![]() That means that, even though these clusters are incredibly dense compared to typical interstellar space, the stars are still separated by many AU and are unlikely to ever experience direct collisions. ![]() So, the stars in the cluster only fill ( 7.2 x 10 23 km 3 ) / ( 1.2 x 10 44 km 3 ) = 6.0 x 10 − 21 of the cluster's total volume. The volume of the entire cluster is 4 / 3 π R 3 = 4 / 3 π ( 3.1 x 10 14 km ) 3 = 1.2 x 10 44 km 3. If a typical globular cluster has 500,000 stars in it, those stars fill a volume of 500, 000 x 1.4 x 10 18 km 3 = 7.2 x 10 23 km 3. Assume the radius of a typical globular cluster is 10 parsecs, or 3.1 x 10 14 km. Nasa picture of the day star clusters full#We can actually do a quick calculation to prove to ourselves the answer to the question: How full is a globular cluster?Īssume all stars in the cluster are the size of the Sun, or roughly 700,000 km in radius. On average, the abundance of all elements heavier than helium is only 1-10% of the abundance of these same elements in the Sun and in the stars in open clusters. Again, this observation reveals another difference between these star clusters and open clusters. When we observe the stars in globular clusters spectroscopically, we can also measure the abundance of chemical elements in their atmospheres. There are several globular clusters visible near the Milky Way in the part of the sky you studied using the S tarry Night file on the last page however, there are many more distributed all over the sky. ![]() They do not seem to have any particular association with the band of light that we call the Milky Way. ![]() Also, unlike open clusters, we find globular clusters in every direction on the sky. Globular clusters are not found to contain any gas, nor are they, in general, associated with reflection or emission nebulae, like we see with open clusters. Thus, if the Sun were inside a globular cluster, we would see many thousands of stars in the sky many times brighter than the brightest stars we see in our night sky. Nasa picture of the day star clusters Pc#As part of a research study in 1996 of the globular cluster M15, astronomers using the Hubble Space Telescope observed about 30,000 stars within 6.7 pc of the core of this globular cluster. If you were to draw this same sphere in the center of the globular cluster M13, it would contain approximately 10,000 stars. Thus, if we were able to draw a sphere around the Sun with a radius of 1.3 parsecs, it would only contain 2 stars: the Sun and Proxima Centauri. We have measured the distance to the nearest star to the Sun, Proxima Centauri, and it is 4.2 light-years, or about 1.3 parsecs. The density of stars inside a globular cluster is significantly higher than the density of stars around the Sun. This has led to the legend of the ‘lost Pleiad’, which faded below naked-eye visibility.Source: LEFT: Penn State Astronomy & Astrophysics, RIGHT: Astronomy Picture of the Day It should remain identifiable for at least 250 million years, however, so we need be in no hurry to observe it before it merges into the general background.Īlthough there are only six stars easily distinguishable with the naked eye by people of average sight, the cluster has traditionally been known as the Seven Sisters. Like all galactic clusters it will eventually be dispersed because of the gravitational pull of non-cluster stars. ![]() Yet the cluster will not survive indefinitely. When studies were first made of the proper motions of the stars in the Pleiades, it was found that they were all moving across the sky in the same direction at the same rate. In 1767, the English clergyman John Michell – a brilliant researcher, too often neglected by modern scientific historians – calculated that the probability of a chance alignment of so many bright stars was only about one in half a million. The Pleiades (1885) by the Symbolist painter Elihu Vedder. ![]()
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