The Milky Way Galaxy over Bosque Alegre Station in Argentina

Bosque Alegre Station, Argentina

September 2015

Image Credit & Copyright: Sebastián D' Alessandro

Spiral Galaxy NGC 4394

Discovered in 1784 by the German–British astronomer William Herschel, NGC 4394 is a barred spiral galaxy situated about 55 million light-years from Earth. The galaxy lies in the constellation of Coma Berenices (Berenice's Hair), and is considered to be a member of the Virgo Cluster.

NGC 4394 is the archetypal barred spiral galaxy, with bright spiral arms emerging from the ends of a bar that cuts through the galaxy’s central bulge. These arms are peppered with young blue stars, dark filaments of cosmic dust, and bright, fuzzy regions of active star formation. At the centre of NGC 4394 lies a region of ionised gas known as a LINER. LINERs are active regions that display a characteristic set of emission lines in their spectra— mostly weakly ionised atoms of oxygen, nitrogen and sulphur.

Although LINER galaxies are relatively common, it’s still unclear where the energy comes from to ionise the gas. In most cases it is thought to be the influence of a black hole at the centre of the galaxy, but it could also be the result of a high level of star formation. In the case of NGC 4394, it is likely that gravitational interaction with a nearby neighbour has caused gas to flow into the galaxy’s central region, providing a new reservoir of material to fuel the black hole or to make new stars.

Image Credit: ESA/Hubble, NASA, Judy Schmidt
Explanation from: https://www.spacetelescope.org/images/potw1618a/

Globular Cluster NGC 339

This NASA/ESA Hubble Space Telescope image reveals the simple beauty of NGC 339, a massive intermediate age star cluster in the southern constellation of Tucana (The Toucan).

NGC 339 is part of the Small Magellanic Cloud (SMC), a dwarf galaxy that lies around 200 000 light-years away from us. Along with our own galaxy, the Milky Way, the SMC is one of a collection of neighbouring galaxies known as the Local Group.

By measuring the brightnesses and colours of the stars of NGC 339, astronomers were able to estimate the overall age of the cluster — a method that places NGC 339 at around 6.5 billion years old. This makes it only half the age of the more common globular clusters. The relationship between massive intermediate age star clusters, such as NGC 339, and the true globular clusters are not fully understood yet. So far none of these type of clusters has been found in the Milky Way.

In this very detailed image, it is also possible to see a number of galaxies. They appear as fuzzy, extended blobs, contrasting with the sharp stars that make up NGC 339. Most obvious here are two elliptical galaxies, one towards the top left of the image and another in the centre right. These galaxies are not associated with NGC 339 but lie far in the background, across the vast expanse of the cosmos.

Image Credit: ESA/Hubble, NASA, Judy Schmidt
Explanation from: https://www.spacetelescope.org/images/potw1617a/