A Smudged Fingerprint: TheNASA/ESA Hubble Space Telescope is no stranger to spiral galaxies. The telescope has brought us some of the most beautiful images ever taken of our spiral neighbours — and this Picture of the Week, which features a galaxy known as NGC 4689, is no exception. However, seen almost face on, NGC 4689 appears less like a majestic spiral and more like a smudged fingerprint on the sky. No matter how good the image quality, there is little contrast between the spiralling arms of stars, gas, and dust, and the less dense areas in between. This is because NGC 4689 is something known as an “anaemic galaxy”, a galaxy that contains only quite small quantities of the raw materials needed to produce stars. This means that star formation is quelled in NGC 4689, and the pinwheeling, bustling arms are less bright than those belonging to other Hubble muses. Despite this subtlety when compared to its brash, spotlight-stealing relatives, NGC 4689 retains an otherworldly charm, its delicately glowing material standing out subtly from the surrounding darkness of space.

Name: NGC 4689
Type: Local Universe : Galaxy : Type : Spiral
Category: Galaxies
Constellation: Coma Berenices
Release date: 17 February 2020, 06:00
Credit: ESA/Hubble & NASA, P. Erwin

Centuries Before Hubble: This image of an archetypal spiral galaxy was captured by the NASA/ESA Hubble Space Telescope. The subject of this image is known as NGC 691, and it can be found some 120 million light-years from Earth. This galaxy was one of thousands of objects discovered by astronomer William Herschel during his prolific decades-long career spent hunting for, characterising, and cataloguing a wide array of the galaxies and nebulae visible throughout the night sky — almost 200 years before Hubble was even launched. The intricate detail visible in this Picture of the Week would likely be extraordinary to Herschel. Hubble was able to capture an impressive level of structure within NGC 691’s layers of stars and spiralling arms — all courtesy of the telescope’s high-resolution Wide Field Camera 3.

Name: NGC 691
Type: Local Universe : Galaxy : Type : Spiral
Category: Galaxies
Constellation: Aries
Distance: 120 million light years
Release date: 24 February 2020, 06:00
Credit: ESA/Hubble & NASA, A. Riess et al.

A Galactic Traffic Jam: The barred spiral galaxy NGC 3887, seen here as viewed by the Wide Field Camera 3 aboard the NASA/ESA Hubble Space Telescope, lies over 60  million light-years away from us in the southern constellation of Crater (The Cup); it was discovered on 31 December 1785 by the German/British astronomer William Herschel. Its orientation to us, while not exactly face-on, allows us to see NGC 3887’s spiral arms and central bulge in detail, making it an ideal target for studying a spiral galaxy’s winding arms and the stars within them. The very existence of spiral arms was for a long time a problem for astronomers. The arms emanate from a spinning core and should therefore become wound up ever more tightly, causing them to eventually disappear after a (cosmologically) short amount of time. It was only in the 1960s that astronomers came up with the solution to this winding problem; rather than behaving like rigid structures, spiral arms are in fact areas of greater density in a galaxy’s disc, with dynamics similar to those of a traffic jam. The density of cars moving through a traffic jam increases at the centre of the jam, where they move more slowly. Spiral arms function in a similar way; as gas and dust move through the density waves they become compressed and linger, before moving out of them again.

Name: NGC 3887
Type: Local Universe : Galaxy : Type : Spiral
Category: Galaxies
Constellation: Crater
Distance: 60 million light years
Release date: 2 March 2020, 06:00
Credit: ESA/Hubble & NASA, P. Erwin et al.

Hunger Pangs: The subject of this Picture of the Week, a spiral galaxy named NGC 1589, was once the scene of a violent bout of cosmic hunger pangs; as astronomers looked on, a poor, hapless star was torn apart and devoured by the ravenous supermassive black hole at the centre of the galaxy. The astronomers are now using the NASA/ESA Hubble Space Telescope to test this interpretation. Hubble has observed such events before so the scientists are confident that Hubble will be able to provide smoking gun evidence in the form of stellar debris that was ejected during the disruption event.

Name: NGC 1589
Type: Local Universe : Galaxy : Type : Spiral
Category: Galaxies
Constellation: Taurus
Distance: 150 million light years
Release date: 9 March 2020, 06:00
Credit: ESA/Hubble & NASA

Cotton Wool Galaxy: This Picture of the Week, taken by the NASA/ESA Hubble Space Telescope, shows the galaxy NGC 4237. Located about 60 million light-years from Earth in the constellation of Coma Berenices (Berenice's Hair), NGC 4237 is classified as a flocculent spiral galaxy. This means that its spiral arms are not clearly distinguishable from each other, as in grand design spiral galaxies, but are instead patchy and discontinuous. This gives the galaxy a fluffy appearance, somewhat resembling cotton wool. Astronomers studying NGC 4237 were actually more interested in its galactic bulge — its bright central region. By learning more about these bulges, we can explore how spiral galaxies have evolved, and study the growth of the supermassive black holes that lurk at the centres of most spirals. There are indications that the mass of the black hole at the centre of a galaxy is related to the mass of its bulge. However, this connection is still uncertain, and why these two components should be so strongly correlated is still a mystery — one that astronomers hope to solve by studying galaxies in the nearby Universe, such as NGC 4237.

Name: NGC 4237
Type: Local Universe : Galaxy : Type : Spiral
Category: Galaxies
Constellation: Coma Berenices
Distance: 60 million light years
Release date: 16 March 2020, 06:00
Credit: ESA/Hubble & NASA, P. Erwin et al.

Single Arm Galaxy: NGC 4618 was discovered on 9 April 1787 by the German-British astronomer, Wilhelm Herschel, who also discovered Uranus in 1781. Only a year before discovering NGC 4618, Herschel theorised that the “foggy” objects astronomers were seeing in the night sky were likely to be large star clusters located much further away then the individual stars he could easily discern. Since Herschel proposed his theory, astronomers have come to understand that what he was seeing was a galaxy. NGC 4618, classified as a barred spiral galaxy, has the special distinction amongst other spiral galaxies of only having one arm rotating around the centre of the galaxy. Located about 21 million light-years from our galaxy in the constellation Canes Venatici, NGC 4618 has a diameter of about one third that of the Milky Way. Together with its neighbour, NGC 4625, it forms an interacting galaxy pair, which means that the two galaxies are close enough to  influence each other gravitationally. These interactions may result in the two (or more) galaxies merging together to form a new formation, such as a ring galaxy.

Name: NGC 4618
Type: Local Universe : Galaxy : Type : Spiral
Category: Galaxies
Constellation: Canes Venatici
Distance: 20 million light years
Release date: 23 March 2020, 06:00
Credit: ESA/Hubble & NASA, I. Karachentsev