The Death of a Star: This atmospheric Picture of the Week, taken with the NASA/ESA Hubble Space Telescope, shows a dark, gloomy scene in the constellation of Gemini (The Twins). The subject of this image confused astronomers when it was first studied — rather than being classified as a single object, it was instead recorded as two objects, owing to its symmetrical lobed structure (known as NGC 2371 and NGC 2372, though sometimes referred to together as NGC 2371/2). These two lobes are visible to the upper right and lower left of the frame, and together form something known as a planetary nebula. Despite the name, such nebulae have nothing to do with planets; NGC 2371/2 formed when a Sun-like star reached the end of its life and blasted off its outer layers, shedding the constituent material and pushing it out into space to leave just a superheated stellar remnant behind. This remnant is visible as the orange-tinted star at the centre of the frame, sitting neatly between the two lobes. The structure of this region is complex. It is filled with dense knots of gas, fast-moving jets that appear to be changing direction over time, and expanding clouds of material streaming outwards on diametrically opposite sides of the remnant star. Patches of this scene glow brightly as the remnant star emits energetic radiation that excites the gas within these regions, causing it to light up. This scene will continue to change over the next few thousand years; eventually the knotty lobes will dissipate completely, and the remnant star will cool and dim to form a white dwarf.

Name: NGC 2371
Type: Milky Way : Nebula : Type : Planetary
Category: Nebulae
Constellation: Gemini
Release date: 19 August 2019, 06:00
Credit: ESA/Hubble & NASA, R. Wade et al.

A Passing Fancy: This Picture of the Week from the NASA/ESA Hubble Space Telescope shows NGC 5307, a planetary nebula which lies about 10000 light years from Earth. It can be seen in the constellation Centaurus (The Centaur), which can be seen primarily in the southern hemisphere.  A planetary nebula is the final stage of a Sun-like star. As such, planetary nebulae allow us a glimpse into the future of our own Solar System. A star like our Sun will, at the end of its life, transform into a red giant. Stars are sustained by the nuclear fusion that occurs in their core, which creates energy. The nuclear fusion processes constantly try to rip the star apart. Only the gravity of the star prevents this from happening. At the end of the red giant phase of a star, these forces become unbalanced. Without enough energy created by fusion, the core of the star collapses in on itself, while the surface layers are ejected outward. After that, all that remains of the star is what we see here: glowing outer layers surrounding a white dwarf star, the remnants of the red giant star’s core. This isn’t the end of this star’s evolution though — those outer layers are still moving and cooling. In just a few thousand years they will have dissipated, and all that will be left to see is the dimly glowing white dwarf.

Name: NGC 5307
Type: Milky Way : Nebula : Type : Planetary
Category: Nebulae
Constellation: Centaurus
Distance: 8000 light years
Release date: 26 August 2019, 06:00
Credit: ESA/Hubble & NASA, R. Wade et al.

Hubble’s Legacy: 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.

Name: UGC 685
Type: Local Universe : Galaxy : Size : Dwarf
Category: Galaxies
Constellation: Pisces
Distance: 15 million light years
Release date: 2 September 2019, 06:00
Credit: ESA/Hubble & NASA; the LEGUS team, B. Tully, D. CalzettiAcknowledgement(s): Judy Schmidt (Geckzilla)

Dark Matter in the Belly of the Whale: 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.

Name: UGC 695
Type: Local Universe : Galaxy
Category: Galaxies
Constellation: Cetus
Distance: 30 million light years
Release date: 9 September 2019, 06:00
Credit: ESA/Hubble & NASA, D. Calzetti

Not So Dead After All: 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.

Name: Messier 110
Type: Local Universe : Galaxy : Type : EllipticalLocal Universe : Galaxy : Size : Dwarf
Category: Galaxies
Constellation: Andromeda
Distance: 3 million light years
Release date: 16 September 2019, 06:00
Credit: ESA/Hubble & NASA, L.Ferrarese et al.