The Universe

What exactly is The Universe?

The universe is commonly defined as totality of everything that exists, including all physical matter and energy, the planets, stars, galaxies and the contents of intergalactic space.
The study of Universe is called ‘Cosmology’.

Theories regarding the Origin of Universe:

1. Big-Bang Theory – by E. George Lamaitre
2. Steady State Theory – by Thomas Gold and Herman Bondy
3. Pulsating Universe Theory – by Dr. Allen Sundes


What is a galaxy?

A galaxy is a vast system of billions of stars, which also contains a large number of gas clouds and dust, isolated in space from similar systems.

How many galaxies are there?

There are about 100 billion galaxies in the Universe and each galaxy has, on average, 100 billion stars.
The total number of stars in the universe is 10 raise to power 22.

In which galaxy we all live in?

The Milky Way Galaxy is the home of the Earth and solar system. It was formed 5 billion years after the Big-Bang. Andromeda is our nearest galaxy.

Parts of the universe:

Universe can be classified into two parts, namely, Atmosphere and Space.

Some Facts and Figures about the Planets

Planet Distance from the Sun (x 10⁶ km) Number of Satellites
Mercury 57.9 None
Venus 108.2 None
Earth 149.6 1
Mars 227.9 2
Jupiter 778.6 67
Saturn 1433.5 62
Uranus 2872.5 27
Neptune 4495.1 14

The Solar System

o The solar system consists of the sun, the eight planets and their satellites (or moons) and thousands of other smaller heavenly bodies such as asteroids, comets and meteors.
o The sun is at the centre of the solar system and all these bodies are revolving around it.
o Planets revolve around the sun in elliptical orbit.
o In the solar system the planet nearest to the sun is Mercury and the planet farthest from the sun is Neptune (not Pluto).

Members of Solar System:

o The size of solar system has been estimated to at about 10 raise to power 5 A.U.
o The Sun is at the centre of Solar System surrounded by planets, stars, asteroids etc.
o The sequence of planets according to their size is Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury.
o Mercury, Venus, Earth, Mars are called terrestrial planets and Jupiter, Saturn, Uranus and Neptune are called gaseous planets.
o Jupiter is the biggest and Mercury is the smallest planets of our solar system.

The Earth

• Earth is the largest of the inner planets.
• The Earth is 23 ½° tilted on its axis and thus makes 66 ½°.
• It takes 23 hours 56 minutes and 4.091 seconds to rotate on its axis.
• It takes 365 days, 5 hours and 48 minutes to revolve around the Sun.
• Earth is known as the 'watery planet' or the 'blue planet' due to the presence of huge amount of water on it.
• Earth is the only known planet which provides sustenance or life on it. It has a large quantity of oxygen which supports life.

The Moon

The moon is the only satellite of the earth having diameter of 3,475 km and its circumsference is 10,864 km while its orbit is elliptical. Only 59% of the total surface of the moon is visible from the earth.

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