Object description:
IC 4628 is a large
H-II emission nebula in the constellation Scorpio and is part of the large SCO
OB 1 stellar association. It is also popularly referred to as the "Shrimp
Nebula". The nebula surrounds the very loose open star cluster Collinder 316
and from its stars it also receives much of the excitation energy that causes
the nebular masses to glow red. IC 4628 contains blue reflection nebula
components in addition to H-II emission. The stars of Collinder 316 are only a
few million years old. The nebular region has a distance of about 5700
light-years to the solar system and covers a size of about 250
light-years.
IC 4628 is the northeastern part of a much larger circular
H-II region, which was cataloged as BBW 31100 by a group of South African
astronomers (B. Bok, M. Bester, and C. Wade) in the 1950s. Independently, the
Australian astronomer Colin Gum published a catalog of H-II regions of the
southern sky in 1951, and in it the region is designated Gum 55. As part of Gum
55, IC 4628 is listed as Gum 56. Another catalog designation is RCW 113. The
apparent size of GUM 55 is about 5 degrees in diameter.
In the center of
Gum 55 is the bright open star cluster NGC 6231,
which appears relatively inconspicuous at first glance. The age of the stars in
NGC 6231 is estimated to be only a few million years. The number of hot blue
stars of spectral class O is given as 15 to 20, whose strong UV radiation
excites the whole region to emission. Violent stellar winds are apparently
responsible for the ring-like structure of Gum 55, possibly there has been at
least one supernova explosion in the vicinity of NGC 6231. The discovery of IC
4628 is based on photographic observations by Edward Barnard.
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