The oldest
83. 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.
Credit: ESA/Hubble & NASA
Barred
84. spiral galaxy
Messier 83
This new Hubble image shows the scatterings of bright stars and
thick dust that make up spiral galaxy Messier 83, otherwise known
as the Southern Pinwheel Galaxy. One of the largest and closest
barred spirals to us, this galaxy is dramatic and mysterious; it has
hosted a large number of supernova explosions, and appears to
have a double nucleus lurking at its core.
Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)