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The first known globular cluster, now called M 22, was discovered in 1665 by Abraham Ihle, a German amateur astronomer. [4] [5] [6] The cluster Omega Centauri, easily visible in the southern sky with the naked eye, was known to ancient astronomers like Ptolemy as a star, but was reclassified as a nebula by Edmond Halley in 1677, [7] then finally as a globular cluster in the early 19th century ...
These are globular clusters within the halo of the Milky Way galaxy. The diameter is in minutes of arc as seen from Earth. For reference, the J2000 epoch celestial coordinates of the Galactic Center are right ascension 17 h 45 m 40.04 s, declination −29° 00′ 28.1″.
Messier 3 is the prototype for the Oosterhoff type I cluster, which is considered "metal-rich". That is, for a globular cluster, Messier 3 has a relatively high abundance of heavier elements. [15] Arcturus and Cor Caroli can be used to help locate M3
Messier 15 or M15 (also designated NGC 7078 and sometimes known as the Great Pegasus Cluster) is a globular cluster in the constellation Pegasus. It was discovered by Jean-Dominique Maraldi in 1746 and included in Charles Messier 's catalogue of comet -like objects in 1764.
Messier 22 or M22, also known as NGC 6656 or the Great Sagittarius Cluster, is an elliptical globular cluster of stars in the constellation Sagittarius, near the Galactic bulge region. It is one of the brightest globulars visible in the night sky.
Messier 28 or M28, also known as NGC 6626, is a globular cluster of stars in the center-west of Sagittarius.It was discovered by French astronomer Charles Messier in 1764. [11] [a] He briefly described it as a "nebula containing no star... round, seen with difficulty in 3 1 ⁄ 2-foot telescope; Diam 2′."
The cluster is located 4.5° WSW of Theta Ophiuchi and is just visible as a fuzzy point of light using 50 mm (2.0 in) binoculars. Using a telescope with a 25.4 cm (10.0 in) aperture, the cluster shows an oval appearance with a 3 ′ × 4 ′ core and a 5 ′ × 7 ′ halo. [6] M19 is one of the most oblate of the known globular clusters. [7]
At 175 light-years in diameter, it is one of the larger globular clusters known. The cluster is rich, compact, and significantly elliptical. It is 12.5 billion years old and one of the older globular clusters associated with the Milky Way galaxy. [7] M2 contains about 150,000 stars, including 21 known variable stars.