M4, 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.
The white dwarfs in M4 are less than one-billionth the apparent brightness of the faintest stars that can be seen with the naked eye. Even the brightest of the detected white dwarfs are no more luminous than a 100-watt light bulb seen at the moon’s distance. The faintest are comparable to a 2.5-watt night-light at the same distance.
Radius: 34.997 light years
Declination: −26° 31′ 32.7″
Right ascension: 16h 23m 35.22s
Metallicity: = −1.07 dex
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