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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.
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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." « Click here or the thumbnail to load a large annoted image and a size comparison to the full moon. |
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