Explanation:
The fifty-fifth entry in Charles
Messier's catalog,
M55 is
a large and lovely
globular cluster
of around 100,000 stars.
Only 20,000 light-years away in the constellation
Sagittarius,
M55 appears to earth-bound observers to be nearly 2/3 the size
of the full moon.
Globular star clusters like M55
roam the halo of our Milky Way Galaxy as gravitationally bound populations of stars
known to be much older than stellar groups found in the Galactic disk.
Astronomers who make
detailed studies
of globular cluster stars
can accurately measure the cluster ages and distances.
Their results ultimately constrain the age
of the
Universe (... it must be older than the stars in it!),
and provide a fundamental rung on the
astronomical
distance ladder.
This stunning color image was made with the 3.6 meter
CFHT telescope and
spans about 100 light-years across the globular cluster M55.
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