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Object description
Messier 28 - a globular cluster in the constellation Sagittarius

Object description:

Messier 28 (NGC 6626) is much smaller than the neighboring globular cluster Messier 22, whose image we show here. Its distance to the solar system is about 18 000 light-years. With an apparent diameter of 14 arcminutes, this gives a true diameter of about 70 light-years. According to Shapley/Sawyer it is assigned to concentration class IV. Messier 28 has a total mass of nearly 600 000 solar masses, it contains at least 50 000 stars.

Messier 28 is the first globular cluster in which a millisecond pulsar, designated PSR B1821-24, was discovered. The neutron star has a rotation period of about 3 milliseconds and was discovered in 1986 with the Lovell radio telescope in England. In later years, astronomers discovered 11 other millisecond pulsars in the same area, making M 28 a globular cluster with the third largest population of pulsars. Only in Terzan 5 (also in the constellation Sagittarius) and 47 Tucanae (see a picture of us here) contain an even larger number of pulsars.

The cluster also contains 18 RR Lyrae stars - periodically variable stars that can be used to measure galactic distances. These stars are commonly found in globular clusters. Other "exotic" stars in Messier 28 include an RV-Tauri variable and a W-Virginis variable with a pulsation period of 17 days. W-Virginis variables are old, low-mass, low-metal Population II stars that exhibit variations in brightness because instabilities cause their diameters to change periodically. W-Virginis stars are a subclass of Type II Cepheids and can be used to determine distances to celestial objects.

The globular cluster was discovered by the French astronomer Charles Messier on July 27, 1764. He described it as a nebula, only Friedrich Wilhelm Herschel was able to resolve the cluster into single stars more than three decades later. On August 1, 1799, he wrote in his observation diary:

"It may be called isolated, although it lies in a part of the sky very rich in stars. It may have a nucleus, for it is much compressed towards the center, and the position is too low to see it well. The stars of the cluster are quite numerous."

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