Picture of the Week

27 May 2024: This week, an image of the broad and sweeping spiral galaxy NGC 4731 is the Hubble Picture of the Week. This galaxy lies among the galaxies of the Virgo cluster, in the constellation Virgo, and is located 43 million light-years from Earth. This highly detailed image was created using six different filters. The abundance of colour illustrates the galaxy's billowing clouds of gas, dark dust bands, bright pink star-forming regions and, most obviously, the long, glowing bar with trailing arms. Barred spiral galaxies outnumber both regular spirals and elliptical galaxies put together, numbering around 60% of all galaxies. The visible bar structure is a result of orbits of stars and gas in the galaxy lining up, forming a dense region that individual stars move in and out of over time. This is the same process that maintains a galaxy's spiral arms, but it is somewhat more mysterious for bars: spiral …

20 May 2024: This Picture of the Week shows the jewel-bright spiral galaxy NGC 4689, which lies 54 million light-years from Earth in the constellation Coma Berenices. This constellation has the distinction of being the only one of the 88 constellations officially recognised by the International Astronomical Union (IAU) to be named after an historical figure, Queen Berenice II of Egypt. The latin word ‘coma’ references her hair, meaning that NGC 4689 can be said to be found in the hair of a queen. Some people of Berenice’s time would have meant this quite literally, as the story goes that her court astronomer thought that a missing lock of Berenice’s hair had been catasterised (a word meaning ‘placed amongst the stars’) by the gods: hence the name of the constellation, Coma Berenices. NGC 4689 holds an interesting — albeit less royal — place in modern astronomy too. The Universe is so incredibly vast …

13 May 2024: Featured in this new image from the NASA/ESA Hubble Space Telescope is a nearly edge-on view of the lenticular galaxy NGC 4753. These galaxies have an elliptical shape and ill-defined spiral arms. This image is the object's sharpest view to date, showcasing Hubble’s incredible resolving power and ability to reveal complex dust structures. NGC 4753 resides around 60 million light-years from Earth in the constellation Virgo and was first discovered by the astronomer William Herschel in 1784. It is a member of the NGC 4753 Group of galaxies within the Virgo II Cloud, which comprises roughly 100 galaxies and galaxy clusters. This galaxy is believed to be the result of a galactic merger with a nearby dwarf galaxy roughly 1.3 billion years ago. NGC 4753’s distinct dust lanes around its nucleus are believed to have been accreted from this merger event. It is now believed that most of the mass …

6 May 2024: The celestial object showcased in this week's Hubble Picture of the Week is the spiral galaxy UGC 9684, which lies around 240 million light-years from Earth in the constellation Boötes. This image shows an impressive example of several classic galactic features, including a clear bar in the galaxy's centre, and a halo surrounding its disc. The impetus for this Hubble image was a study into the host galaxies of Type-II supernovae. These cataclysmic stellar explosions take place throughout the Universe, and are of great interest to astronomers, so automated surveys scan the night sky and attempt to catch sight of them. The supernova which brought UGC 9684 to Hubble's attention occurred during 2020. It has faded from view in this image, which was taken in 2023. Remarkably, the 2020 supernova in this galaxy isn't the only one that's been seen there — four supernova-like events have been spotted in UGC …

29 April 2024: Featured in this Hubble Picture of the Week this week is the dwarf galaxy IC 776. This swirling collection of stars new and old is located in the constellation Virgo — in fact, in the Virgo galaxy cluster — 100 million light-years from Earth. While a dwarf galaxy, it's also been classified as an SAB-type or ‘weakly barred’ spiral, one study naming it a “complex case” in morphology. This highly detailed view from Hubble demonstrates that complexity well. IC 776 has a ragged, disturbed disc that nevertheless looks to spiral around the core, and arcs of star-forming regions. This image is from an observation programme dedicated to the study of dwarf galaxies in the Virgo cluster, searching for sources of X-rays in such galaxies. X-rays are often emitted by accretion discs, where material that is drawn into a compact object by gravity crashes together and forms a hot, glowing disc. …

22 April 2024: This Hubble Picture of the Week depicts the spiral galaxy ESO 422-41, which lies about 34 million light-years from Earth in the constellation Columba. The patchy, star-filled structure of the galaxy’s spiral arms and the glow from its dense core are laid out in intricate detail here by Hubble’s Advanced Camera for Surveys. Images of this galaxy have, however, a decades-long history. The name ESO 422-41 comes from its identification in the European Southern Observatory (B) Atlas of the Southern Sky. In the times before automated sky surveys with space observatories such as ESA’s Gaia, many stars, galaxies and nebulae were discovered by means of large photographic surveys. Astronomers used the most advanced large telescopes of the time to produce hundreds of photographs, covering an area of the sky. They later studied the resulting photographs, attempting to catalogue all the new astronomical objects revealed. In the 1970s a new telescope …

15 April 2024: This image features NGC 3783, a bright barred spiral galaxy about 130 million light-years from Earth, that also lends its name to the eponymous NGC 3783 galaxy group. Like galaxy clusters, galaxy groups are aggregates of gravitationally bound galaxies. Galaxy groups, however, are less massive and contain fewer members than galaxy clusters do: where galaxy clusters can contain hundreds or even thousands of constituent galaxies, galaxy groups do not typically include more than 50. The Milky Way is actually part of a galaxy group, known as the Local Group, which contains two other large galaxies (Andromeda and the Triangulum galaxy), as well as several dozen satellite and dwarf galaxies. The NGC 3783 galaxy group, meanwhile, contains 47 galaxies. It also seems to be at a fairly early stage of its evolution, making it an interesting object of study. Whilst the focus of this image is the spiral galaxy NGC 3783, …

8 April 2024: The subject of this week’s Picture of the Week from Hubble is the spiral galaxy IC 4633, located 100 million light-years away from us in the constellation Apus. IC 4633 is a galaxy rich in star-forming activity, as well as hosting an active galactic nucleus at its core. From our point of view, the galaxy is tilted mostly towards us, giving astronomers a fairly good view of its billions of stars. However, we can’t fully appreciate the features of this galaxy — at least in visible light — because it’s partially concealed by a stretch of dark dust. This dark nebula is part of the Chamaeleon star-forming region, itself located only around 500 light-years from us, in a nearby part of the Milky Way galaxy. The dark clouds in the Chamaeleon region occupy a large area of the southern sky, covering their namesake constellation but also encroaching on nearby constellations, …

1 April 2024: This image features Arp 72, a very selective galaxy group that only includes two interacting galaxies: NGC 5996 (the large spiral galaxy) and NGC 5994 (its smaller companion, in the lower left of the image). Both galaxies lie approximately 160 million light-years from Earth, and their cores are separated from each other by a distance of around 67 thousand light-years. Moreover, the distance between the galaxies at their closest points is even smaller, closer to 40 thousand light-years. Whilst this might still sound vast, in galactic separation terms it is really very cosy! For comparison, the distance between the Milky Way and its nearest independent galactic neighbour Andromeda is around 2.5 million light-years. Alternatively, the distance between the Milky Way and its largest and brightest satellite galaxy, the Large Magellanic Cloud (satellite galaxies are galaxies that are bound in orbit around another galaxy), is about 162 thousand light-years. Given this, …

25 March 2024: This image shows a globular cluster known as NGC 1651. Like the object in another recent Picture of the Week, it is located about 162 000 light-years away in the largest and brightest of the Milky Way’s satellite galaxies, the Large Magellanic Cloud (LMC). A notable feature of this image is that the globular cluster almost fills the entire image, even though globular clusters are only about 10 to 300 light-years in diameter (NGC 1651 has a diameter of roughly 120 light-years). In contrast, there are numerous Hubble Pictures of the Week that feature entire galaxies — which can be tens or hundreds of millions of light-years in diameter — that also more or less fill the whole image. A common misconception is that Hubble and other large telescopes manage to observe wildly differently sized celestial objects by zooming in on them, as one would with a specialised camera here …

18 March 2024: This gauzy-looking celestial body is UGC 5829, an irregular galaxy that lies about 30 million light-years away. Despite there not being many observations of this relatively faint galaxy, it has the distinction of having a descriptive soubriquet: the Spider Galaxy. Perhaps the distorted galactic arms with their glowing, star-forming tips bring to mind the clawed legs of an arachnid. Somewhat confusingly, there is another, very similarly nicknamed but otherwise entirely distinct, galaxy known as the Spiderweb Galaxy. This galaxy has also been more extensively imaged (notably by Hubble), despite the fact that it lies about 300 times further from Earth than the Spider Galaxy does. Fortunately, correct galaxy identification does not depend on casual given names. Rather, known galaxies are recorded in at least one catalogue — and often in several — such as the Uppsala General Catalogue of Galaxies, which gives the Spider Galaxy its more formal title of …

11 March 2024: This image shows LEDA 42160, a galaxy about 52 million light-years from Earth in the constellation Virgo. The dwarf galaxy is one of many forcing its way through the comparatively dense gas in the Virgo cluster, a massive cluster of galaxies. The pressure exerted by this intergalactic gas, known as ram pressure, has dramatic effects on star formation in LEDA 42160, which are presently being studied using the Hubble Space Telescope. LEDA 42160 falls into the category of ‘Magellanic spiral galaxy’, or type Sm for short, under the de Vaucouleurs galaxy classification system. Magellanic spiral galaxies can be further sub-categorised as barred (SBm), unbarred (SAm) and weakly barred (SABm), where a ‘bar’ is an elongated bar-shape at a galaxy’s core. Generally speaking, Magellanic spiral galaxies are dwarf galaxies with only one single spiral arm. They are named after their prototype, the Large Magellanic Cloud, which is an SBm galaxy. Magellanic …

4 March 2024: Here we see NGC 4423, a galaxy that lies about 55 million light-years away in the constellation Virgo. In this image NGC 4423 appears to have quite an irregular, tubular form, so it might be surprising to find out that it is in fact a spiral galaxy. Knowing this, we can make out the denser central bulge of the galaxy, and the less crowded surrounding disc (the part that comprises the spiral arms). If NGC 4423 were viewed face-on it would resemble the shape that we most associate with spiral galaxies: the spectacular curving arms sweeping out from a bright centre, interspersed with dimmer, darker, less populated regions. But when observing the skies we are constrained by the relative alignments between Earth and the objects that we are observing: we cannot simply reposition Earth so that we can get a better face-on view of NGC 4423! Of course, celestial objects …

26 February 2024: This densely populated group of stars is the globular cluster known as NGC 1841, which is found within the Large Magellanic Cloud (LMC), a satellite galaxy to the Milky Way galaxy that lies about 162 000 light-years away. Satellite galaxies are galaxies that are bound by gravity in orbits around a more massive host galaxy. We typically think of our galaxy’s nearest galactic companion as being the Andromeda Galaxy, but it would be more accurate to say that Andromeda is the nearest galaxy that is not in orbit around the Milky Way galaxy. In fact, our galaxy is orbited by tens of known satellite galaxies that are far closer than Andromeda, the largest and brightest of which is the LMC, which is easily visible to the naked eye from the southern hemisphere (although this is decreasingly the case thanks to light pollution). The LMC is home to many globular clusters. …

19 February 2024: This image features IC 3476, a dwarf galaxy that lies about 54 million light-years from Earth in the constellation Coma Berenices. Whilst this image does not look very dramatic — if we were to anthropomorphise the galaxy, we might say it looks almost serene — the actual physical events taking place in IC 3476 are highly energetic. In fact, the little galaxy is undergoing a process known as ram pressure stripping, which is driving unusually high levels of star formation within regions of the galaxy itself. We tend to associate the letters ‘ram’ with the acronym RAM, which refers to Random Access Memory in computing. However, ram pressure has a totally distinct definition in physics: it is the pressure exerted on a body when it moves through some form of fluid, due to the overall resistance of the fluid. In the case of entire galaxies experiencing ram pressure, the galaxies …

12 February 2024: If the Hubble Picture of the Week from two weeks ago was somewhat dim and subtle in appearance, then this week’s image is a veritable riot of colour and activity! It features a relatively close-by star-forming region known as IRAS 16562-3959 that lies within the Milky Way in the constellation Scorpius, about 5900 light-years from Earth. This image was compiled using observations from Hubble’s Wide Field Camera 3 (WFC 3). The detailed nuances of colour are possible because of the four separate filters that were used to collect the data. Filters are thin slivers of highly specialised material that only allow very specific wavelengths of light through. They can be slid in front of the part of the telescope that is sensitive to light, letting astronomers control which wavelengths of light the telescope collects with each observation. This is useful not only for specific scientific research, but also for the …

5 February 2024: This image shows a densely packed field of stars, laid on top of a background of dust, gas, and light from more distant celestial objects. The stars take up so much of the field of view in this image that it is a little tricky to discern that you are in fact looking at most of a galaxy, known as ESO 245-5. This galaxy is a relatively close neighbour of the Milky Way, lying at the fairly modest distance of 15 million light-years from Earth in the constellation Phoenix. Another reason that it is perhaps a little tricky to spot that ESO 245-5 is a galaxy is its apparent lack of structure. We frequently enjoy Hubble’s spectacular images of spiral galaxies, which are so interesting to look at in part because of their seemingly extraordinarily ordered arms of stars, gas and dust. ESO 245-5, in contrast, is classified as an …

29 January 2024: This image of the spiral galaxy UGC 11105 is not as bright and vivid as some other Hubble Pictures of the Week. This softly luminous galaxy — lying in the constellation Hercules, about 110 million light-years from Earth — seems outshone by the sparkling foreground stars that surround it. The type II supernova which took place in this galaxy in 2019, while no longer visible in this image, definitely outshone the galaxy at the time! To be more precise, UGC 11105 has an apparent magnitude of around 13.6 in the optical light regime (this image was created using data that covers the heart of the optical regime, in addition to ultraviolet data). Astronomers have different ways of quantifying how bright celestial objects are, and apparent magnitude is one of them. Firstly, the ‘apparent’ part of this quantity refers to the fact that apparent magnitude only describes how bright objects appear …

22 January 2024: This image shows the spiral galaxy IC 438, which lies about 130 million light-years from Earth in the constellation Lepus (the Hare). Lepus lies just south of the celestial equator (the ring around the middle of Earth that falls at right angles to its rotation axis). Appropriately, Lepus is flanked by the constellations Canis Major (the Greater Dog) and Orion (the Hunter), whilst Canis Minor (the Lesser Dog) lies very nearby, meaning that in artistic representations of the constellations, Lepus is often shown as being pursued by Orion and his two hunting dogs. Lepus is one of the 88 constellations that are officially recognised by the International Astronomical Union (IAU). It is worth clarifying that, whilst the actual constellations themselves only comprise a handful of stars, the area of sky covered by those stars is often referred to using the name of the constellation. For example, when we say that …

15 January 2024: This image features a relatively small galaxy known as UGC 5189A, which is located about 150 million light-years away in the constellation Leo. This galaxy was observed by Hubble to study a supernova explosion in 2010 known as SN 2010jl. This particular supernova was notable for having been an exceptionally luminous supernova event. In fact, over a period of three years, SN 2010jl released at least 2.5 billion times more visible energy alone than our Sun emitted over the same timeframe across all wavelengths. Even after supernovae have faded to non-observable levels, it can still be of interest to study the environments where they occurred. This can provide astronomers with valuable information: supernovae can take place for a variety of reasons, and understanding the environments in which they took place can help improve our understanding of the conditions necessary for them to be triggered. Furthermore, follow-up studies after supernovae can …