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We have discovered a large amount of information about stars, about their birth, life and their death.. Star, large celestial body composed of gravitationally contained hot gases emitting electromagnetic radiation, especially light, as a result of nuclear reactions inside the star. The Sun is a star. With the sole exception of the Sun, the stars appear to be fixed, maintaining the same pattern in the skies year after year. In fact the stars are in rapid motion, but their distances are so great that their relative changes in position become apparent only over the centuries.

Globular Star Cluster

Hundreds of thousands of stars are packed into a sphere only a few tens of light-years across in the globular cluster M13 in the northern-sky constellation of Hercules. Globular clusters are spread through a huge spherical halo around the Galaxy, and were probably born as the Galaxy itself was taking shape.

The number of stars visible to the naked eye from Earth has been estimated to total 8,000, of which 4,000 are in the northern hemisphere of the sky and 4,000 in the southern hemisphere. At any one time during the night in either hemisphere, only about 2,000 stars are visible. The others are obscured by atmospheric haze, especially near the horizon, and by faint sky light. Astronomers have calculated that the stars in the Milky Way, the galaxy to which the Sun belongs, number in the hundreds of billions. The Milky Way, in turn, is only one of several hundred million such galaxies visible through large modern telescopes. The individual stars visible in the sky are simply those that lie closest to the solar system in the Milky Way.

The star nearest to our solar system is Proxima Centauri, one component of the triple star Alpha Centauri, which is about 40 trillion km (25 trillion mi) from the Earth. In terms of the speed of light, the common standard used by astronomers for expressing distance, this triple-star system is about 4.29 light years distant; light travelling at about 300,000 km/s (186,000 mi/s) takes more than four years and three months to travel from this star to the Earth.


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