An open cluster, sometimes called a Galactic Cluster, is a group
of 10s or 100s of stars that were born from the same initial cloud of
gas (mainly Hydrogen) and dust. When they are young - a few million
or tens of millions of years old - these clusters contain some very
large, bright stars (called O or B-type stars). The very youngest
clusters (usually less than 10 million years old) often still contain
the remains of the gas cloud from which the stars were born – this
is seen as nebulosity.
Cluster stars are very useful as they were all formed from the
same giant cloud , so they have the same chemistry, and they are all
at about the same distance from us, although they are typically
hundreds or thousands of light years away. By observing a group of
stars in a cluster, we can assume they are all made of the same
stuff, and they are all the same distance away from us – so any
differences between them are really caused by their different mass.
Astronomers measure the intensity of light from the stars in a
cluster through different filters with a process known as photometry
and plot the colors of the stars on a color-magnitude diagram. Once a
measure of how “red” or “blue” the stars are is made, more
information about them can be obtained – massive stars are usually
very blue and hot, intermediate mass stars (like the Sun) are yellow,
and the very lowest mass stars are red and cool.
