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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 ...
Being much less densely populated than globular clusters, they are much less tightly gravitationally bound, and over time, are disrupted by the gravity of giant molecular clouds and other clusters. Close encounters between cluster members can also result in the ejection of stars, a process known as "evaporation".
One hundred and five stars in M5 are known to be variable in brightness, 97 of them belonging to the RR Lyrae type. [11] RR Lyrae stars, sometimes referred to as "Cluster Variables", are somewhat similar to Cepheid type variables and as such can be used as a tool to measure distances in outer space since the relation between their luminosities and periods are well known.
The brightest star in the cluster is a red giant, the variable star V11, also known as V1554 Herculis, [16] with an apparent visual magnitude of 11.95. M13 is 22,200 to 25,000 light-years away from Earth , [ 9 ] and the globular cluster is one of over one hundred that orbit the center of the Milky Way.
Based on the method of comparing the luminosities of globular clusters (located in galactic halos) from distant galaxies to that of the Virgo Cluster, the globular cluster luminosity function carries an uncertainty of distance of about 20% (or 0.4 magnitudes).
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′."
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Located at a distance of 17,090 light-years (5,240 parsecs), it is the largest known globular cluster in the Milky Way at a diameter of roughly 150 light-years. [10] It is estimated to contain approximately 10 million stars, with a total mass of 4 million solar masses , [ 11 ] making it the most massive known globular cluster in the Milky Way.