Picture of the Week

13 April 2020: This image depicts a swirling spiral galaxy named NGC 2906. The blue speckles seen scattered across this galaxy are massive young stars, which emit hot, blue-tinged radiation as they burn through their fuel at an immense rate. The swathes of orange are a mix of older stars that have swollen and cooled, and low-mass stars that were never especially hot to begin with. Owing to their lower temperatures, these stars emit a cooler, reddish, radiation. This image of NGC 2906 was captured by the NASA/ESA Hubble Space Telescope’s Wide Field Camera 3, an instrument installed on Hubble in 2009 during the telescope’s fourth servicing mission. Hubble observed this galaxy on the hunt for fading light from recent, nearby occurrences of objects known as supernovae.

6 April 2020: At first glance, the subject of this NASA/ESA Hubble Space Telescope image looks to be a simple spiral galaxy, with two pinwheeling arms emerging from a central bar of stars and material that cuts through the galactic centre. In fact, there are rings within these spiral arms, too: spirals within a spiral. This kind of morphology is known as a multiring structure. As this description suggests, this galaxy, named NGC 2273, hosts an inner ring and two outer “pseudorings” — having so many distinct rings is rare, and makes NGC 2273 unusual. Rings are created when a galaxy’s spiral arms appear to loop around to nearly close upon one another, combined with a trick of cosmic perspective. NGC 2273’s two pseudorings are formed by two swirling sets of spiral arms coming together, and the inner ring by two arcing structures nearer to the galactic centre, which seem to connect in …

30 March 2020: This remarkable spiral galaxy, known as NGC 4651, may look serene and peaceful as it swirls in the vast, silent emptiness of space, but don’t be fooled — it keeps a violent secret. It is believed that this galaxy consumed another smaller galaxy to become the large and beautiful spiral that we observe today. Although only a telescope like the NASA/ESA Hubble Space Telescope, which captured this image, could give us a picture this clear, NGC 4651 can also be observed with an amateur telescope — so if you have a telescope at home and a star-gazing eye, look out for this glittering carnivorous spiral.

23 March 2020: NGC 4618 was discovered on 9 April 1787 by the German-British astronomer, Wilhelm Herschel, who also discovered Uranus in 1781. Only a year before discovering NGC 4618, Herschel theorised that the “foggy” objects astronomers were seeing in the night sky were likely to be large star clusters located much further away then the individual stars he could easily discern. Since Herschel proposed his theory, astronomers have come to understand that what he was seeing was a galaxy. NGC 4618, classified as a barred spiral galaxy, has the special distinction amongst other spiral galaxies of only having one arm rotating around the centre of the galaxy. Located about 21 million light-years from our galaxy in the constellation Canes Venatici, NGC 4618 has a diameter of about one third that of the Milky Way. Together with its neighbour, NGC 4625, it forms an interacting galaxy pair, which means …

16 March 2020: This Picture of the Week, taken by the NASA/ESA Hubble Space Telescope, shows the galaxy NGC 4237. Located about 60 million light-years from Earth in the constellation of Coma Berenices (Berenice's Hair), NGC 4237 is classified as a flocculent spiral galaxy. This means that its spiral arms are not clearly distinguishable from each other, as in grand design spiral galaxies, but are instead patchy and discontinuous. This gives the galaxy a fluffy appearance, somewhat resembling cotton wool. Astronomers studying NGC 4237 were actually more interested in its galactic bulge — its bright central region. By learning more about these bulges, we can explore how spiral galaxies have evolved, and study the growth of the supermassive black holes that lurk at the centres of most spirals. There are indications that the mass of the black hole at the centre of a galaxy is related to the mass of its bulge. However, …

9 March 2020: The subject of this Picture of the Week, a spiral galaxy named NGC 1589, was once the scene of a violent bout of cosmic hunger pangs; as astronomers looked on, a poor, hapless star was torn apart and devoured by the ravenous supermassive black hole at the centre of the galaxy. The astronomers are now using the NASA/ESA Hubble Space Telescope to test this interpretation. Hubble has observed such events before so the scientists are confident that Hubble will be able to provide smoking gun evidence in the form of stellar debris that was ejected during the disruption event.

2 March 2020: The barred spiral galaxy NGC 3887, seen here as viewed by the Wide Field Camera 3 aboard the NASA/ESA Hubble Space Telescope, lies over 60 million light-years away from us in the southern constellation of Crater (The Cup); it was discovered on 31 December 1785 by the German/British astronomer William Herschel. Its orientation to us, while not exactly face-on, allows us to see NGC 3887’s spiral arms and central bulge in detail, making it an ideal target for studying a spiral galaxy’s winding arms and the stars within them. The very existence of spiral arms was for a long time a problem for astronomers. The arms emanate from a spinning core and should therefore become wound up ever more tightly, causing them to eventually disappear after a (cosmologically) short amount of time. It was only in the 1960s that astronomers came up with the solution to this winding problem; rather …

24 February 2020: This image of an archetypal spiral galaxy was captured by the NASA/ESA Hubble Space Telescope. The subject of this image is known as NGC 691, and it can be found some 120 million light-years from Earth. This galaxy was one of thousands of objects discovered by astronomer William Herschel during his prolific decades-long career spent hunting for, characterising, and cataloguing a wide array of the galaxies and nebulae visible throughout the night sky — almost 200 years before Hubble was even launched. The intricate detail visible in this Picture of the Week would likely be extraordinary to Herschel. Hubble was able to capture an impressive level of structure within NGC 691’s layers of stars and spiralling arms — all courtesy of the telescope’s high-resolution Wide Field Camera 3.

17 February 2020: 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 …

10 February 2020: The spiral galaxy NGC 2008 sits centre stage, its ghostly spiral arms spreading out towards us, in this image captured by the NASA/ESA Hubble Space Telescope. 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. …

3 February 2020: 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 …

27 January 2020: The galaxy depicted in this Picture of the Week is a barred spiral known as NGC 7541, seen here as viewed by the NASA/ESA Hubble Space Telescope, in the constellation of Pisces (The Fishes). 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 …

20 January 2020: 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 …

13 January 2020: 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 …

6 January 2020: 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 …

30 December 2019: 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. …

23 December 2019: 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. The data comprising this image were gathered by the Wide Field Camera 3 aboard the NASA/ESA Hubble Space Telescope.

16 December 2019: 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 …

9 December 2019: 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, …

2 December 2019: 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 …