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Messier 21
Messier 21 (M21) is an open cluster located in the constellation Sagittarius. The cluster has an apparent magnitude of 6.5 and lies at an approximate distance of 4,250 light years from Earth. Its designation in the New General Catalogue is NGC 6531.
Messier 21 is only 4.6 million years old, which makes it relatively young for an open star cluster. It is only 10 percent of the age of Messier 45, the famous Pleiades cluster in Taurus, and 1 percent the age of the Sun.
M21 consists mainly of small, faint stars, but is also home to a few blue giants.
The cluster contains about 57 confirmed members, but is pretty densely packed. It is classified as of Trumpler class I 3 r, which means that is a detached cluster with strong central concentation (I), consists of both bright and faint stars (3), and contains more than 100 stars (r).
The stars of M21 are believed to be members of the Sagittarius OB1 Association, a group of massive O and B-type stars that share a common motion, age and origin. The eight of the 10 brightest stars in the cluster are spectroscopic binaries with periods shorter than 6 days.
Messier 21 contains about 35 stars with a visual magnitude between 8 and 12. Many of these stars can easily be seen in a small telescope. 6-inch or larger telescopes will show many more stars tightly packed within an area 13 arc minutes in apparent size.
The open cluster lies just beyond naked eye visibility, but can easily be found even in the smallest binoculars. It is located 2.5 degrees northwest of Messier 8 (the Lagoon Nebula) and only 0.75 degrees to the northeast of Messier 20 (the Trifid Nebula). The cluster can be found using the Teapot asterism, formed by the brightest stars in Sagittarius, to first find either of the two famous nebulae and then use them to locate M21.
The best time of year to observe M21 is in the months of June, July and August.
Messier 21
Messier 21 (M21) is an open cluster located in the constellation Sagittarius. The cluster has an apparent magnitude of 6.5 and lies at an approximate distance of 4,250 light years from Earth. Its designation in the New General Catalogue is NGC 6531.
Messier 21 is only 4.6 million years old, which makes it relatively young for an open star cluster. It is only 10 percent of the age of Messier 45, the famous Pleiades cluster in Taurus, and 1 percent the age of the Sun.
M21 consists mainly of small, faint stars, but is also home to a few blue giants.
The cluster contains about 57 confirmed members, but is pretty densely packed. It is classified as of Trumpler class I 3 r, which means that is a detached cluster with strong central concentation (I), consists of both bright and faint stars (3), and contains more than 100 stars (r).
The stars of M21 are believed to be members of the Sagittarius OB1 Association, a group of massive O and B-type stars that share a common motion, age and origin. The eight of the 10 brightest stars in the cluster are spectroscopic binaries with periods shorter than 6 days.
Messier 21 contains about 35 stars with a visual magnitude between 8 and 12. Many of these stars can easily be seen in a small telescope. 6-inch or larger telescopes will show many more stars tightly packed within an area 13 arc minutes in apparent size.
The open cluster lies just beyond naked eye visibility, but can easily be found even in the smallest binoculars. It is located 2.5 degrees northwest of Messier 8 (the Lagoon Nebula) and only 0.75 degrees to the northeast of Messier 20 (the Trifid Nebula). The cluster can be found using the Teapot asterism, formed by the brightest stars in Sagittarius, to first find either of the two famous nebulae and then use them to locate M21.
The best time of year to observe M21 is in the months of June, July and August.