Basic Definitions
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Milky Way Galaxy:
Milky Way Galaxy
The whole solar system orbits the center of our home galaxy, a spiral disk of 200 billion stars, which is known as the Milky Way. The Milky Way has two small galaxies orbiting it, which are visible from the southern hemisphere. The two galaxies are known as the Large Magellanic Cloud and the Small Magellanic Cloud. Our galaxy is on in billions of galaxies.

Black Hole:
Black Hole

With the death explosion of a massive star (Supernova Explosion), a massive amount of burnt remains of the star is left behind. These remains collapse on each other and gain infinite density. Any ray of light emitted is trapped within an orbit around the star due to the high gravitational pull. Light once entering into this orbit can never escape out after the star reaches the infinite density mark. Hence it is called a Black Hole.

Planetary Orbit:
It refers to the path in which a planet revolves around the Sun. Earth has a near circular orbit such that the difference between Earth's farthest point from the Sun and its closest point is very small. It takes 365 â¼ days for the Earth to go around the Sun once.

Astronomical Unit (AU):

Astronomers have introduced the concept of AU i.e. Astronomical Units in order to measure distances between various bodies of our Solar System. 1 AU is roughly 150 million kilometers which also the distance between the Earth and the Sun.

Light Year:

It refers to the distance that light can travel in one year. Astronomers use the concept of a light year for distances to other parts of the Milky Way.

1 Light year = 9,500,000,000,000 kilometers

Light moves at a velocity of about 300,000 kilometers per second. Therefore, light can travel 10 trillion kms in one year.

Gravitational Pull:
Gravitational Pull is the force exerted by a body due to the invisible force of gravity which pulls an object near to it. Gravitational Pull depends upon the mass of the object. The bigger the object, greater is its gravitational pull. For example, the gravitational pull of the Sun keeps the planets in orbit around it.

How are Planets classified?

Planets can be classified on the basis of:
  1. Composition:

    Terrestrial or Rocky planets (Mercury, Venus, Earth and Mars):
    composed primarily of rock and metal and have relatively high densities, slow rotation, solid surfaces, no rings and few satellites.

    Jovian or gas planets (Jupiter, Saturn, Uranus and Neptune):
    composed primarily of hydrogen and helium and generally have low densities, rapid rotation, deep atmospheres, rings and lots of satellites.

  2. Size:

    Small planets (Mercury, Venus, Earth and Mars):
    These planets have diameters less than 13000 km.

    Giant planets (Jupiter, Saturn, Uranus and Neptune):
    These planets have diameters greater than 48000 km.

  3. History:

    Classical planets (Mercury, Venus, Mars, Jupiter, and Saturn):
    These planets known since pre-historical times and are visible to the naked eye.

    Modern planets (Uranus and Neptune):
    These planets were discovered in the modern times and are visible only with the help of a powerful telescope.
    - Earth is neither a classical nor a modern planet.

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