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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 new-found star clusters contain hundreds of thousands of stars, a similar number to globular clusters. The clusters also share other characteristics with globular clusters, e.g. the stellar populations and metallicity. What distinguishes them from the globular clusters is that they are much larger – several hundred light-years across ...
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. [17] [18]
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.
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).
These pulsating horizontal-branch stars are known as RR Lyrae variable stars and they are obviously variable in brightness with periods of up to 1.2 days. [10] It requires an extended observing program to establish the star's true (that is, averaged over a full period) apparent magnitude and color. Such a program is usually beyond the scope of ...
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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.