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Messier 15 - a globular cluster in the constellation
Pegasus |
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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.
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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.
« Click here or the thumbnail to load a large annoted image
and a size comparison to the full moon. |
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All Images and all Content are ©
by Franz Hofmann + Wolfgang Paech |