|Picture of The Week|
|A Smudged Fingerprint|
|Mon, 17 Feb 2020 06:00:00 +0100|
TheNASA/ESA Hubble Space Telescope is no stranger to spiral galaxies. The telescope has brought us some of the most beautiful images ever taken of our spiral neighbours — and this Picture of the Week, which features a galaxy known as NGC 4689, is no exception.
However, seen almost face on, NGC 4689 appears less like a majestic spiral and more like a smudged fingerprint on the sky. No matter how good the image quality, there is little contrast between the spiralling arms of stars, gas, and dust, and the less dense areas in between. This is because NGC 4689 is something known as an “anaemic galaxy”, a galaxy that contains only quite small quantities of the raw materials needed to produce stars. This means that star formation is quelled in NGC 4689, and the pinwheeling, bustling arms are less bright than those belonging to other Hubble muses.
Despite this subtlety when compared to its brash, spotlight-stealing relatives, NGC 4689 retains an otherworldly charm, its delicately glowing material standing out subtly from the surrounding darkness of space.
|Mon, 10 Feb 2020 06:00:00 +0100|
This galaxy is located about 425 million light-years from Earth in the constellation of Pictor (The Painter’s Easel). Discovered in 1834 by astronomer John Herschel, NGC 2008 is categorised as a type Sc galaxy in the Hubble sequence, a system used to describe and classify the various morphologies of galaxies. The “S” indicates that NGC 2008 is a spiral, while the “c” means it has a relatively small central bulge and more open spiral arms. Spiral galaxies with larger central bulges tend to have more tightly wrapped arms, and are classified as Sa galaxies, while those in between are classified as type Sb.
Spiral galaxies are ubiquitous across the cosmos, comprising over 70% of all observed galaxies — including our own, the Milky Way. However, their ubiquity does not detract from their beauty. These grand, spiralling collections of billions of stars are among the most wondrous sights that have been captured by telescopes such as Hubble, and are firmly embedded in astronomical iconography.
|Nature’s Grand Design|
|Mon, 03 Feb 2020 06:00:00 +0100|
This eye-catching galaxy is known as NGC 5364.
Unmistakably a spiral, NGC 5364 is also something known as a grand design spiral galaxy — a descriptive name deserved by only one-tenth of spirals. While all spirals have a structure that is broadly similar, there is quite a bit of variation amongst individual galaxies; some have patchy, oddly-shaped arms, some have bars of stars cutting through their core, some are colossal and radiant, and others are dim and diminutive. Grand designs like NGC 5364 are in many ways the archetype of a spiral galaxy. They are characterised by their prominent, well-defined arms, which circle outwards from a clear core.
Despite being classified in this way, NGC 5364 is far from perfect. Its arms are asymmetrical compared to other grand design spirals — this is thought to be due to interactions with a nearby neighbour. This neighbour and NGC 5364 are tugging on one another, warping and moving their stars and gas around and causing this misshapen appearance.
|Bars and Baby Stars|
|Mon, 27 Jan 2020 06:00:00 +0100|
A barred spiral is a galaxy with whirling, pinwheeling, spiral arms, and a bright centre that is intersected by a bar of gas and stars. This bar cuts directly through the galaxy’s central region, and is thought to invigorate the region somewhat, sparking activity and fuelling myriad processes that may otherwise have never occurred or have previously ground to a halt (star formation and active galactic nuclei being key examples). We think bars exist in up to two-thirds of all spiral galaxies, including our own home, the Milky Way.
NGC 7541 is actually observed to have a higher-than-usual star formation rate, adding weight to the theory that spiral bars act as stellar nurseries, corralling and funnelling inwards the material and fuel needed to create and nurture new baby stars. Along with its nearby companion NGC 7537, the galaxy makes up a pair of galaxies located about 110 million light-years away from us.
|Mon, 20 Jan 2020 06:00:00 +0100|
This peculiar galaxy, beautifully streaked with tendrils of reddish dust, is captured here in wonderful detail by the NASA/ESA Hubble Space Telescope.
The galaxy is known as NGC 1022, and is officially classified as a barred spiral galaxy. You can just about make out the bar of stars in the centre of the galaxy in this image, with swirling arms emerging from its ends. This bar is much less prominent than in some of the galaxy’s barred cousins and gives the galaxy a rather squat appearance; but the lanes of dust that swirl throughout its disc ensure it is no less beautiful.
Hubble observed this image as part of a study into one of the Universe’s most notorious residents: black holes. These are fundamental components of galaxies, and are thought to lurk at the hearts of many — if not all — spirals. In fact, they may have quite a large influence over their cosmic homes. Studies suggest that the mass of the black hole sitting at a galaxy’s centre is linked with the larger-scale properties of the galaxy itself. However, in order to learn more, we need observational data of a wider and more diverse range of galaxies — something Hubble’s study aims to provide.
|Mon, 13 Jan 2020 06:00:00 +0100|
This bright, somewhat blob-like object — seen in this Picture of the Week as observed by the NASA/ESA Hubble Space Telescope — is a galaxy named NGC 1803. It is about 200 million light-years away, in the southern constellation of Pictor (The Painter’s Easel).
NGC 1803 was discovered in 1834 by astronomer John Herschel. Herschel is a big name in astronomy; John, his father William, and his aunt Caroline all made huge contributions to the field, and their legacies remain today. William systematically catalogued many of the objects he viewed in the night sky, named many moons in the Solar System, discovered infrared radiation, and more. Caroline discovered several comets and nebulae. John took this aforementioned catalogue of night-sky objects and reworked and expanded it into his General Catalogue of Nebulae and Clusters of Stars. This was the basis for the cataloguing system still used today by astronomers (Dreyer’s New General Catalogue of Nebulae and Clusters of Stars, or the New General Catalogue for short).
This gives rise to the NGC names assigned to a vast number of galaxies — including NGC 1803. This galaxy is one of a galactic pair. It was described by Dreyer as being “faint, small, [and] round”, and located near to a very bright star to the southeast. This star is in fact the nebulous lenticular galaxy PGC 16720 — not visible in this image.
|Imposter or the Real Deal?|
|Mon, 06 Jan 2020 06:00:00 +0100|
This Picture of the Week, taken by the NASA/ESA Hubble Space Telescope, shows a close-up view of a galaxy named NGC 2770. NGC 2770 is intriguing, as over time it has hosted four different observed supernovae (not visible here).
Supernovae form in a few different ways, but always involve a dying star. These stars become unbalanced, lose control, and explode violently, briefly shining as brightly as an entire galaxy before slowly fading away.
One of the four supernovae observed within this galaxy, SN 2015bh, is especially interesting. This particular supernova initially had its identity called into question. When it was first discovered in 2015, astronomers classified SN 2015bh as a supernova imposter, believing it to be not an exploding star but simply an unpredictable outburst from a massive star in its final phase of life. Thankfully, astronomers eventually discovered the truth and the object was given its correct classification as a Type II supernova, resulting from the death of a star between eight and 50 times the mass of the Sun.
|Placed Among the Stars|
|Mon, 30 Dec 2019 06:00:00 +0100|
This smattering of celestial sequins is a spiral galaxy named NGC 4455, located in the northern constellation of Coma Berenices (Berenice’s Hair). This might sound like an odd name for a constellation — and in fact it is somewhat unusual; it’s the only modern constellation to be named in honour of a real person from history: Queen Berenice II of Egypt.
The story of Queen Berenice II is an interesting one. A ruling queen of the ancient Greek city of Cyrene in modern-day Libya, and later a queen of Ptolemaic Egypt through her marriage to her cousin Ptolemy III Euergetes, Berenice became known for sacrificing locks of her hair as an offering to ensure her husband’s safe return from battle. Her husband did indeed return safely and her hair, which she had left in a Zephyrium temple, had disappeared — it had apparently been stolen and placed among the stars.
|An Active Centre|
|Mon, 23 Dec 2019 06:00:00 +0100|
This swirling mass of celestial gas, dust, and stars is a moderately luminous spiral galaxy named ESO 021-G004, located just under 130 million light-years away.
This galaxy has something known as an active galactic nucleus. While this phrase sounds complex, this simply means that astronomers measure a lot of radiation at all wavelengths coming from the centre of the galaxy. This radiation is generated by material falling inwards into the very central region of ESO 021-G004, and meeting the behemoth lurking there — a supermassive black hole. As material falls towards this black hole it is dragged into orbit as part of an accretion disc; it becomes superheated as it swirls around and around, emitting characteristic high-energy radiation until it is eventually devoured.
|Discs and Bulges|
|Mon, 16 Dec 2019 06:00:00 +0100|
This image from the NASA/ESA Hubble Space Telescope shows IC 2051, a galaxy in the southern constellation of Mensa (The Table Mountain), lying about 85 million light-years away. It is a spiral galaxy, as evidenced by its characteristic whirling, pinwheeling arms, and it has a bar of stars slicing through its centre.
This galaxy was observed for a Hubble study on galactic bulges, the bright round central region of spiral galaxies. Spiral galaxies like IC 2051 are shaped a bit like flying saucers when seen from the side; they comprise a thin, flat disc, with a bulky bulge of stars in the centre that extends above and below the disc. These bulges are thought to play a key role in how galaxies evolve, and to influence the growth of the supermassive black holes lurking at the centres of most spirals. While more observations are needed in this area, studies suggest that some, or even most, galactic bulges may be complex composite structures rather than simple ones, with a mix of spherical, disc-like, or boxy components, potentially leading to a wide array of bulge morphologies in the Universe.
This image comprises data from Hubble’s Wide Field Camera 3 at visible and infrared wavelengths.
|Mon, 09 Dec 2019 06:00:00 +0100|
NGC 3175 is located around 50 million light-years away in the constellation of Antlia (The Air Pump). The galaxy can be seen slicing across the frame in this image from the NASA/ESA Hubble Space Telescope, with its mix of bright patches of glowing gas, dark lanes of dust, bright core, and whirling, pinwheeling arms coming together to paint a beautiful celestial scene.
The galaxy is the eponymous member of the NGC 3175 group, which has been called a nearby analogue for the Local Group. The Local Group contains our very own home galaxy, the Milky Way, and around 50 others — a mix of spiral, irregular, and dwarf galaxies. The NGC 3175 group contains a couple of large spiral galaxies — the subject of this image, and NGC 3137 — and numerous lower-mass spiral and satellite galaxies. Galaxy groups are some of the most common galactic gatherings in the cosmos, and they comprise 50 or so galaxies all bound together by gravity.
This image comprises observations from Hubble’s Wide Field Camera 3.
|A Dramatic Demise|
|Mon, 02 Dec 2019 06:00:00 +0100|
Some of the most dramatic events in the Universe occur when certain stars die — and explode catastrophically in the process.
Such explosions, known as supernovae, mainly occur in a couple of ways: either a massive star depletes its fuel at the end of its life, become dynamically unstable and unable to support its bulk, collapses inwards, and then violently explodes; or a white dwarf in an orbiting stellar couple syphons more mass off its companion than it is able to support, igniting runaway nuclear fusion in its core and beginning the supernova process. Both types result in an intensely bright object in the sky that can rival the light of a whole galaxy.
In the last 20 years the galaxy NGC 5468, visible in this image, has hosted a number of observed supernovae of both the aforementioned types: SN 1999cp, SN 2002cr, SN2002ed, SN2005P, and SN2018dfg. Despite being just over 130 million light-years away, the orientation of the galaxy with respect to us makes it easier to spot these new ‘stars’ as they appear; we see NGC 5468 face on, meaning we can see the galaxy’s loose, open spiral pattern in beautiful detail in images such as this one from the NASA/ESA Hubble Space Telescope.
|A Close Relationship|
|Mon, 25 Nov 2019 06:00:00 +0100|
Some galaxies are closer friends than others. While many live their own separate, solitary lives, others stray a little too close to a near neighbour and take their relationship to the next level.
The two galaxies featured in this Picture of the Week, named NGC 6285 (left) and NGC 6286 (right), have done just that! Together, the duo is named Arp 293 and they are interacting, their mutual gravitational attraction pulling wisps of gas and streams of dust from them, distorting their shapes, and gently smudging and blurring their appearances on the sky — to Earth-based observers, at least.
The NASA/ESA Hubble Space Telescope has viewed a number of interacting pairs. These can have distinctive, beautiful, and downright odd shapes, ranging from sheet music to a spaceship entering a sci-fi-esque wormhole, a bouquet of celestial blooms, and a penguin fiercely guarding its precious egg.
Arp 293 is located in the constellation of Draco (The Dragon), and lies over 250 million light-years from Earth.
|Emission Versus Absorption|
|Mon, 18 Nov 2019 06:00:00 +0100|
When astronomers explore the contents and constituent parts of a galaxy somewhere in the Universe, they use various techniques and tools. One of these is to spread out the incoming light from that galaxy into a spectrum and explore its properties. This is done in much the same way as a glass prism spreads white light into its constituent wavelengths to create a rainbow. By hunting for specific signs of emission from various elements within a galaxy’s spectrum of light — so-called emission lines — or, conversely, the signs of absorption from other elements — so-called absorption lines — astronomers can start to deduce what might be happening within.
If a galaxy’s spectrum shows many absorption lines and few emission lines, this suggests that its star-forming material has been depleted and that its stars are mainly old, while the opposite suggests it might be bursting with star formation and energetic stellar newborns. This technique known as spectroscopy, can tell us about a galaxy’s type and composition, the density and temperature of any emitting gas, the star formation rate, or how massive the galaxy’s central black hole might be.
While not all galaxies display strong emission lines, NGC 3749 does! It lies over 135 million light-years away, and is moderately luminous. The galaxy has been used a “control” in studies of especially active and luminous galaxies — those with centres known as active galactic nuclei, which emit copious amounts of intense radiation. In comparison to these active cousins, NGC 3749 is classified as inactive, and has no known signs of nuclear activity.
|A Rival to the Milky Way|
|Mon, 11 Nov 2019 06:00:00 +0100|
The Universe is simply so vast that it can be difficult to maintain a sense of scale. Many galaxies we see through telescopes such as the NASA/ESA Hubble Space Telescope, the source of this beautiful Picture of the Week, look relatively similar: spiralling arms, a glowing centre, and a mixture of bright specks of star formation and dark ripples of cosmic dust weaving throughout.
This galaxy, a spiral galaxy named NGC 772, is no exception. It actually has much in common with our home galaxy, the Milky Way. Each boasts a few satellite galaxies, small galaxies that closely orbit and are gravitationally bound to their parent galaxies. One of NGC 772’s spiral arms has been distorted and disrupted by one of these satellites (NGC 770 — not visible in the image here), leaving it elongated and asymmetrical.
However, the two are also different in a few key ways. For one, NGC 772 is both a peculiar and an unbarred spiral galaxy; respectively, this means that it is somewhat odd in size, shape, or composition, and that it lacks a central feature known as a bar, which we see in many galaxies throughout the cosmos — including the Milky Way. These bars are built of gas and stars, and are thought to funnel and transport material through the galactic core, possibly fueling and igniting various processes such as star formation.
|Mon, 04 Nov 2019 06:00:00 +0100|
Within a galaxy hosting around 300 billion stars, here the NASA/ESA Hubble Space Telescope has captured a mere handful or two — just about enough to form a single football team. These stellar “teammates” play under the banner of NGC 1333, the cloud of gas and dust which formed them and that they continue to call home.
NGC 1333 is located about 1000 light-years away in the constellation of Perseus (The Hero). The cool gas and dust concentrated in this region is generating new stars whose light is then reflecting off the surrounding material, lighting it up and making this object’s lingering presence known to us. NGC 1333 is accordingly classified as a reflection nebula.
This image shows just a single region of NGC 1333. Hubble has imaged NGC 1333 more widely before, revealing that the smattering of stars seen here has ample company. Seen in a broader context, this team of stars is but one gathering amongst many in NGC 1333’s celestial Champions League.
|Lonely Hearts Club|
|Mon, 28 Oct 2019 06:00:00 +0100|
Galaxies may seem lonely, floating alone in the vast, inky blackness of the sparsely populated cosmos — but looks can be deceiving. The subject of this Picture of the Week, NGC 1706, is a good example of this. NGC 1706 is a spiral galaxy, about 230 million light-years away, in the constellation of Dorado (The Swordfish).
NGC 1706 is known to belong to something known as a galaxy group, which is just as the name suggests — a group of up to 50 galaxies which are gravitationally bound and hence relatively close to each other. Around half of the galaxies we know of in the Universe belong to some kind of group, making them incredibly common cosmic structures. Our home galaxy, the Milky Way, belongs to the Local Group, which also contains the Andromeda Galaxy, the Large and Small Magellanic Clouds, and the Triangulum Galaxy.
Groups are the smallest of galactic gatherings; others are clusters, which can comprise hundreds of thousands of galaxies bound loosely together by gravity, and subsequent superclusters, which bring together numerous clusters into a single entity.
|A Familiar Sight|
|Mon, 21 Oct 2019 06:00:00 +0200|
This image from the NASA/ESA Hubble Space Telescope shows IC 4653, a galaxy just above 80 million light-years from Earth. That may sound like quite a distance, but it’s not that far on a cosmic scale. At these kinds of distances, the types and structures of the objects we see are similar to those in our local area.
Thie galaxy's whirling arms tells us a story about what’s happening inside this galaxy. Stars are generally brighter and bluer when they are younger, so the blue patches mark sites of new star formation. Studying the structures of other galaxies is a key way to learn about the structure of our own, given that humans can’t leave the Milky Way to look back and see what it looks like from the outside. It helps to compare our observations of our home galaxy with those of nearby galaxies we can see in their entirety.
|A Galactic Portal|
|Mon, 14 Oct 2019 06:00:00 +0200|
The galaxy NGC 4380 looks like a special effect straight out of a science fiction or fantasy film in this Hubble Picture of the Week, swirling like a gaping portal to another dimension.
In the grand scheme of things, though, the galaxy is actually quite ordinary. Spiral galaxies like NGC 4380 are one of the most common types of galaxy in the Universe. These colossal collections of stars, often numbering in the hundreds of billions, are shaped like a flat disc, sometimes with a rounded bulge in the centre. Graceful spiral arms outlined by dark lanes of dust wind around the bulging core, which glows brightly and has the highest concentration of stars in the galaxy.
|A Spiral in Profile|
|Mon, 07 Oct 2019 06:00:00 +0200|
The NASA/ESA Hubble Space Telescope sees galaxies of all shapes, sizes, brightnesses, and orientations in the cosmos. Sometimes, the telescope gazes at a galaxy oriented sideways — as shown here. The spiral galaxy featured in this Picture of the Week is called NGC 3717, and it is located about 60 million light-years away in the constellation of Hydra (The Sea Serpent).
Seeing a spiral almost in profile, as Hubble has here, can provide a vivid sense of its three-dimensional shape. Through most of their expanse, spiral galaxies are shaped like a thin pancake. At their cores, though, they have bright, spherical, star-filled bulges that extend above and below this disc, giving these galaxies a shape somewhat like that of a flying saucer when they are seen edgeon.
NGC 3717 is not captured perfectly edge-on in this image; the nearer part of the galaxy is tilted ever so slightly down, and the far side tilted up. This angle affords a view across the disc and the central bulge (of which only one side is visible).
|Snakes and Stones|
|Mon, 30 Sep 2019 06:00:00 +0200|
The galaxy pictured in this Hubble Picture of the Week has an especially evocative name: the Medusa merger.
Often referred to by its somewhat drier New General Catalogue designation of NGC 4194, this was not always one entity, but two. An early galaxy consumed a smaller gas-rich system, throwing out streams of stars and dust out into space. These streams, seen rising from the top of the merger galaxy, resembles the writhing snakes that Medusa, a monster in ancient Greek mythology, famously had on her head in place of hair, lending the object its intriguing name.
The legend of Medusa also held that anyone who saw her face would transform into stone. In this case, you can feast your eyes without fear on the centre of the merging galaxies, a region known as Medusa's eye. All the cool gas pooling here has triggered a burst of star formation, causing it to stand out brightly against the dark cosmic backdrop.
The Medusa merger is located about 130 million light-years away in the constellation of Ursa Major (The Great Bear).
|Beacon of Light|
|Mon, 23 Sep 2019 06:00:00 +0200|
This image from the NASA/ESA Hubble Space Telescope shows the galaxy Messier 86. Despite its being discovered over 235 years ago by astronomer Charles Messier, the morphological classification of Messier 86 remains unclear; astronomers are still debating over whether it is either elliptical or lenticular (the latter being a cross between an elliptical and spiral galaxy).
Messier 86 is part of the Virgo Cluster of galaxies and is situated about 50 million light-years from Earth. The galaxy is moving through space remarkably quickly — its current trajectory is bringing it in our direction, back towards the centre of its cluster from the far side, at the incredible speed of over 875 000 kilometres per hour! Because of the speed with which it is moving through the cluster, Messier 86 is undergoing a process known as ram-pressure stripping; the resistive material filling the gaps between individual cluster galaxies is pulling at the gas and dust in Messier 86 and stripping them out as the galaxy moves, creating a long trail of hot gas that is emitting X-ray radiation.
Astronomers are using Hubble observations such as this to study elliptical and lenticular galaxies, both of which are often found at the centres of galaxy clusters. By studying the cores of these galaxies, astronomers hope to determine details of the central structure and to analyse both the history of the galaxy and the formation of its core.
|Not So Dead After All|
|Mon, 16 Sep 2019 06:00:00 +0200|
Many of the best-loved galaxies in the cosmos are remarkably large, close, massive, bright, or beautiful, often with an unusual or intriguing structure or history. However, it takes all kinds to make a Universe — as demonstrated by this Hubble Picture of the Week of Messier 110.
Messier 110 may not look like much, but it is a fascinating near neighbour of our home galaxy, and an unusual example of its type. It is a member of the Local Group, a gathering of galaxies comprising the Milky Way and a number of the galaxies closest to it. Specifically, Messier 110 is one of the many satellite galaxies encircling the Andromeda Galaxy, the nearest major galaxy to our own, and is classified as a dwarf elliptical galaxy, meaning that it has a smooth and almost featureless structure. Elliptical galaxies lack arms and notable pockets of star formation — both characteristic features of spiral galaxies. Dwarf ellipticals are quite common in groups and clusters of galaxies, and are often satellites of larger galaxies.
Because they lack stellar nurseries and contain mostly old stars, elliptical galaxies are often considered ‘dead’ when compared to their spiral relatives. However, astronomers have spotted signs of a population of young, blue stars at the centre of Messier 110 — hinting that it may not be so dead after all.
|Dark Matter in the Belly of the Whale|
|Mon, 09 Sep 2019 06:00:00 +0200|
This image, taken with the NASA/ESA Hubble Space Telescope, focuses on an object named UGC 695, which is located 30 million light-years away within the constellation Cetus (The Sea Monster), also known as The Whale.
UGC 695 is a low-surface-brightness (LSB) galaxy. These galaxies are so faint that their brightness is less than the background brightness of Earth’s atmosphere, which makes them tricky to observe. This low brightness is the result of the relatively small number of stars within them — most of the baryonic matter in these galaxies exists in the form of huge clouds of gas and dust. The stars are also distributed over a relatively large area.
LSB galaxies, like dwarf galaxies, have a high fraction of dark matter relative to the number of stars they contain. Astronomers still debate about how LSB galaxies formed in the first place.
|Mon, 02 Sep 2019 06:00:00 +0200|
This Picture of the Week shows a dwarf galaxy named UGC 685. Such galaxies are small and contain just a tiny fraction of the number of stars in a galaxy like the Milky Way. Dwarf galaxies often show a hazy structure, an ill-defined shape, and an appearance somewhat akin to a swarm or cloud of stars — and UGC 685 is no exception to this. Classified as an SAm galaxy — a type of unbarred spiral galaxy — it is located about 15 million light-years from Earth.
These data were gathered under the NASA/ESA Hubble Space Telescope’s LEGUS (Legacy ExtraGalactic UV Survey) Program, the sharpest and most comprehensive ultraviolet survey of star-forming galaxies in the nearby Universe.
LEGUS is imaging 50 spiral and dwarf galaxies in our cosmic neighbourhood in multiple colours using Hubble’s Wide Field Camera 3. The survey is picking apart the structures of these galaxies and resolving their constituent stars, clusters, groups, and other stellar associations. Star formation plays a huge role in shaping its host galaxy; by exploring these targets in detail via both new observations and archival Hubble data, LEGUS will shed light on how stars form and cluster together, how these clusters evolve, how a star’s formation affects its surroundings, and how stars explode at the end of their lives.