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.
