M4

Credits: Keith Turnecliff, Nerja, Spain

M4, (also designated NGC 6121) located in the constellation Scorpius, is a huge, spherical collection of stars known as a globular cluster. Just 5,500 light-years away, it is the closest globular cluster to Earth. Because of its apparent magnitude of 5.9 and proximity to Antares, one of the brightest stars in the night sky, M4 is relatively easy to find with a small telescope. The cluster is best spotted in July.
M4 was discovered in 1746 by the Swiss astronomer Jean-Philippe Loys de Chéseaux. Home to more than 100,000 stars, the cluster is predicted to contain up to 40,000 white dwarfs — the cores of ancient, dead stars whose outer layers have drifted away into space. As white dwarfs age, they grow cooler, fainter, and more difficult to detect. Therefore, a globular cluster’s age can be inferred from the age of its faintest white dwarf. Because the stars in these clusters are some of the oldest in the universe, up to 13 billion years old, astronomers are able to use them to estimate the age of the universe.
It was the first globular cluster in which individual stars were resolved.

Facts about M4 by Keith Turnecliff

M4 is conspicuous in even the smallest of telescopes as a fuzzy ball of light. It appears about the same size as the Moon in the sky. It is one of the easiest globular clusters to find, being located only 1.3 degrees west of the bright star Antares, with both objects being visible in a wide-field telescope. Modestly sized telescopes will begin to resolve individual stars, of which the brightest in M4 are of apparent magnitude 10.8.