WBCS

The limitless expanse of space around us consisting of solar system, galaxies, stars and planets etc, is called universe. The age of the universe is estimated to be (1-2)X10¹⁰ years.
The vast collection of billions of stars along with the vast amount of hydrogen and dust in an isolated in the universe is called galaxy. There are nearly 10¹⁰ galaxies which are the building block of the vast universe. Galaxies are not fixed in the universe but are moving outwards.
Our Solar System is a part og thr galaxy called the "milky Way". The nearest galaxy to our own galaxy is Andromeda galaxy. The Milky way has three main parts: a nuecleus, a disc and a halo. It contains about 100,000 milion stars.The diameter of the Milky way is neraly 120,000 light years.
Nebula, which appear in the sky as bright spots, are actually clusters of stars and gaseous clouds. The gases in a nebula gradually gather together into spinning balls. They spins more and more quickly, untill they get amazingly hot and a big blast, called a nuclear reaction, begins. When this happens, a baby star begins to glow. Stars are mostly made of two gases, hydrogen and helium.
A group of a few stars whose arrangement can be compared to the figure of some animal or any other known thing is called costellations. There are in all 89 constellations. The largest of this is Hydra.
One of the most famous constellations which you can see during summer time in the early part of the night is Ursa Major. It is also known as the Big Dipper, the Great Bear or the Saptarshi.There are seven prominent stars in this constellation. It appears like a big ladle or a question mark. There are three stars in the handle of the ladle and four in its bowl.
Orion is another well-known constellation that can be seen during winter in the late evenings.It also has seven or eight bright stars. Orion is also called the Hunter. The three middle stars represent the belt of the hunter. The four bright stars appear to be arranged in the form of a quadrilateral.
Cassiopeia is another prominent constellation in the northern sky. It is visible during winter in the early part of the night. It looks like a distorted letter W or M.
Solar System: The Sun and the celestial bodies which revolve around it form the solar system. It consists of large number of bodies such as planets, comets, asteroids and meteors. The gravitational attraction between the Sun and these objects keeps them revolving around it.
There are eight planets that revolve around the Sun. The eight planets in their order of distance from the Sun are: Mercury, Venus, Earth, Mars , Jupiter, Saturn, Uranus and Neptune.
Till 2006 there were nine planets in the solar system. Pluto was the farthest planet from the Sun. In 2006, the International Astronomical Union (IAU) adopted a new definition of a planet. Pluto does not fit this definition. It is no longer a planet of the solar system.
- The Sun: The Sun is 150 milion kilometer away from the earth. Its diameter is 1.4X10⁶ km and is approximately 3.0X10⁵ times heavier than earth. The core of the sun is made of mostly hydrogen gas at extremely high pressure and its temperature in the centre can be as high as 15 milion degree celsius. The atmosphere of sun consists three parts:
  1. Corona at a temperature of 1.7 X 10⁶ degree celsius.
  2. Chromosphere at a temperature of 27800 degree celsius.
  3. Photosphere at a temperature of 6000 degree celsius.
- Planets: The planets look like stars, but they do not have light of their own. They merely reflect the sunlight that falls on them. A planet has a definite path in which it revolves around the Sun. This path is called an orbit. All the planets revolve round the sun in elliptical orbit. The time taken by a planet to complete one revolution is called its period of revolution. The period of revolution increases as the distance of the planet increases from the sun. Besides revolving around the Sun, a planet also rotates on its own axis like a top. The time taken by a planet to complete one rotation is called its period of rotation.
  1. Mercury (Budh):The planet mercury is nearest to the Sun. It is the smallest planet of our solar system.Mercury has no satellite of its own.
  2. Venus (Shukra): Venus is earth’s nearest planetary neighbour. It is the brightest planet in the night sky.Venus has no moon or satellite of its own. Rotation of Venus on its axis is somewhat unusual. It rotates from east to west while the Earth rotates from west to east.
  3. The Earth:The Earth is the only planet in the solar system on which life is known to exist.The axis of rotation of the Earth is not perpendicular to the plane of its orbit. The tilt is responsible for the change of seasons on the Earth. The average diameter of earth is about 12.800 kilometer or its radius is about 6400 kilometer. The earth takes 365 days and 6 hours to complete one revolution around sun. It also rotates on its own axis once in 24 hours. The Earth has only one satellite(Moon).
  4. Mars (Mangal):The next planet, the first outside the orbit of the Earth is Mars. It appears slightly reddish and, therefore, it is also called the red planet. Mars has two small natural satellites.
  5. Jupiter (Brihaspati):Jupiter is the largest planet of the solar system. Jupiter has a large number of satellites. It also has faint rings around it. Ganymede which is the sattelite of jupiter is the largest satellite of the solar system.
  6. Saturn (Shani):Beyond Jupiter is Saturn which appears yellowish in colour. What makes it unique in the solar system is its beautiful rings. These rings are not visible with the naked eye. Saturn also has a large number of satellites. Titan of saturn is the second largest sattellite of the solar system. One interesting thing about Saturn is that it is the least dense among all the planets. Its density is less than that of water.
  7. Uranus:The first planet found with the aid of a telescope, Uranus was discovered in 1781 by astronomer William Herschel. The seventh planet from the Sun is so distant that it takes 84 years to complete one orbit. Uranus rotates east to west. Uranus' rotation axis is tilted almost parallel to its orbital plane, so Uranus appears to be rotating on its side.Uranus has two sets of rings. The inner system of nine rings, discovered in 1977, consists mostly of narrow, dark rings. Voyager found two additional inner rings. An outer system of two more-distant rings was discovered in Hubble Space Telescope images in 2003. In 2006, Hubble and Keck observations showed that the outer rings are brightly colored. Uranus has 27 known moons, named for characters from the works of William Shakespeare or Alexander Pope. Miranda is the strangest-looking Uranian moon: its complex surface may indicate partial melting of the interior, with icy material drifting to the surface.
  8. Neptune: Nearly 4.5 billion kilometers (2.8 billion miles) from the Sun, Neptune orbits the Sun once every 165 years. It is invisible to the naked eye because of its extreme distance from Earth.The main axis of Neptune's magnetic field is tipped over by about 47 degrees compared with the planet's rotation axis. Like Uranus, whose magnetic axis is tilted about 60 degrees from the axis of rotation, Neptune's magnetosphere undergoes wild variations during each rotation because of this misalignment. The magnetic field of Neptune is about 27 times more powerful than that of Earth.Neptune has six known rings. Voyager 2's observations confirmed that these unusual rings are not uniform but have four thick regions (clumps of dust) called arcs. The rings are thought to be relatively young and short-lived.Neptune has 13 known moons, six of which were discovered by Voyager 2. Triton, Neptune's largest moon, orbits the planet in the opposite direction compared with the rest of the moons, suggesting that it may have been captured by Neptune in the distant past. Triton is extremely cold - temperatures on its surface are about -235 degrees Celsius (-391 degrees Fahrenheit).
- The first four planets, Mercury, Venus, Earth and Mars are much nearer the Sun than the other four planets. They are called the inner planets(terrestial planets).The planets outside the orbit of Mars, namely Jupiter, Saturn, Uranus and Neptune are much farther off than the inner planets. They are called the outer planets( Jovian planets).
- Asteroids: There is a large gap in between the orbits of Mars and Jupiter . This gap is occupied by a large number of small objects that revolve around the Sun. These are called asteroids.
- Comets : Comets are also members of our solar system. They revolve around the Sun in highly elliptical orbits. However, their period of revolution round the Sun is usually very long. A Comet appears generally as a bright head with a long tail. The length of the tail grows in size as it approaches the sun. The tail of a comet is always directed away from the sun.One such comet is Halley’s comet, which appears after nearly every 76 years.
- Meteors and Meteorites: A meteor is usually a small object that occasionally enters the earth’s atmosphere. At that time it has a very high speed. The friction due to the atmosphere heats it up. It glows and evaporates quickly.These are commonly known as shooting stars, although they are not stars.Some meteors are large so that they can reach the Earth before they evaporate completely. The body that reaches the Earth is called a meteorite.
- The Moon: Any celestial body revolving around another celestial body is called its satellite.The Earth has only one satellite, moon.The day on which the whole disc of the moon is visible is known as the full moon day.Thereafter, every night the size of the bright part of the moon appears to become thinner and thinner. On the fifteenth day the moon is not visible. This day is known as the new moon day. The next day, only a small portion of the moon appears in the sky. This is known as the crescent moon. Then again the moon grows larger every day. On the fifteenth day once again we get a full view of the moon.The moon completes one rotation on its axis as it completes one revolution around the Earth. The moon revolves around the earth once in about 27 days and 8 hours. The moon’s surface is dusty and barren. There are many craters of different sizes. It also has a large number of steep and high mountains. Some of these are as high as the highest mountains on the Earth.The moon has no atmosphere. It has no water. On July 21, 1969 (Indian time) the American astronaut Neil Armstrong landed on the moon for the first time followed by Edwin Aldrin