian catalogue is quite diverse. An excess of ultraviolet emissions can be the result of the nucleus of an “active” galaxy, powered by a supermassive black hole at its centre. It can also be due to events of intense star formation, called starbursts, possibly triggered by galactic collisions. Markarian galaxies are, therefore, often the subject of studies aimed at understanding active galaxies, starburst activity, and galaxy interactions and mergers.
Looking like a hoard of gems fit for an emperor’s collection, this deep sky object called NGC 6752 is in fact far more worthy of admiration. It is a globular cluster, and at over 10 billion years old is one the most ancient collections of stars known. It has been blazing for well over twice as long long as our Solar System has existed.
NGC 6752 contains a high number of “blue straggler” stars, some of which are visible in this image. These stars display characteristics of stars younger than their neighbours, despite models suggesting that most of the stars within globular clusters should have formed at approximately the same time. Their origin is therefore something of a mystery.
Studies of NGC 6752 may shed light on this situation. It appears that a very high number — up to 38% — of the stars within its core region are binary systems. Collisions between stars in this turbulent area could produce the blue stragglers that are so prevalent.
Lying 13 000 light-years distant, NGC 6752 is far beyond our reach, yet the clarity of Hubble’s images brings it tantalisingly close.
This NASA/ESA Hubble Space Telescope picture may trick you into thinking that the galaxy in it — known as UZC J224030.2+032131 — has not one but five different nuclei. In fact, the core of the galaxy is only the faint and diffuse object seen at the centre of the cross-like structure formed by the other four dots, which are images of a distant quasar located in the background of the galaxy.
The picture shows a famous cosmic mirage known as the Einstein Cross, and is a direct visual confirmation of the theory of general relativity. It is one of the best examples of the phenomenon of gravitational lensing — the bending of light by gravity as predicted by Einstein in the early 20th century. In this case, the galaxy’s powerful gravity acts as a lens that bends and amplifies the light from the quasar behind it, producing four images of the distant object.
The quasar is seen as it was around 11 billion light-years ago, in the direction of the constellation of Pegasus, while the galaxy that works as a lens is some ten times closer. The alignment between the two objects is remarkable (within 0.05 arcseconds), which is in part why such a special type of gravitational lensing is observed.
This image is likely the sharpest image of the Einstein Cross ever made, and was produced by Hubble’s Wide Field and Planetary Camera 2, and has a field of view of 26 by 26 arcseconds.
Messier 100 is a perfect example of a grand design spiral galaxy, a type of galaxy with prominent and very well-defined spiral arms. These dusty structures swirl around the galaxy’s nucleus, and are marked by a flurry of star formation activity that dots Messier 100 with bright blue, high-mass stars.
This image from the NASA/ESA Hubble Space Telescope, the most detailed made to date, shows the bright core of the galaxy and the innermost parts of its spiral arms. Messier 100 has an active galactic nucleus — a bright region at the galaxy’s core caused by a supermassive black hole that is actively swallowing material, which radiates brightly as it falls inwards.
The galaxy’s spiral arms also host smaller black holes, including the youngest ever observed in our cosmic neighbourhood, the result of a supernova observed in 1979.
Messier 100 is located in the direction of the constellation of Coma Berenices, about 50 million light-years distant.
The galaxy became famous in the early 1990s with the release of two images of the object taken with Hubble before and after a major repair to the telescope, which illustrated the dramatic improvement in Hubble’s observations.
This image, taken with the high resolution channel of Hubble’s Advanced Camera for Surveys demonstrates the continued evolution of Hubble’s capabilities over two decades in orbit. This image, like all high resolution channel images, has a relatively small field of view: only around 25 by 25 arcseconds.
This classic shot of a galaxy in the constellation of Ursa Major was taken by the NASA/ESA Hubble Space Telescope. NGC 3259 is a bright barred spiral galaxy located approximately 110 million light-years from Earth.
Being a fully-formed active galaxy, its bright central bulge hosts a supermassive black hole, whose huge appetite for matter explains the high luminosity of the galaxy’s core: as it devours its surroundings, the black hole emits intense radiation across the whole electromagnetic spectrum, including in visible light.
The beautiful spiral arms of the galaxy are not left out either as they contain dark lanes of dust and gas, ideal spawning grounds for stars. These bright, young, hot stars appear in rich clusters in the galaxy’s arms and are what gives the galaxy its blueish hue.
Interestingly, the galaxy has a small companion (visible to the left of the image), a much smaller galaxy that may be orbiting NGC 3259. In the background, numerous distant galaxies can be seen, easily identifiable by their elliptical shapes. They are visible here mainly in infrared light, which is shown in red in this image.
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.
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.
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.
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.
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.
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.
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.
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.
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.