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A star is a celestial body composed of incandescent gas which can emit light by itself.  Stars are formed from the condensed hydrogen gas and dust of massive nebulae (clouds). As the gas and dust become more dense, the gravitational force increases and attracts more matter.  As the cloud's gravity increases, so does the pressure, which also increases the temperature.  At some critical point, fusion begins, and the "proto-star" begins to emit light.

The closest star to the earth is the sun, which is about 150 million kilometers (93 million miles) away.  This distance is referred to as 1 AU ( Astronomical Unit ). Therefore, we can see clearly the disk of the sun without using any equipment.  Besides that, the light released by the sun is beneficial for life on earth.


Classification

In order to make an easier observation and study of stars, a classification is needed. Star based on its temperature is divided into :

  • 0 class -- 33.000 K or more
  • B class -- 10.500 – 30.000 K
  • A class -- 7.500 – 10.000 K
  • F class -- 6.000 – 7.200 K
  • G class -- 5.500 – 6.000 K
  • K class -- 4.000 – 5.250 K
  • M class -- 2.600 – 3.850 K

A good example for G class star is our own Sun. There are many other classes--Red Dwarfs, Blue Giants, and Yellow Stars. 400px|right

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