Notes
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Topics to be learn :
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Galaxy : A group of innumerable stars and their planetary systems is called a galaxy.
Milky Way : Milky Way is also a galaxy. Our solar system is present in a milky way called Mandakini. Our milky way can be seen on a dark and clear night. It is spread north-south in the sky.
Local group of galaxies : The Milky Way is a part of local group of galaxies. There are many such galaxies in the universe. The galaxy closest to our Milky Way is Andromeda.
Celestial bodies : In our Milky Way, there are many celestial bodies such as clusters of stars, nebulae, clouds of gases, clouds of dust, dead stars, newly born stars, etc.
Universe : The universe includes innumerable galaxies, the spaces between them and also energy.
Types of galaxies : According to the shapes the galaxies are identified as follows : Spiral galaxy, elliptical galaxy, barred spiral galaxy and irregular galaxy. Various types identified according to their shapes
Hubble Telescope : In 1990, American space agency, NASA launched ‘Hubble telescope‘ in the orbit of earth. The scientist Edwin Hubble had shown that there are many galaxies beyond our Milky way. With the help of Hubble telescope stars are observed, photographs are taken and spectrums are obtained.
Stars : There is varied luminance among the stars; some are bright while some are faint. They show radiance of different colours, viz. blue, white, yellow and reddish.
The birth place of stars are the huge nebulae, made up of dust particles and gases.
Surface temperature of stars varies from 3500°C; to 5000O°C. According to the temperature the colours of the stars change.
Types of Stars : (1) Sun-like stars (2) Red Giants stars (3) Super Nova (4) Binary or Twin stars (5) Variable stars. (1) Sun-like stars : (2) Red Giants : (3) Super Nova : (4) Binary or Twin Stars : (5) Variable Stars :
The solar system : The solar system consists of the sun, the planets revolving around the sun, their satellites, dwarf planets, asteroids, comets and meteors.
- Stars have light of their own. Therefore, they appear to twinkle.
- Planets do not have light of their own; they receive the light from the stars such as sun. Thus their light looks stable.
- The stars have their own heat and light,
- Planets do not have their own heat and light.
- Planets move around the stars.
Our solar system : There are eight planets in our solar system
- Mercury, Venus, Earth and Mars are called inner planets
- Jupiter, Saturn, Uranus and Neptune are called outer planets.
- Inner planets have hard crust
- Outer planets have gaseous outer cover and rings around them.
- Between Mars and Jupiter there is a belt of asteroids
The sun :
- Sun is yellow coloured star and at the centre of the solar system.
- Surface temperature is around 6000°C.
- The diameter of the sun is approximately 13,92,000 km.
- The size of the sun is so huge that around 13 lakh planets of the size of the earth can be easily placed within it.
- Due to the gravitational force of the sun, the celestial bodies in the solar system revolve around it.
- The sun rotates around its axis and while doing so, it revolves around the centre of the Milky Way taking the solar system along with it.
Planets of the solary system : (1) Mercury : Facts and figures : (2) Venus : Facts and figures : (3) Earth : Facts and figures : (4) Mars : Facts and figures : (5) Jupiter : Facts and figures : (6) Saturn : Facts and figures : (7) Uranus : Facts and figures : (8) Neptune : Facts and figures :
Satellite : The celestial bodies revolving around the planets are called satellites.
- They do not revolve around the sun independently.
- Satellites also rotate around their axes.
- The satellite of the earth is the moon, which completes its rotation and revolution both in 27.3 days.
- Mercury and Venus do not have satellites, remaining planets have satellites.
Asteroid : Some small sized bodies formed when the solar system was created, are asteroids.
- They too revolve around the sun but they could not become planets.
- An asteroid belt is seen between the Mars and Jupiter.
Dwarf planet : Dwarf planet is a celestial body revolving independently around the sun, but is of small size. e.g. Pluto : 248 years revolution period and 6.39 days rotation period.
Comet :
- A comet is a celestial body formed of ice and dust particles. It too revolves around the sun and is a part of the solar system.
- When they are away from the sun, they appear as small points. But when they come closer to the sun, they are easily visible to us.
- When they come closer to the sun, the frozen matter on them is converted to gases due to solar heat. These gases are thrown in the opposite direction away from the sun. This appears as a long feathery tail.
- They have long elliptical orbits, their appearance in the sky is very rare and taking place after a very long time period.
Types of comets :
- Long period comets : Comets that take more than 200 years to complete one revolution around the sun.
- Short period comets : Comets that take less than 200 years to complete one revolution around the sun.
| Fred Whipple was the American astronomer who gave the details of internal structure of a comet. He proposed that comets consist of icy cluster of various constituents. |
Till 1950, Whipple discovered 6 comets. Comets were called dirty snowballs because of Whipple’s research.
Hailey’s comet was seen in Indian sky in 1986.
Meteor fall ; Rocky pieces originating from the asteroid belt, fall on the earth's surface. This is called a meteor fall. When we see a falling star, it is actually a meteor falling down.
Meteor : When small rocky pieces enter the earth’s atmosphere, they get completely burnt due to friction with air. Sometimes they do not burn but fall down. They are called meteorites. The ‘Lonar’ lake in Maharashtra is formed by the impact of such meteorite.
Q. Why do we see only one side of the moon ?
We can see only one side of the moon because the moon rotates about its axis at the same rate that the moon orbits the Earth. The moon is illuminated by the sun and thus we can see only one side of the moon.