Credit: ESA/Hubble & NASA
Globular clusters are relatively common in our sky, and generally
look similar. However, this image, taken using the NASA/ESA Hubble
Space Telescope, shows a unique example of such a cluster — Palomar 2.
Palomar 2 is part of a group of 15 globulars known as the Palomar
clusters. These clusters, as the name suggests, were discovered in
survey plates from the first Palomar Observatory Sky Survey in the
1950s, a project that involved some of the most well-known astronomers
of the day, including Edwin Hubble. They were discovered quite late
because they are so faint — each is either extremely remote, very
heavily hidden behind blankets of dust, or has a very small number of
remaining stars.
This particular cluster is unique in more than one way. For one, it
is the only globular cluster that we see in this part of the sky, the
northern constellation of Auriga (The Charioteer). Globular clusters
orbit the centre of a galaxy like the Milky Way in the same way that
satellites circle around the Earth. This means that they normally lie
closer in to the galactic centre than we do, and so we almost always
see them in the same region of the sky. Palomar 2 is an exception to
this, as it is around five times further away from the centre of the
Milky Way than other clusters. It also lies in the opposite direction —
further out than Earth — and so it is classed as an “outer halo”
globular.
It is also unusual due to its brightness. The cluster is veiled by a
mask of dust, dampening the apparent brightness of the stars within it
and making it appear as a very faint burst of stars. The stunning
NASA/ESA Hubble Space Telescope image above shows Palomar 2 in a way
that could not be captured from smaller or ground-based telescopes —
some amateur astronomers with large telescopes attempt to observe all
of the obscure and well-hidden Palomar 15 as a challenge, to see how
many they can pick out from the starry sky.
Source: NASA/ESA Hubble Space Telescope
