Sky & Telescope - May 2024 - 75

M13: BOB FERA / ERIC COLES
The Southern Hemisphere boasts
two of the finest globular clusters in
the Milky Way: Omega Centauri (the
galaxy's biggest) and 47 Tucanae. One
of the best in Northern Hemisphere
skies is the Great Globular Cluster in
Hercules. Also known as M13, it has a
diameter of about 150 light-years and
lies some 25,000 light-years from Earth.
Globulars' density makes them
GLOBULAR CLUSTER M13, the Great Globular Cluster in Hercules,
contains more than 100,000 stars. It is readily visible at this
time of year with small scopes, binoculars, or even the naked eye.
harder to study than open clusters,
whose individual stars are more easily
observable. But these extremely
ancient clusters can help astronomers
understand the conditions
under which their host galaxy
came together in the early universe
(S&T: July 2021, p. 14).
All told, the Milky Way contains
about 150 globular clusters.
Other galaxies have many more.
M87, for instance, a supergiant elliptiGlobular
Clusters
Globular clusters are roughly spherical
knots of hundreds of thousands to
millions of stars. These stellar metropolises
can be hundreds of light-years
across and are thus much larger than
open clusters. The stars in them are
very tightly bound together by gravity,
resulting in many globulars looking like
heaps of spilled sugar.
Globular clusters generally orbit the
center of the Milky Way in our galaxy's
halo, the outer regions above and below
the galactic plane. This is why observers
in the Northern Hemisphere see more
globulars on summer nights, when we
face toward the galactic core, than in
winter, when we look in the opposite
GLOBULAR CLUSTERS
Concentrations
vary: Near right: NGC
5053 in Coma Berenices
is very loose toward
the center. Middle right:
M53, also in Coma
Berenices, is denser
at the core. Far right:
M75 in Sagittarius has a
super-dense heart.
direction. Because of their location in
the less crowded halo, globular clusters
are extremely stable and can survive
intact for billions of years. Those in the
Milky Way formed as early as a few billion
years after the Big Bang.
These super-dense agglomerations
comprise mostly ancient yellow and red
stars with masses less than two times
that of our Sun. The larger stars have
long since exploded as supernovae or
ended their lives as white dwarfs (dense,
planet-size former stars). Globulars
might hold a few rare blue stars as
well. Known as blue stragglers, they are
thought to have arisen through stellar
mergers or by one star in a binary
siphoning off gas from its neighbor.
cal galaxy in Virgo, the Maiden, has
more than 1,000 globulars.
Observing Star Clusters
Star clusters are favorite targets for
telescopes and binoculars because many
are bright, easy to locate, and visually
appealing. Some are even visible to the
naked eye, though optics will help bring
out breathtaking detail.
Open clusters range pleasingly in
breadth, density, and brightness, while
globular clusters dazzle with their
extreme compactness. As Contributing
Editor Bob King has written of globulars,
" These stellar treasure-troves rate
right up there with Saturn and Jupiter
when it comes to making an impression
on first-time stargazers. " ¢
sk yand tele scope .o r g * MAY 2024 75
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Sky & Telescope - May 2024

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