Brief Notes on Space



It is a shining spherical heavenly body around which the planets rotate. It is one of some 10,000,000 stars which constitute our galaxy.



They revolve round the Sun. They are not self-luminous but shine by radiating the light received from the Sun. Their orbits are elliptical.

Their sizes, speeds and distances from the Sun are dissimilar. In all there are nine known planets.


These are secondary bodies that revolve around the planets as planets revolve around the Sun. A satellite is said to have been formed of the matter whirled off from a planet when still in molten state. Except Venus and Mercury, all the other planets have satellites. Moon is Earth’s natural satellite.



These are minor planets whose orbits lie between Jupiter and Mars. These are said to be the fragments of a larger planet disrupted long ago. More than 1,400 have been named. Ceres, the first to be discovered is the largest asteroid having a diameter of 670 km.


These are small bodies coming from inter-planetary space. They become luminous by friction on entering the earth’s atmosphere and are popularly called Shooting stars.



These are the larger meteors that reach the earth and become meteorites.


These are suns of self-luminous bodies, situated at enormous distances from the solar system. There are millions of stars in the universe. There are four kinds of stars:


Fixed Stars:

These are stars which do not appear to alter their relative positions in the sky. These are also called Dog stars.

Binary Stars:

These are groups of two stars revolving round a common centre under mutual gravitational attraction.


Temporary Stars:

These are those which suddenly flare up to greatly increased brightness and fade away after a short time. They are also called Nova.

Variable Stars:

These are stars the brightness of which varies from time to time.

Red Giants:

These are stars which have consumed about 10 per cent of their hydrogen on account of which they appear reddish. Red giants consume their hydrogen at an increasing rate and eventually contract to become White dwarfs.


It is a luminous celestial body which moves about the solar system in elliptical or hyperbolic orbits. Comets are usually accompanied by a long shining tail. Comets have a definite periodicity as the Haley’s comet which is spotted every 76 years. The period of Encke’s Comet is only 3.3 years and of Hale-Bopp is about 4000 years.


These are highly compact stellar objects, distinct from the stars one sees in the night sky, rotating rapidly and emitting electromagnetic radiation in pulses much in the manner of a lighthouse flashing light.

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