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Object description
Messier 15 - a globular cluster in the constellation Pegasus

Object description:

Messier 15 (NGC 7078) is a spectacular globular cluster in the constellation Pegasus. Its distance to the solar system is nearly 34 000 light-years. With an apparent diameter of 18 arcminutes this results in a true diameter of about 175 lightyears. According to Shapley/Sawyer it is assigned to concentration class IV. It is one of the densest globular clusters in our galaxy with very hot blue stars in the core region and cooler orange stars in the outer regions.

The inner core region of Messier 15 is very small, only about 1.5 light-years across. Half of the cluster's mass is concentrated within the central 10 light-years. About 30,000 stars have been found in the inner 22 light-years of the cluster alone. The total number of stars in the tidal radius of about 210 light-years across is estimated at 500,000. A medium-gravity black hole of about 4,000 solar masses is thought to be at the center.

The cluster contains the considerable number of 112 variable stars, 8 pulsars - neutron stars, remnants of supernova explosions which took place when the cluster and the universe were still young. It also contains a double neutron star and the planetary nebula Planetarischen Nebel Pease 1.

M15 was the first globular cluster in which a planetary nebula was discovered. This nebula - Pease 1 - was discovered in 1928 by Francis G. Pease and is one of only 5 planetary nebulae within globular clusters. Pease 1 has an apparent diameter of only 3 arcseconds and is of course not visible in our image scale. An image can be found here.

Messier 15 was discovered on September 7, 1746 by Italian astronomer Jean-Dominique Maraldi while searching for comets. He described it as a

"rather bright nebulous spot composed of many stars." Charles Messier included the object in his catalog on June 3, 1764.

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