Mysterious supernovae: 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 near star-forming regions, but Ca-rich SNe are often observed at large distances from the nearest galaxy, raising curious questions about their progenitors.

Name: NGC 5559
Type: Local Universe : Galaxy : Type : SpiralEarly Universe : Galaxy : Type : Spiral
Category: Galaxies
Constellation: Bootes
Distance: 220 million light years
Release date: 4 September 2017, 06:00
Credit: ESA/Hubble & NASA

Starbursts in NGC 5398: 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 simply “A” and “B”, appear to have undergone two bursts of star-forming activity — “starbursts” — roughly 4 million and less than 3 million years ago respectively. Tol 89-A is thought to contain a number of particularly bright and massive stars known as Wolf-Rayet stars, which are known for their high temperatures and extreme stellar winds.

Name: NGC 5398
Type: Local Universe : Galaxy : Type : Barred
Category: Galaxies
Constellation: Centaurus
Distance: 55 million light years
Release date: 11 September 2017, 06:00
Credit: ESA/Hubble & NASA

More than meets the eye: 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 Space Telescope, as one of only two known spiral galaxies that were both massive enough and close enough to permit detailed observations of their coronas. Of course, NGC 6753 is only close in astronomical terms — the galaxy is nearly 150 million light-years from Earth. NGC 6753 is a whirl of colour in this image — the bursts of blue throughout the spiral arms are regions filled with young stars glowing brightly in ultraviolet light, while redder areas are filled with older stars emitting in the cooler near-infrared.

Name: NGC 6753
Type: Local Universe : Galaxy : Type : Spiral
Category: Galaxies
Constellation: Pavo
Distance: 150 million light years
Release date: 18 September 2017, 06:00
Credit: ESA/Hubble & NASAAcknowledgement: Judy Schmidt

Mapping the nearby Universe: 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 look far more chaotic. On the basis of its rather ill-defined morphology, ESO 376-16 is thought to be either a late-type spiral or a dwarf irregular galaxy. Despite its mystique, observations of ESO 376-16 have been useful in several studies, including one made with the NASA/ESA Hubble Space Telescope that aimed to create a 3D map of galaxies lying in the vicinity of Earth. Researchers used Hubble to gauge the distance to galaxies including ESO 376-16 by measuring the luminosities of especially bright red-giant-branch stars sitting within the galaxies. They then used their data to generate and calibrate 3D maps of the distribution of galaxies throughout the nearby cosmos.

Name: ESO 376-16
Type: Local Universe : Galaxy
Category: Galaxies
Constellation: Antlia
Distance: 23 million light years
Release date: 25 September 2017, 06:00
Credit: ESA/Hubble & NASA