![]() This picture shows what the precession of a single star might look like: The Kepler orbits undergo precession because of the influence of the other stars (there is precession in our solar system, but is more pronounced in a globular cluster). These motions cannot be predicted by the simple methods I explained in part 2, but astronomers have found an alternative, models that provide a rough guide to the motions.Ī typical model goes something like this: all stars in the cluster move in Kepler orbits around the center of mass. If a million stars reside inside of a gravitational field, those stars must move. The globular cluster generates a gravitational field. We now know of approximately 150 globular clusters in our galaxy and have found globular clusters in other galaxies (including G1 in the andromeda galaxy). (I won’t say anything more about open clusters). Globular clusters have between 50,000 and a million stars open clusters have less than 50,000 stars. William Herschel divided star clusters into two subcategories, open clusters and globular clusters. They identified many star clusters (including some previously believed to be nebulae including Omega Centauri, 47 Tucanae and M13). That’s when William Herschel, his sister Caroline, son John and other family members started a systematic survey. However most clusters would remain undiscovered until the 1780s. In the year 1751, Abbe Nicholas Louis de Lacaille looked at one of these stars (47 Tucanae)-it was too fuzzy to be a star as well.Ī number of other astronomers, including Pierre Méchain, Barnabus Oriani and Charles Messier, found new nebulae and/or were able to resolve nebulae into clusters. The stars in the constellation Tucana also seemed ordinary. At least not until the year 1677, when Edmond Halley looked at the star named Omega Centauri with a telescope-the image was too fuzzy to be a star. There are dozens of stars in the constellation Centaurus, none of which seemed unusual. He was only partially correct: as telescopes improved more nebulae were discovered many nebulae were examined and eventually resolved into star clusters, but other nebulae never resolved into clusters. Galileo was convinced that all nebulae would eventually turn out to be star clusters. ![]() ![]() He saw it was a group or “star cluster” of 40 stars. In the early 1600’s Galileo looked at this object with a telescope. One of these nebulae was Praesepe (also called the Beehive). Nebulae (or the singular nebula) referred to fuzzy spots in the night sky-no one knew what they were. Objects like stars, planets, comets and nebulae. Gravity, Part 4: Globular Clusters & Galaxies.īefore the telescope was invented, astronomers were limited to objects ![]()
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