The Milky Way. The satellites of the Solar system revolve round the sun which is stationary with respect to the satellites. It is not correct to say that the sun is absolutely stationary. In fact, the Sun is part of a larger system of stars. This system of stars appear as an arch in a clear night. It is known as Milky Way in Europe and Galaxy in Greek.
This Milky Way spreads in the northern sky from horizon to horizon. Its circle ends in the southern sky. In this way the Milky Way divides the sky into two equal parts. It is bright at some places and appears less bright at other places.
The Milky Way has been compared to a flat disc whose diameter is 100,000 light years and its thickness is 5,000 light years. It has a nucleus at its centre. A large number of stars exist in the spiral arms which emerge from its nucleus and revolve round it.
The Milky Way Galaxy is composed of approximately 100 billion stars. Relatively to the Milky Way, if the galaxy was reduced to 130 km (80 mi) in diameter, the solar system would be a mere 2 mm (0.08 in) in width.
The stars of the Milky Way are not revolving around the centre with the same speed. The stars closer to the centre are revolving faster than the stars away from it. In fact, the whole of the Milky Way is revolving around its centre and completes one revolution in 500 million years.
The sun completes one revolution around the centre of the Milky Way in 250 million years. If we assume that the life of the earth is 3 billion years, we can say that the Sun. since the birth of the earth, has made 12 revolutions around the centre of the Milky Way. The length of the Sun's orbit is 2.30 light years.
The stars of the Milky Way appear in some groups. The smallest one is a group of only two stars known as Binary system. 68% of the stars in the Milky Way belong to this category and are larger than our Sun. Similarly, there are star-groups having 3,4,5,6 and more stars.
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