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Visual magnitude is one way of classifying stars.
- Magnitude refers to the brightness of a star.
- Apparent magnitude is the brightness that is seen on Earth.
- Absolute magnitude is the brightness that would be seen from a standard distance of 10 parsecs (32.6 light years) away.
The lower a star's apparent magnitude, the brigter it appears on Earth. The lower the apparent magnitude, the brighter the star actually is compared to others, because it is measured from a standard distance. Objects that have an apparent magnitude higher than +5.5 cannot be seen with the naked eye. The brightest star in the night sky, Sirius A, has an apparent magnitude of -1.4, while Rigel has an apparent magnitude of +0.12. Their Absolute magnitudes, however, are quite different. While Rigel's is -7.1, Sirius' is +1.4.
Magnitude can also be affected by neblulas and interstellar dust, which blocks an scatters light, or reflects it.
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Stars range greatly in size, anywhere from 150 minnion km to 3 000 km. Larger stars are not nessecarily brighter, but most often, this is the case. There are four main classifications of stars:
- Main sequence stars - Main sequence stars can have absolute magnitudes anywhere between about -6.2 to +16, at which point, they are very, very faint. These stars have average diameters, and fuse hydrogen into helium in a process called Nuclear Fusion.
- Giants - Giants are large stars, and are usually fairly bright. They are old small Main sequence stars, and are running out of hydrogen. These large stars then begin to fuse helium into carbon, and start to cool off. They are at least 70 million km in diameter. Once the star's temperature falls enough, the outer layers drift away, forming a Planetary Nebula. The core is now very dense, and is cooling rapidly.
- Supergiants - Supergiants, as their name implies, are massive celestial bodies that are old large Main Sequence stars. They begin producing iron th their cores when they run out of other fuel. They are about 100 million km in diameter, and are cooling and expanding. These stars eventually die in a cataclysmic explosion called a supernova, that send the outer layers of the star flying. If the stellar core is left behind, the star may take one of two routes: if the stellar core is less than three solar masses, the core becomes a neutron star. If it is greater than three solar masses, it may continue to contract to the point where it becomes the most bizzare formation in the known universe: a Black Hole.
- White Dwarfs - White Dwarfs are the remanents of Main Sequence stars. The cores begin to cool, and loose luminosity, to the point where it stops glowing all together.
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