The M4 globular cluster

The M4 globular cluster

We continue our marathon of the catalog of Charles Messier. Today we are talking about another globular cluster, M4.

The M4 globular cluster is located in the constellation Scorpio, very close to the star Antares. It is the globular cluster closest to us, at an estimated distance of 7,200 light years. It has a diameter of 95 light years and contains over 100,000 stars.

The M4 globular cluster
The position of M4 in the constellation of Scorpio.

To observe it, a pair of binoculars or a small telescope is sufficient, even if with the increase of the power of the instrument it will be possible to see resolved the stars that compose it. To find it, just look at just over from the star Antares. Under really dark skies, although with some difficulty, it is also visible to the naked eye.

The cluster was discovered by De Chéseaux in 1746, while a few years later, in 1764, it was observed by Charles Messier who noted:

Cluster of tiny stars; with a smaller telescope it looks more like a nebula; this cluster is located near Antares and on the same parallel. Observed by M. de La Caille and reported in his catalog … diam. 2½.

The M4 globular cluster was the first cluster to be solved in its component stars, ie it was possible to observe it in great detail and not as a blurred group of stars. More than half of its stars are concentrated in just 8 light years from the center, and magnitude and color of over 660 stars have been identified. Furthermore, 43 variable stars have been cataloged.

Now, some curiosities. The M4 globular cluster orbits the center of the Milky Way along an orbit inclined at 23° to the disk, taking about 116 million years to complete a complete revolution. Inside M4 a pulsar with a rotation period of just 3 milliseconds, about 10 times faster than the Crab pulsar, was discovered in 1987. Inside the cluster, the Hubble Space Telescope has also taken a number of white dwarfs, with an estimated age of 13 billion years, among the oldest stars of the Milky Way. Astronomers have also discovered a very exotic solar system, consisting of a pulsar, PSR B1620-26, and a white dwarf. Inside there is a planet, among the oldest that is known, having an estimated age of 13 billion years.

The M4 globular cluster
M4’s photography taken from the Hubble space telescope.

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