Picture of the Week

2 April 2018: This image is packed full of galaxies! A keen eye can spot exquisite ellipticals and spectacular spirals, seen at various orientations: edge-on with the plane of the galaxy visible, face-on to show off magnificent spiral arms, and everything in between. The vast majority of these specks are galaxies, but to spot a foreground star from our own galaxy, you can look for a point of light with tell-tale diffraction spikes. The galaxies within the image are almost all part of a galaxy cluster — a monstrous collection of hundreds of galaxies all shackled together in the unyielding grip of gravity — with the charming name of SDSSJ0146-0929. The mass of this galaxy cluster is large enough to severely distort the spacetime, creating the odd, looping curves that almost encircle the central regions of the cluster. These graceful arcs are examples of a cosmic phenomenon known as an Einstein ring. The …

26 March 2018: This image, captured by the Advanced Camera for Surveys (ACS) on the NASA/ESA Hubble Space Telescope, shows the spiral galaxy NGC 5714, about 130 million light-years away in the constellation of Boötes (the Herdsman). NGC 5714 is classified as a Sc spiral galaxy, but its spiral arms — the dominating feature of spiral galaxies — are almost impossible to see, as NGC 5714 presents itself at an almost perfectly edge-on angle. Discovered by William Herschel in 1787, NGC 5714 was host to a fascinating and rare event in 2003. A faint supernova appeared about 8000 light-years below the central bulge of NGC 5714. Supernovae are the huge, violent explosions of dying stars, and the one that exploded in NGC 5714 — not visible in this much later image — was classified as a Type Ib/c supernova and named SN 2003dr. It was particularly interesting because its spectrum showed strong signatures …

12 March 2018: This stunning image from Hubble shows the majestic galaxy NGC 1015, found nestled within the constellation of Cetus (The Whale) 118 million light-years from Earth. In this image, we see NGC 1015 face-on, with its beautifully symmetrical swirling arms and bright central bulge creating a scene akin to a sparkling Catherine wheel firework. NGC 1015 has a bright, fairly large centre and smooth, tightly wound spiral arms and a central “bar” of gas and stars. This shape leads NGC 1015 to be classified as a barred spiral galaxy — just like our home, the Milky Way. Bars are found in around two-thirds of all spiral galaxies, and the arms of this galaxy swirl outwards from a pale yellow ring encircling the bar itself. Scientists believe that any hungry black holes lurking at the centre of barred spirals funnel gas and energy from the outer arms into the core via these …

5 March 2018: This enchanting spiral galaxy can be found in the constellation of Ursa Major (the Great Bear). Star-studded NGC 3972 lies about 65 million light-years away from the Earth, meaning that the light that we see now left it 65 million years ago, just when the dinosaurs became extinct. NGC 3972 has had its fair share of dramatic events. In 2011 astronomers observed the explosion of a type Ia supernova in the galaxy (not visible in this image). These dazzling objects all peak at the same brightness, and are brilliant enough to be seen over large distances. NGC 3972 also contains many pulsating stars called Cepheid variables. These stars change their brightness at a rate matched closely to their intrinsic luminosity, making them ideal cosmic lighthouses for measuring accurate distances to relatively nearby galaxies. Astronomers search for Cepheid variables in nearby galaxies which also contain a type Ia supernova so they …

26 February 2018: Discovered in 1900 by astronomer DeLisle Stewart and here imaged by the NASA/ESA Hubble Space Telescope, IC 4710 is an undeniably spectacular sight. The galaxy is a busy cloud of bright stars, with bright pockets — marking bursts of new star formation — scattered around its edges. IC 4710 is a dwarf irregular galaxy. As the name suggests, such galaxies are irregular and chaotic in appearance, lacking central bulges and spiral arms — they are distinctly different from spirals or ellipticals. It is thought that irregular galaxies may once have been spirals or ellipticals, but became distorted over time through external gravitational forces during interactions or mergers with other galaxies. Dwarf irregulars in particular are important to our overall understanding of galactic evolution, as they are thought to be similar to the first galaxies that formed in the Universe. IC 4710 lies roughly 25 million light-years away in the southern …

5 February 2018: Roughly 50 million light-years away lies a somewhat overlooked little galaxy named NGC 1559. Pictured here by Hubble’s Wide Field Camera 3, this barred spiral lies in the little-observed southern constellation of Reticulum (The Reticule). NGC 1559 has massive spiral arms chock-full of star formation, and is receding from us at a speed of about 1300 km/s. The galaxy contains the mass of around ten billion Suns — while this may sound like a lot, that is almost 100 times less massive than the Milky Way. Although NGC 1559 appears to sit near one of our nearest neighbours in the sky — the Large Magellanic Cloud (LMC), this is just a trick of perspective. In reality, NGC 1559 is physically nowhere near the LMC in space — in fact, it truly is a loner, lacking the company of any nearby galaxies or membership of any galaxy cluster. Despite its lack …

29 January 2018: This NASA/ESA Hubble Space Telescope image shows a spiral galaxy known as NGC 7331. First spotted by the prolific galaxy hunter William Herschel in 1784, NGC 7331 is located about 45 million light-years away in the constellation of Pegasus (The Winged Horse). Facing us partially edge-on, the galaxy showcases it’s beautiful arms which swirl like a whirlpool around its bright central region. Astronomers took this image using Hubble’s Wide Field Camera 3 (WFC3), as they were observing an extraordinary exploding star — a supernova — which can still be faintly seen as a tiny red dot near the galaxy’s central yellow core. Named SN2014C, it rapidly evolved from a supernova containing very little Hydrogen to one that is Hydrogen-rich — in just one year. This rarely observed metamorphosis was luminous at high energies and provides unique insight into the poorly understood final phases of massive stars. NGC 7331 is similar …

1 January 2018: This image, captured by the NASA/ESA Hubble Space Telescope’s Wide Field Camera 3 (WFC3), shows a galaxy named UGC 6093. As can be easily seen, UGC 6093 is something known as a barred spiral galaxy — it has beautiful arms that swirl outwards from a bar slicing through the galaxy’s centre. It is classified as an active galaxy, which means that it hosts an active galactic nucleus, or AGN: a compact region at a galaxy’s centre within which material is dragged towards a supermassive black hole. As this black hole devours the surrounding matter it emits intense radiation, causing it to shine brightly. But UGC 6093 is more exotic still. The galaxy essentially acts as a giant astronomical laser that spews out light at microwave, not visible, wavelengths — this type of object is dubbed a megamaser (maser being the term for a microwave laser). Megamasers such as UGC 6093 …

4 December 2017: Don’t be fooled! The subject of this Picture of the Week, ESO 580-49, may seem tranquil and unassuming, but this spiral galaxy actually displays some explosive tendencies. In October of 2011, a cataclysmic burst of high-energy gamma-ray radiation — known as a gamma-ray burst, or GRB — was detected coming from the region of sky containing ESO 580-49. Astronomers believe that the galaxy was the host of the GRB, given that the chance of a coincidental alignment between the two is roughly 1 in 10 million. At a distance of around 185 million light-years from Earth, it was the second-closest gamma-ray burst (GRB) ever detected. Gamma-ray bursts are among the brightest events in the cosmos, occasionally outshining the combined gamma-ray output of the entire observable Universe for a few seconds. The exact cause of the GRB that probably occurred within this galaxy, catalogued as GRB 111005A, remains a mystery. Several …

13 November 2017: This new Picture of the Week, taken by the NASA/ESA Hubble Space Telescope, shows the dwarf galaxy NGC 4625, located about 30 million light-years away in the constellation of Canes Venatici (The Hunting Dogs). The image, acquired with the Advanced Camera for Surveys (ACS), reveals the single spiral arm of the galaxy, which gives it an asymmetric appearance. But why is there only one spiral arm, when spiral galaxies normally have at least two? Astronomers looked at NGC 4625 in different wavelengths in the hope of solving this cosmic mystery. Observations in the ultraviolet provided the first hint: in ultraviolet light the disc of the galaxy appears four times larger than on the image depicted here. An indication that there are a large number of very young and hot — hence mainly visible in the ultraviolet — stars forming in the outer regions of the galaxy. These young stars are …

25 September 2017: The distances to objects in the Universe can differ enormously. The nearest star to us — Proxima Centauri — lies some 4.2 light-years from us, while some incredibly distant galaxies are so far away — 13 billion light-years or more — that they are only visible to us as a result of cosmic tricks of magnification. The subject of this image, a galaxy called ESO 376-16, sits nearly 23 million light-years from Earth — not that great a distance on a cosmic scale. However, given the galaxy’s relative proximity to us, we know surprisingly little about it. Astronomers are still debating about many of the properties of ESO 376-16, including its morphology. Galaxies are divided into types based on their visual appearance and characteristics; spiral galaxies, like the Milky Way, are flattened discs with curved arms sweeping out from a central nucleus, while irregular galaxies lack a distinct structure and …

18 September 2017: Despite the advances made in past decades, the process of galaxy formation remains an open question in astronomy. Various theories have been suggested, but since galaxies come in all shapes and sizes — including elliptical, spiral, and irregular — no single theory has so far been able to satisfactorily explain the origins of all the galaxies we see throughout the Universe. To determine which formation model is correct (if any), astronomers hunt for the telltale signs of various physical processes. One example of this is galactic coronas, which are huge, invisible regions of hot gas that surround a galaxy’s visible bulk, forming a spheroidal shape. They are so hot that they can be detected by their X-ray emission, far beyond the optical radius of the galaxy. Because they are so wispy, these coronas are extremely difficult to detect. In 2013, astronomers highlighted NGC 6753, imaged here by the NASA/ESA Hubble …

11 September 2017: This NASA/ESA Hubble Space Telescope picture shows NGC 5398, a barred spiral galaxy located about 55 million light-years away. The galaxy is famous for containing an especially extensive HII region, a large cloud composed of ionised hydrogen (or HII, pronounced “H-two”, with H being the chemical symbol for hydrogen and the “II” indicating that the atoms have lost an electron to become ionised). NGC 5398’s cloud is named Tol 89 and sits at the lower left end of the galaxy’s central “bar” of stars, a structure that cuts through the galactic core and funnels material inwards to maintain the star formation occurring there. Tol 89 is conspicuous in being the only large massive star forming complex in the entire galaxy, with an extension of roughly 5000 times 4000 light-years; it contains at least seven young and massive star clusters. The two brightest clumps within Tol 89, which astronomers have named …

4 September 2017: Like firecrackers lighting up the sky on New Year’s Eve, the majestic spiral arms of NGC 5559 are alight with new stars being born. NGC 5559 is a spiral galaxy, with spiral arms filled with gas and dust sweeping out around the bright galactic bulge. These arms are a rich environment for star formation, dotted with a festive array of colours including the newborn stars glowing blue as a result of their immensely high temperatures. NGC 5559 was discovered by astronomer William Herschel in 1785 and lies approximately 240 million light-years away in the northern constellation of Boötes (the herdsman) In 2001, a calcium-rich supernova called 2001co was observed in NGC 5559. Calcium-rich supernovae (Ca-rich SNe) are described as “fast-and-faint”, as they're less luminous than other types of supernovae and also evolve more rapidly, to reveal spectra dominated by strong calcium lines. 2001co occurred within the disc of NGC 5559 …

7 August 2017: The subject of this NASA/ESA Hubble Space Telescope image is a dwarf galaxy named NGC 5949. Thanks to its proximity to Earth — it sits at a distance of around 44 million light-years from us, placing it within the Milky Way’s cosmic neighbourhood — NGC 5949 is a perfect target for astronomers to study dwarf galaxies. With a mass of about a hundredth that of the Milky Way, NGC 5949 is a relatively bulky example of a dwarf galaxy. Its classification as a dwarf is due to its relatively small number of constituent stars, but the galaxy’s loosely-bound spiral arms also place it in the category of barred spirals. This structure is just visible in this image, which shows the galaxy as a bright yet ill-defined pinwheel. Despite its small proportions, NGC 5949’s proximity has meant that its light can be picked up by fairly small telescopes, something that facilitated …

24 July 2017: This beautiful clump of glowing gas, dark dust, and glittering stars is the spiral galaxy NGC 4248, located about 24 million light-years away in the constellation of Canes Venatici (The Hunting Dogs). This image was produced by the NASA/ESA Hubble Space Telescope as it embarked upon compiling the first Hubble ultraviolet “atlas”, for which the telescope targeted 50 nearby star-forming galaxies. A sample spanning all kinds of different morphologies, masses, and structures. Studying this sample can help us to piece together the star-formation history of the Universe. By exploring how massive stars form and evolve within such galaxies, astronomers can learn more about how, when, and where star formation occurs, how star clusters change over time, and how the process of forming new stars is related to the properties of both the host galaxy and the surrounding interstellar medium (the “stuff” that fills the space between individual stars). This image …

17 July 2017: Tucked away in the small northern constellation of Canes Venatici (The Hunting Dogs) is the galaxy NGC 4242, shown here as seen by the NASA/ESA Hubble Space Telescope. The galaxy lies some 30 million light-years from us. At this distance from Earth, actually not all that far on a cosmic scale, NGC 4242 is visible to anyone armed with even a basic telescope (as British astronomer William Herschel found when he discovered the galaxy in 1788). This image shows the galaxy’s bright centre and the surrounding dimmer and more diffuse “fuzz”. Despite appearing to be relatively bright in this image, studies have found that NGC 4242 is actually relatively dim (it has a moderate-to-low surface brightness and low luminosity) and also supports a low rate of star formation. The galaxy also seems to have a weak bar of stars cutting through its asymmetric centre, and a very faint and poorly-defined …

10 July 2017: Discovered by British astronomer William Herschel over 200 years ago, NGC 2500 lies about 30 million light-years away in the northern constellation of Lynx. As this NASA/ESA Hubble Space Telescope image shows, NGC 2500 is a particular kind of spiral galaxy known as a barred spiral, its wispy arms swirling out from a bright, elongated core. Barred spirals are actually more common than was once thought. Around two-thirds of all spiral galaxies — including the Milky Way — exhibit these straight bars cutting through their centres. These cosmic structures act as glowing nurseries for newborn stars, and funnel material towards the active core of a galaxy. NGC 2500 is still actively forming new stars, although this process appears to be occurring very unevenly. The upper half of the galaxy — where the spiral arms are slightly better defined — hosts many more star-forming regions than the lower half, as indicated …

3 July 2017: IC 342 is a challenging cosmic target. Although it is bright, the galaxy sits near the equator of the Milky Way’s galactic disc, where the sky is thick with glowing cosmic gas, bright stars, and dark, obscuring dust. In order for astronomers to see the intricate spiral structure of IC 342, they must gaze through a large amount of material contained within our own galaxy — no mean feat! As a result IC 342 is relatively difficult to spot and image, giving rise to its intriguing nickname: the “Hidden Galaxy”. Located very close (in astronomical terms!) to the Milky Way, this sweeping spiral galaxy would be among the brightest in the sky were it not for its dust-obscured location. The galaxy is very active, as indicated by the range of colours visible in this NASA/ESA Hubble Space Telescope image, depicting the very central region of the galaxy. A beautiful mixture …

19 June 2017: The object in the middle of this image, sitting alone within a star-studded cosmos, is a galaxy known as ESO 486-21. ESO 486-21 is a spiral galaxy — albeit with a somewhat irregular and ill-defined structure — located some 30 million light-years from Earth. The NASA/ESA Hubble Space Telescope observed this object while performing a survey — the Legacy ExtraGalactic UV Survey (LEGUS) — of 50 nearby star-forming galaxies. The LEGUS sample was selected to cover a diverse range of galactic morphologies, star formation rates, galaxy masses, and more. Astronomers use such data to understand how stars form and evolve within clusters, and how these processes affect both their home galaxy and the wider Universe. ESO 486-21 is an ideal candidate for inclusion in such a survey as it is known to be in the process of forming new stars, which are created when large clouds of gas and dust …