The darkness within?: This atmospheric image shows a galaxy named Messier 85, captured in all its delicate, hazy glory by the NASA/ESA Hubble Space Telescope. Messier 85 slants through the constellation of Coma Berenices (Berenice’s Hair), and lies around 50 million light-years from Earth. It was first discovered by Charles Messier’s colleague Pierre Méchain in 1781, and is included in the Messier catalogue of celestial objects. Messier 85 is intriguing — its properties lie somewhere between those of a lenticular and an elliptical galaxy, and it appears to be interacting with two of its neighbours: the beautiful spiral NGC 4394, located out of frame to the upper left, and the small elliptical MCG 3-32-38, located out of frame to the centre bottom. The galaxy contains some 400 billion stars, most of which are very old. However, the central region hosts a population of relatively young stars of just a few billion years in age; these stars are thought to have formed in a late burst of star formation, likely triggered as Messier 85 merged with another galaxy over four billion years ago. Messier 85 has a further potentially strange quality. Almost every galaxy is thought to have a supermassive black hole at its centre, but from measurements of the velocities of stars in this galaxy, it is unclear whether Messier 85 contains such a black hole. This image combines infrared, visible and ultraviolet observations from Hubble’s Wide Field Camera 3.

Name: Messier 85
Type: Local Universe : Galaxy : Type : Elliptical
Category: Galaxies
Constellation: Coma Berenices
Distance: 50 million light years
Release date: 4 February 2019, 06:00
Credit: ESA/Hubble & NASA, R. O'Connell

The two mysterious populations of NGC 2419: Globular clusters like NGC 2419, visible in this image taken with the NASA/ESA Hubble Space Telescope, are not only beautiful, but also fascinating. They are spherical groups of stars which orbit the centre of a galaxy; in the case of NGC 2419, that galaxy is the Milky Way. NGC 2419 can be found around 300 000 light-years from the Solar System, in the constellation Lynx (the Lynx). The stars populating globular clusters are very similar to one another, with similar properties such as metallicity. The similarity of these stellar doppelgängers is due to their formation early in the history of the galaxy. As the stars in a globular cluster all formed at around the same time, they tend to display reasonably homogeneous properties. It was believed that this similarity also extended to the stellar helium content; that is, it was thought that all stars in a globular cluster would contain comparable amounts of helium. However, Hubble’s observations of NGC 2419 have shown that this is not always the case. This surprising globular cluster turns out to be made up of two separate populations of red giant stars, one of which is unusually helium-rich. Other elements within the different stars in NGC 2419 vary too — nitrogen in particular. On top of this, these helium-rich stars were found to be predominantly in the centre of the globular cluster, and to be rotating. These observations have raised questions about the formation of globular clusters; did these two drastically different groups of stars form together? Or did this globular cluster come into being by a different route entirely?

Name: NGC 2419
Type: Local Universe : Star : Grouping : Cluster : Globular
Category: Star Clusters
Constellation: Lynx
Distance: 300000 light years
Release date: 25 February 2019, 06:00
Credit: ESA/Hubble & NASA, S. Larsen et al.

Colliding galaxies: Located in the constellation of Hercules, about 230 million light-years away, NGC 6052 is a pair of colliding galaxies. They were first discovered in 1784 by William Herschel and were originally classified as a single irregular galaxy because of their odd shape. However, we now know that NGC 6052 actually consists of two galaxies that are in the process of colliding. This particular image of NGC 6052 was taken using the Wide Field Camera 3 on the NASA/ESA Hubble Space Telescope. A long time ago gravity drew the two galaxies together into the chaotic state we now observe. Stars from within both of the original galaxies now follow new trajectories caused by the new gravitational effects. However, actual collisions between stars themselves are very rare as stars are very small relative to the distances between them (most of a galaxy is empty space). Eventually things will settle down and one day the two galaxies will have fully merged to form a single, stable galaxy. Our own galaxy, the Milky Way, will undergo a similar collision in the future with our nearest galactic neighbour, the Andromeda Galaxy. Although this is not expected to happen for around 4 billion years so there is nothing to worry about just yet. This object was previously observed by Hubble with its old WFPC2 camera. That image was released in 2015.

Name: NGC 6052
Type: Local Universe : Galaxy : Type : Interacting
Category: Galaxies
Constellation: Hercules
Distance: 230 million light years
Release date: 4 March 2019, 06:00
Credit: ESA/Hubble & NASA, A. Adamo et al.

Nebulous, but no nebula: This Hubble Picture of the Week shows Messier 28, a globular cluster in the constellation of Sagittarius (The Archer), in jewel-bright detail. It is about 18 000 light-years away from Earth. As its name suggests, this cluster belongs to the Messier catalogue of objects — however, when astronomer Charles Messier first added Messier 28 to his list in 1764, he catalogued it incorrectly, referring to it as a “[round] nebula containing no star”. While today we know nebulae to be vast, often glowing clouds of interstellar dust and ionised gases, until the early twentieth century a nebula represented any astronomical object that was not clearly localised and isolated. Any unidentified hazy light source could be called a nebula. In fact, all 110 of the astronomical objects identified by Messier were combined under the title of the Catalogue of Nebulae and Star Clusters. He classified many objects as diverse as star clusters and supernova remnants as nebulae. This includes Messier 28, pictured here — which, ironically, is actually a star cluster. Messier’s mistake is understandable. Whilst Messier 28 is easily recognisable as a globular stellar cluster in this image, it is far less recognisable from Earth. Even with binoculars it is only visible very faintly, as the distorting effects of the Earth’s atmosphere reduce this luminous ancient cluster to a barely visible smudge in the sky. One would need larger telescopes to resolve single stars in Messier 28. Fortunately, from space Hubble allows Messier 28 to be seen in all its beauty — far more than a faint, shapeless, nebulous cloud.

Name: Messier 28
Type: Milky Way : Star : Grouping : Cluster : Globular
Category: Star Clusters
Constellation: Sagittarius
Distance: 18000 light years
Release date: 11 March 2019, 06:00
Credit: ESA/Hubble & NASA, J. E. Grindlay et al.