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Basic Astronomy
Astronomy is the Study of heavenly bodies such as the stars, the moon and the sun including their origins, evolution, composition, motions, relative positions, and sizes.
The Stars
Stars are self-luminous celestial objects seen as points of light in the sky. It has continuous nuclear reactions which send heat, and light. Stars are large incandescent balls of gases held together by its own gravity. The Sun is a star. Every one of the vast number of twinkling points of light in the sky is a gigantic atomic furnace, just as the sun is. And among the stars, the sun is not a particularly impressive example. It is about an average-size star. The distances of stars are hard to imagine. The best way to deal with such distances is in terms of time, and that is what astronomers have done. Those stars that are brighter are not necessarily any bigger than the others.
A star's brightness depends on three things: its size, its distance, and the kind of star it is. One of the things that determine the apparent brightness of a star is the actual intensity of its light. If you watch a piece of iron being melted, you will notice that it first begins to glow a dull red, gradually grows more orange and then yellow and finally white. The colours of the stars indicate in much the same way how hot and, therefore, how bright they are. The red stars are the coolest. The yellow stars, like the sun, are moderately hot in the scale, and the white and blue-white stars are the hottest. Here are some examples of stars and their color:
Rigel - Bluish white
Sirius - White
Vega - White
Procyon - Yellowish White
Sun - Yellow
Capella - Yellow
Arcturus - Orange
Betelgeuse - Red
Antares - Red
Cygni - Orange
Bearnard's Star - Red
Year after year, the stars seem to remain unchanged in the sky. Even so, stars do not last forever. In the year 1054 a brilliant star suddenly appeared in the sky. For two years it shone so brightly that it was visible during the day. Then it faded away. All that remains now is a mass of faintly glowing gas --- the Crab Nebula.
Stars that end their lives in a tremendous explosion is called a Supernova. As we look at them in the sky, the stars vary in brightness, a few being very bright indeed while others are so faint that they can hardly be seen. The brightness of a star is called its magnitude. It is not a reliable guide to the star's real brightness, because the farther away a star is the fainter it looks.
Thus a very bright star which is very distant may look much fainter than a not very bright star which is comparatively close to us. The second nearest star to us is Proxima Centauri which is about 26 trillion miles away. It is 4 light years away from us. This means that the light of this star reaches us in 4 years. Sirius is a very bright star and is very near the solar system.
It is about 8½ light years away therefore it takes 8½ years for the light of Sirius to reach our eyes. Betelgeuse, which is another bright star, is 500 light years away. Rigel, the star nearer Betelgeuse is also one of the brightest star in the constellation Orion. The farthest stars that can be seen in the sky with the naked eye are those in the group of stars of Andromeda. They are 1,500,000 light years away.
The Sun
The Sun is the center of the Solar System which the earth and the other planets revolve. Compared with the millions of other stars we know about, the sun is small and of only average brightness. But because it is much nearer than any other star it appears larger and bigger. The sun rotates on its own axis. The sun is made up of a mass of hat gases. Without the sun, life as we know it would be impossible.
The Moon
The Moon is the celestial body that revolves around the earth from west to east. It is a natural satellite of the planets. It accompanies the earth in its revolution around the sun.
The Galaxies
A system of stars and planets like the one containing our solar system is called a galaxy. There are different galaxies in the universe. The earth and the solar system belongs to the Milky Way Galaxy. The nearest galaxy to ours is the Andromeda Galaxy or the Great Nebula.
Constellations
A Constellation is a group of stars that form patterns. Ancient people who studied the stars imagined that they saw likeness of objects, animals and people in the groups of stars. In the northern hemisphere they saw a group of stars which they thought look like a dragon. They named this group Draco. According to Greek legends, Cassiopeia was a queen who had a very beautiful daughter. A constellation has been named after her. Orion was a great hunter, according to Greek legends. When the ancient people saw this group of stars, they seemed to see the figure of Orion, thus, its name. Twelve constellations make up the zodiac, a belt which circles the sky close to the plane of the earth's orbit around the sun. They are Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, and Pisces.
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