The oldest cluster in its cloud: This image shows NGC 121, a globular cluster in the constellation of Tucana (The Toucan). Globular clusters are big balls of old stars that orbit the centres of their galaxies like satellites — the Milky Way, for example, has around 150. NGC 121 belongs to one of our neighbouring galaxies, the Small Magellanic Cloud (SMC). It was discovered in 1835 by English astronomer John Herschel, and in recent years it has been studied in detail by astronomers wishing to learn more about how stars form and evolve. Stars do not live forever — they develop differently depending on their original mass. In many clusters, all the stars seem to have formed at the same time, although in others we see distinct populations of stars that are different ages. By studying old stellar populations in globular clusters, astronomers can effectively use them as tracers for the stellar population of their host galaxies. With an object like NGC 121, which lies close to the Milky Way, Hubble is able to resolve individual stars and get a very detailed insight. NGC 121 is around 10 billion years old, making it the oldest cluster in its galaxy; all of the SMC's other globular clusters are 8 billion years old or younger. However, NGC 121 is still several billions of years younger than its counterparts in the Milky Way and in other nearby galaxies like the Large Magellanic Cloud. The reason for this age gap is not completely clear, but it could indicate that cluster formation was initially delayed for some reason in the SMC, or that NGC 121 is the sole survivor of an older group of star clusters. This image was taken using Hubble’s Advanced Camera for Surveys (ACS). A version of this image was submitted to the Hubble’s Hidden Treasures image processing competition by contestant Stefano Campani.

Name: NGC 121
Type: Milky Way : Star : Grouping : Cluster : Globular
Category: Stars
Constellation: Tucana
Distance: 200000 light years
Release date: 14 July 2014, 10:00
Credit: ESA/Hubble & NASA Acknowlegement: Stefano Campani

A slice of stars: The thin, glowing streak slicing across this image cuts a lonely figure, with only a few foreground stars and galaxies in the distant background for company. However, this is all a case of perspective; lying out of frame is another nearby spiral. Together, these two galaxies make up a pair, moving through space together and keeping one another company. The subject of this Hubble image is called NGC 3501, with NGC 3507 as its out-of-frame companion. The two galaxies look very different — another example of the importance of perspective. NGC 3501 appears edge-on, giving it an elongated and very narrow appearance. Its partner, however, looks very different indeed, appearing face-on and giving us a fantastic view of its barred swirling arms. While similar arms may not be visible in this image of NGC 3501, this galaxy is also a spiral — although it is somewhat different from its companion. While NGC 3507 has bars cutting through its centre, NGC 3501 does not. Instead, its loosely wound spiral arms all originate from its centre. The bright gas and stars that make up these arms can be seen here glowing brightly, mottled by the dark dust lanes that trace across the galaxy. A version of this image was entered into the Hubble's Hidden Treasures image processing competition by contestant Nick Rose.

Name: NGC 3501
Type: Local Universe : Galaxy : Type : Spiral
Category: Galaxies
Constellation: Leo
Distance: 80 million light years
Release date: 21 July 2014, 10:00
Credit: ESA/Hubble & NASA Acknowledgement: Nick Rose

Galaxy gets a cosmic hair ruffling: From objects as small as Newton's apple to those as large as a galaxy, no physical body is free from the stern bonds of gravity, as evidenced in this stunning picture captured by the Wide Field Camera 3 and Advanced Camera for Surveys onboard the NASA/ESA Hubble Space Telescope. Here we see two spiral galaxies engaged in a cosmic tug-of-war — but in this contest, there will be no winner. The structures of both objects are slowly distorted to resemble new forms, and in some cases, merge together to form new, super galaxies. This particular fate is similar to that of the Milky Way Galaxy, when it will ultimately merge with our closest galactic partner, the Andromeda Galaxy. There is no need to panic however, as this process takes several hundreds of millions of years. Not all interacting galaxies result in mergers though. The merger is dependent on the mass of each galaxy, as well as the relative velocities of each body. It is quite possible that the event pictured here, romantically named 2MASX J06094582-2140234, will avoid a merger event altogether, and will merely distort the arms of each spiral without colliding — the cosmic equivalent of a hair ruffling! These galactic interactions also trigger new regions of star formation in the galaxies involved, causing them to be extremely luminous in the infrared part of the spectrum. For this reason, these types of galaxies are referred to as LIRGs, or Luminous Infrared Galaxies. This image was taken as part of as part of a Hubble survey of the central regions of LIRGs in the local Universe, which also used the NICMOS instrument. A version of this image was entered into the Hubble's Hidden Treasures image processing competition by contestant Luca Limatola.

Name: 2MASX J06094582-2140234
Type: Local Universe : Galaxy : Activity : Ultraluminous
Category: Galaxies
Constellation: Lepus
Release date: 28 July 2014, 10:00
Credit: ESA/Hubble & NASA Acknowledgement: Luca Limatola

IC 4499: A globular cluster’s age revisited: This new NASA/ESA Hubble Space Telescope image shows the globular cluster IC 4499. Globular clusters are big balls of old stars that orbit around their host galaxy.  It has long been believed that all the stars within a globular cluster form at the about same time, a property which can be used to determine the cluster's age. For more massive globulars however, detailed observations have shown that this is not entirely true — there is evidence that they instead consist of multiple populations of stars born at different times. One of the driving forces behind this behaviour is thought to be gravity: more massive globulars manage to grab more gas and dust, which can then be transformed into new stars. IC 4499 is a somewhat special case. Its mass lies somewhere between low-mass globulars, which show a single generation build-up, and the more complex and massive globulars which can contain more than one generation of stars. By studying objects like IC 4499 astronomers can therefore explore how mass affects a cluster's contents. Astronomers found no sign of multiple generations of stars in IC 4499 — supporting the idea that less massive clusters in general only consist of a single stellar generation. Hubble observations of IC 4499 have also helped to pinpoint the cluster's age: observations of this cluster from the 1990s suggested a puzzlingly young age when compared to other globular clusters within the Milky Way. However, since those first estimates new Hubble data been obtained, and it has been found to be much more likely that IC 4499 is actually roughly the same age as other Milky Way clusters at approximately 12 billion years old.

Name: IC 4499
Type: Milky Way : Star : Grouping : Cluster : Globular
Category: Stars
Constellation: Apus
Distance: 55000 light years
Release date: 4 August 2014, 10:00
Credit: ESA/Hubble & NASA