Globular Clusters, the Ancient Outliers

By Rod Kennedy

When the summer makes the days unbearable, we often seek refuge indoors.  Anyplace from museums to the local library can be a welcome oasis from the heat of the sun.  However, in Wyoming the evenings are often pleasantly cool and comfortable; a great opportunity to get outdoors and enjoy the night sky.  The challenge is to find something worth observing. Fortunately the southern half of the July evening skies offers a wealth of interesting targets for telescopes and binoculars.  Among these gems are the globular clusters and they are some of the oldest structures in our galaxy.

Globular clusters are huge balls of very ancient stars.  There are so many stars that their common gravity pulls the cluster into a spherical shape or globe, hence the name globular cluster.  Globular clusters are not contained within the disc of the Milky Way galaxy, but rather in the halo of around our galaxy.  To most small amateur telescopes, globular clusters appear as fuzzy balls with a few distinct stars around the edges.  Telescopes larger than 8 inches in diameter are needed to “resolve” individual stars within the core of most globular clusters.

In the skies of July a number of globulars are within the range of most telescopes.  The most famous of these is M13 in Hercules.  M13 is a great target because it is almost directly overhead after sunset.  Under very dark skies, M13 is just barely visible to the naked eye.  M13 is 145 light years in diameter and is an astonishing 25,000 light years from Earth.

Another easy to spot globular is just 1 degree west of the star Antares in Scorpius.  This is M4 and both it and Antares are visible in a wide field eyepiece.  M4 is the first globular cluster in which individual stars were resolved.  In contrast to M13, M4 is only 75 light years across.

One of the closest globulars to Earth is M22 just East of the star Kaus Borealis (the top star in the lid of the Teapot Asterism) in Sagittarius.  M22 was one of the first globulars to be closely studied by Harlow Shapley in the 1930’s.  He discovered that M22 has about 70,000  stars.  M22 is only about 10,600 light years from Earth.  M22 is also interesting because it is one of only a few globulars that contains a planetary nebula.  A planetary nebula is the left over remains of a star roughly the mass of our sun.  When these stars die their outer layers are puffed off into space.  A planetary nebula in M22 tells us that there was once at least one star very much like our sun in mass and temperature.  M22 also appears to contain 2 black holes, detected with the Very Large Array in New Mexico and also with Chandra X-ray telescope in Earth orbit.

The constellation of Sagittarius is a treasure trove for hunting globular clusters.  Since globulars reside in the halo around the bulge of our galaxy they are more densely concentrated in that direction.  When we look toward the constellation Sagittarius we are looking toward the center of our Milky Way, therefore we find a great variety of globulars.  There are so many that an observer could spend many nights observing and not see them all.  What better way to spend a warm summer evening recovering from the day’s heat?

July2013 Chart

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