The Universe - Part 3

THE UNIVERSE - PART 3

ASTEROIDS
•    Asteroids are small heavenly bodies which orbit round the Sun between the orbits of Mars and Jupiter.
•    They are the pieces of much larger planet which broke up due to the gravitational effect of Jupiter.
•    About 1600 asteroids are revolving around the Sun.
•    The largest among them has a diameter of about 700 km is called Ceres. It circles the Sun once in every 4½ years.

COMETS
•    A comet consists of a small mass of rock−like material surrounded by large masses of substances such as water, ammonia and methane.
•    These substances are easily vapourised.
•    Comets move round the Sun in highly elliptical orbits and most of the time they keep far away from the Sun.
•    As the comet approaches the Sun, it is heated by the Sun’s radiant energy and vapourises and forms a head of about 10000 km in diameter.
•    The comet also develops a tail pointing away from the Sun.
•    Some comets are seen at fixed regular intervals of time. Halley’s Comet is a periodic comet which made its appearance in 1910 and in 1986. It would appear again in 2062.

METEORS AND METEORITES
•    The comets break into pieces as they approach very close to the Sun.
•    When Earth’s orbit crosses the orbit of comet, these broken pieces fall on the Earth.
•    Most of the pieces are burnt up by the heat generated due to friction in the Earth’s atmosphere. They are called meteors (shooting stars).
•    We can see these meteors in the sky on a clear moonless night.
•    Some bigger size meteors may survive the heat produced by friction and may not be completely burnt. These blazing objects which manage to reach the Earth are called meteorites.
•    The formation of craters on the surface of the moon, Mercury and Mars is due to the fact that they have been bombarded by large number of meteorites.

STARS
•    A star is a huge, more or less spherical mass of glowing gas emitting large amount of radiant energy. Billions of stars form a galaxy.
•    There are three types of stars.
     (i)    Double and multiple stars
     (ii)    Intrinsically variable stars
     (iii)    Novae and super novae.
•    In a galaxy, there are only a few single stars like the Sun.
•    Majority of the stars are either double stars (binaries) or multiple stars. The binary stars are pairs of stars moving round their common centre of gravity in stable equilibrium.
•    Some stars suddenly attain extremely large brightness, which they may be seen even during daytime and then they slowly fade away. Such stars are called novae.
•    A supernova is a large novae.
•    The night stars in the sky have been given names such as Sirius(Vyadha), Canopas (Agasti), Spica (Chitra), Arcturus (Swathi), Polaris (Dhruva) ... etc.
•    After the Sun, the star Alpha Centauri is nearest to Earth.

THE SUN
•    The Sun is extremely hot and self−luminous body. It is made of hydrogeneous matter. It is the star nearest to the Earth.
•    Its mass is about 1.989 × 1030 kg.
•    Its radius is about 6.95 × 108 m.
•    Its distance from the Earth is 1.496 × 1011 m. This is known as astronomical unit (AU).
•    Light of the sun takes 8 minutes 20 seconds to reach the Earth.
•    The gravitational force of attraction on the surface of the Sun is about 28 times that on the surface of the Earth.
•    Sun rotates about its axis from East to West. The period of revolution is 34 days at the pole and 25 days at the equator.
•    The density of material is one fourth that of the Earth. The inner part of the Sun is a bright disc of temperature 14 × 106 K known as photosphere.
•    The outer most layer of the Sun of temperature 6000 K is called chromosphere.

CONSTELLATIONS
•    The configurations or groups of star formed in the patterns of animals and human beings are called constellations.
•    There are 88 constellations into which the whole sky has been divided.
•    Ursa Major or Saptarishi or Great Bear is a constellation that resembles a bear.
•    The constellation Orion resembles the figure of a hunter and Taurus (Vrishabha) resembles the shape of a bull.

GALAXY
•    A large band of stars, gas and dust particles held together by gravitational forces is called a galaxy.
•    Galaxies are really complex in nature consisting of billions of stars.
•    Some galaxies emit a comparatively small amount of radio radiations compared to the total radiations emitted. They are called normal galaxies.
•    Our galaxy Milky Way is a normal galaxy spiral in shape.
•    The nearest galaxy to us known as Andromeda galaxy, is also a normal galaxy.
•    It is at a distance of 2 × 106 light years. (The distance travelled by the light in one year [9.467 × 1012 km] is called light year).
•    Some galaxies are found to emit millions of times more radio waves compared to normal galaxies. They are called radio galaxies.