Since stars with higher masses evolve more quickly, we can find clusters in which massive stars have already completed their main-sequence phase of evolution and become red giants, while stars of lower mass in the same cluster are still on the main sequence, or even—if the cluster is very young—undergoing pre-main-sequence gravitational contraction. We can see many stages of stellar evolution among the members of a single cluster, and we can see whether our models can explain why the H—R diagrams of clusters of different ages look the way they do.
The three basic types of clusters astronomers have discovered are globular clusters , open clusters , and stellar associations. Their properties are summarized in Table 1. As we will see in the next section of this chapter, globular clusters contain only very old stars, whereas open clusters and associations contain young stars. Globular clusters were given this name because they are nearly symmetrical round systems of, typically, hundreds of thousands of stars.
The most massive globular cluster in our own Galaxy is Omega Centauri , which is about 16, light-years away and contains several million stars Figure 1. Note that the brightest stars in this cluster, which are red giants that have already completed the main-sequence phase of their evolution, are red-orange in color. These stars have typical surface temperatures around K. As we will see, globular clusters are among the oldest parts of our Milky Way Galaxy.
Figure 1. Omega Centauri: a Located at about 16, light-years away, Omega Centauri is the most massive globular cluster in our Galaxy. It contains several million stars. The image is about 6. The most numerous stars in the image, which are yellow-white in color, are main-sequence stars similar to our Sun. The brightest stars are red giants that have begun to exhaust their hydrogen fuel and have expanded to about times the diameter of our Sun. The blue stars have started helium fusion.
What would it be like to live inside a globular cluster? In the dense central regions, the stars would be roughly a million times closer together than in our own neighborhood. If Earth orbited one of the inner stars in a globular cluster, the nearest stars would be light-months, not light-years, away.
They would still appear as points of light, but would be brighter than any of the stars we see in our own sky. The Milky Way would probably be difficult to see through the bright haze of starlight produced by the cluster. About globular clusters are known in our Galaxy. All the globular clusters are very far from the Sun, and some are found at distances of 60, light-years or more from the main disk of the Milky Way.
Those introduced by Bayer were created by the Dutch-Flemish cartographer Petrus Plancius based on observations of Dutch explorers and are named after various animals they encountered on their journeys. The constellations created by Lacaille, on the other hand, represent various scientific instruments. As many of these constellations are invisible to northern observers living north of tropical latitudes, they are still largely unknown in the northern hemisphere.
Smallest Constellations. Triangulum Australe. Corona Australis. Corona Borealis. Canis Minor. Leo Minor. Its area is calculated at about 68 square degrees. Updated February 28, Infoplease Staff. The Answer: The largest of the 88 recognized constellations is Hydra , a long, slender, winding region of stars named after the many-headed sea serpent Hercules encountered in Greek mythology.
See also:. Ask the Editors: The 88 Recognized Constellations. Trending Here are the facts and trivia that people are buzzing about.
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