Globular clusters are roughly spherical collections of extremely old
stars, and around 150 of them are scattered around our galaxy. Hubble
is one of the best telescopes for studying these, as its extremely high
resolution lets astronomers see individual stars, even in the crowded
core. The clusters all look very similar, and in Hubble’s images it can be quite hard to tell them apart – and they all look much like NGC 411, pictured here.
And yet appearances can be deceptive: NGC 411 is in fact not a
globular cluster, and its stars are not old. It isn’t even in the Milky
Way.
NGC 411 is classified as an open cluster. Less tightly bound than a
globular cluster, the stars in open clusters tend to drift apart over
time as they age, whereas globulars have survived for well over 10
billion years of galactic history. NGC 411 is a relative youngster — not
much more than a tenth of this age. Far from being a relic of the
early years of the Universe, the stars in NGC 411 are in fact a
fraction of the age of the Sun.
The stars in NGC 411 are all roughly the same age, having formed in
one go from one cloud of gas. But they are not all the same size.
Hubble’s image shows a wide range of colours and brightnesses in the
cluster’s stars. These tell astronomers many facts about the stars,
including their mass, temperature and evolutionary phase. Blue stars,
for instance, have higher surface temperatures than red ones.
The image is a composite produced from ultraviolet, visible and
infrared observations made by Hubble’s Wide Field Camera 3. This filter
set lets the telescope “see” colours slightly further beyond red and
the violet ends of the spectrum.
