{"id":14489,"date":"2026-05-01T09:37:27","date_gmt":"2026-05-01T13:37:27","guid":{"rendered":"https:\/\/www.fas37.org\/wp\/?p=14489"},"modified":"2026-05-01T09:37:27","modified_gmt":"2026-05-01T13:37:27","slug":"globular-clusters","status":"publish","type":"post","link":"https:\/\/www.fas37.org\/wp\/globular-clusters\/","title":{"rendered":"Globular Clusters"},"content":{"rendered":"\n

FAS Astronomers Blog, Volume 34, Number 4.<\/strong><\/p>\n\n\n\n

An Introduction to Globular Clusters<\/strong><\/p>\n\n\n\n

Globular Clusters (aka Globulars) are some of the more interesting objects viewed by amateur astronomers, although they do tend to the look the same. There are more than 150 of these visible in the Milky Way galaxy. The Andromeda galaxy does us one better with around 300. Galaxy M87 might have as many as 10,000 or more.\u00a0<\/p>\n\n\n\n

The first globular cluster was identified by Abraham Ihle back in 1665. One of the more well known, the great Hercules globular cluster, was initially classified by Edmond Halley of the comet fame, as a nebula in 1714. Later, William Herschel correctly reclassified it as a globular cluster. Charles Messier might have been the first to identify individual stars in a globular cluster in 1764 when he observed M4. The term globular cluster was first introduced by William Herschel in his 1789 Catalogue of a Second Thousand New Nebulae and Clusters of Stars<\/em>. Beginning in 1914, Harlow Shapely mapped the rough distribution and distance to globular clusters in the halo of the Milky Way. Later, in 1918, Shapley used globular clusters to first estimate the shape of the Milky Way when he positioned the Earth off to the side rather than at the galactic center. (This paragraph was adapted from Wikipedia<\/a>.) <\/p>\n\n\n\n

Globular clusters are densely packed spheres of stars generally found in the halo of the Milky Way. They contain thousands to millions of stars and range in size up to a few hundred light years across. This compares to Open Star Clusters, which are found in the Milky Way\u2019s disk, and are much smaller, more irregular, and contain much younger stars. The density near the center of a globular cluster can range from 100 to 1,000 stars per cubic parsec. A parsec is 3.26 light-years. As a comparison, there are only four stars (the three stars of the Alpha Centauri system and our Sun) in the 4 \u00bc light-year sphere around the Sun. <\/p>\n\n\n\n

One theory is that globular clusters developed during the early stage of galactic formation when a galaxy was mostly spherical and contained older \u201cnon-metallic\u201d stars. Over time, the galaxy flattened out into a spiral while the globular clusters remained in a central sphere, which became the halo. Globular clusters could also be small ancient galaxies that were absorbed by larger galaxies. In July 2024, astronomers found evidence of an intermediate sized black hole at the center of Omega Centauri. There also may be black holes within M22.<\/p>\n\n\n\n

Globular clusters, although spherical, can have a slightly different core density. In the 1920s, Harlow Shapely and Helen Sawyer developed a classification for this ranging from most concentrated (I) to least concentrated (XII). <\/p>\n\n\n\n

Globulars are mostly collections of older (Population II) stars, which lack the \u201cmetals\u201d found in younger stars like the Sun. Some date back close to the formation of the Milky Way around 12 to 13 billion years ago. Although, some have been found to contain younger\/newer stars indicating that some limited stellar formation might still be going on. <\/p>\n\n\n\n

Astronomers have also found \u201cblue stragglers\u201d within globular clusters. These are large stars probably resulting from stellar collisions or from binary stars where a star syphons off material from its partner. Older stars leave the so-called Herzsprung-Russell main sequence as they become larger, but redder (dimmer). Blue stragglers, however, tend to be bigger and bluer (brighter) than other stars of the same age and \u201cstagger\u201d behind the shift to a more reddish color. <\/p>\n\n\n\n

Globular Clusters in the Night Sky<\/strong><\/p>\n\n\n\n

Of the 150 or so visible globular clusters, there are several that are prime targets for amateurs. Most are classified as Messier objects (those categorized by Charles Messier back in the eighteenth century) or Caldwell objects (the 109 brightest objects not included in Messier\u2019s list). <\/p>\n\n\n\n

The four most well-known are probably these below. All are relatively bright and have angular dimensions close to that of a full Moon (at \u00bd a degree or 30\u2018).<\/p>\n\n\n\n