The universe is defined as the vast surrounding space which includes everything that exists from the earth to the most distant parts of space that one can see.

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The Universe
 The universe is defined as the vast surrounding space which includes everything that exists from the
earth to the most distant parts of space that one can see.
 The earth, star, planets, satellites and all the objects in it are the parts of the universe.
 The objects in the sky that appear to twinkle are called stars. They are bigger than the earth and
some are even bigger than the sun.
 They are only visible at night and cannot be seen during the day because of the bright light of the sun.
 There are around 3,000 stars which can be seen through our naked eyes. The other stars which are
far away can be seen through a telescope.
 Stars are the celestial bodies that emit heat and light.
 The distance of stars is expressed in terms of light years.
 A light year is the distance covered by light in one year at the speed of 300,000 kilometres per
second.

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 All heavenly bodies like the planets, the moon, satellites, comets, asteroids and meteors constitute
the universe to move around the sun to form a solar system.
 The sun is a major source of heat and light for all the planets in the solar system.
 The sun is also a star. It is located at the centre of the solar system.

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 It is nearly 150 million km away from the earth.
 The light from the sun reaches the earth in approximately 8 minutes and 20 seconds.
 The sun is the source of almost all energy on the earth.
 The internal structure of the sun is made up of four layers: Corona, Photoshpere, Chromospheres
and Core.
 Natural satellite is a non-luminous celestial body that orbits a planet.
 The moon is a natural satellite. It completes one rotation on its axis as it completes one revolution
around the earth.
 The moon is the brightest object in the night sky. It’s surface is dusty & barren.
 The American astronaut, Neil Armstrong, landed on the moon for the first time on July 21, 1969 and
he was followed by Edwin Aldrin.

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 The various shapes of the bright part of the moon seen during a month are called phases of the
moon.
 The day on which the whole disc of the moon is visible is known as the full moonday.
 On the fifteenth day, the moon is not visible and this day is known as the ‘new moon day’.
 Only a small portion of the moon that appears in the sky, it is known as the crescent moon.
 Tides are defined as the periodic rise and fall of the sea water level in large water bodies like seas,
oceans, etc.
 High tide is caused due to the rise of water to its highest level and it is caused only after 12 hours
and 24 minutes. Whereas low tide is caused when the water stops falling.
 The time interval between a high tide and a low tide is about 6 hours and 12 minutes.
 A group of very large number of stars is called galaxy.
 There are about 1011
stars in each galaxy and about 1011
galaxies in the universe. Our galaxy is
known as Milky Way (also called as Akashganga).

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 A group of stars that can be identified with the shape of an identifiable object like an animal or a
known object is called a constellation.
 Types of Constellations: Orion, Leo Major, Scorpius, Ursa Major and Cassiopeia.
 A constellation that can be seen during winter in the late evenings is Orion. It was an ancient Greek
hunter and warrior and the constellation resembles this figure, with a club and a shield, and a
sword dangling from his belt.
 Leo Major contains bright stars that outline the form of a lion.
 Scorpius can seem reminiscent of a scorpion.

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 Ursa Major (Big Dipper, Great Bear, Saptarshi) is one of the most famous constellations that can
be seen in the summer sky during night. It is also known as the Big Dipper, the Great Bear or the
Saptarshi.
 Cassiopeia is another prominent constellation in the northern sky. It is visible during winter in the
early part of the night.
Galaxy Constellation
It is a collection of billions of stars. It is a group of only few stars.
It does not form any fixed pattern of any
animal or an object.
It forms the shape of an animal or an
object.
There are around 1011
galaxies in the
universe.
There are only 88 constellations.
 Planets look like stars, but they do not have light of their own. They reflect sunlight that is incident on
them.
 The time taken by a planet to complete one full revolution around the sun is called its period of
revolution.

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 The time taken by a planet to rotate a full 360 degrees on its axis is called its period of rotation.
 A celestial body that revolves around another celestial body is called a satellite.
 There are eight classical planets called as Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus,
Neptune. The ninth planet is Pluto which is called as dwarf planet.
 A classical planet is a celestial body that
o Orbits around the sun.
o Has sufficient mass for its self quantity to pull it into a nearly spherical shape.
A. Classical Planets
 Mercury (Budh)
 It is the smallest and the hottest planet in the solar system as well as the closest to the sun.
 It takes about 88 days to complete one revolution around the sun.
 Venus (Shukra)
 It is the second closest planet to the sun and the brightest planet in the entire solar system.
 It takes about 225 days to complete one revolution around the sun.

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 Earth (Prithvi)
 It is the third planet from the sun. It has a large quantity of water.
 It is surrounded by a thick layer of atmosphere which contains nitrogen, oxygen, carbon dioxide, etc.
 It rotates around the sun in 365.25 days.
 Mars (Mangal)
 It appears slightly reddish in colour and therefore, it is also called the red planet.
 It completes one revolution around the sun in about 687 days.
 It has two moons (natural satellites) named Phobos and Deimos.
 Jupiter (Brihaspati)
 It is the largest planet in the solar system.
 The mass of Jupiter is about 318 times that of our earth.
 Its atmosphere consists of hydrogen and helium.

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 Saturn (Shani)
 Saturn is the second largest planet of the solar system. It is yellowish in colour.
 The rings of Saturn are made of ice particles and dust.
 It is the only planet that is lighter than water.
 Uranus
 It is a very cold planet, much bigger than the earth. It has 27 natural satellites.
 Its atmosphere consists of hydrogen and methane.
 It rotates from east to west.
 Neptune
 It is very far away from the sun and is very cold.
 It looks like a small bluish circle through a powerful telescope.
 Mercury, Venus, Earth and Mars are called the inner planets. Jupiter, Saturn, Uranus and Neptune
are called the outer planets.

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B. Dwarf Planets
The International Astronomical Union (IAU) defines a dwarf planet as a celestial body in direct orbit
of the Sun.
 Pluto
 Its orbit overlaps the orbit of Neptune.
 Its size is about 0.002 times the mass of the earth and takes 248 earth days to orbit.
 Asteroids
 Asteroids mean ‘star-like’. These are large pieces of rock or rock and metal.
 They are the pieces of a much larger planet which broke due to gravitational pull of Jupiter.
 They are found in the gap between the orbits of Mars and Jupiter. This gap is called as the
‘Asteroid belt’.
 The first and largest asteroids discovered in our solar system are Ceres.

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 Comets
 It is a luminous heavenly body that revolves around the sun in an elliptical orbit.
 It appears as a bright head with a long tail.
 The tail of a comet is always directed away from the sun.
 It is named after Edmund Halley and was last seen in 1986.
 Meteors and Meteorites
 A meteor is made up of debris. It enters the earth’s atmosphere at a very high speed.
 As it falls to the ground, it glows brightly and therefore it is called a shooting star.
 Some meteors reach the ground before they burn completely and evaporate and these are
called meteorites.

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 Eclipse
 Eclipses are caused by shadows cast by heavenly body.
 When the moon passes into the earth’s shadow i.e., when the earth comes in between the moon and
the sun then it is called as lunar eclipse.
 If the moon is in the umbral core of the earth’s shadow then it is called as total lunar eclipse.
 If the moon is partially in the penumbral core and partially in the umbral core then it is known as
partial lunar eclipse.

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 When the moon casts its shadow on the earth’s surface then it is called as solar eclipse.
 Artificial Satellites
 Artificial satellites are man-made devices that orbit the earth, moon and the sun.
 They gather information about the bodies they orbit.
 They are used for transmission of television and radio signals, tele-communication, weather
forecasting and remote sensing.

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 Aryabhatta was the first Indian satellite.
 The other Indian satellites are INSAT, IRS, Kalpana-1, EDUSAT, etc.
 Expansion of Universe
 Edwin Hubble observed that galaxies are not stationary and are moving away from each other.
 The larger the distance between the two galaxies, the faster they move away from each other.
 Big Bang Theory
 The total matter in the space was concentrated into a mass of about 100 million light years wide.
 About 15 billion years ago, the matter flew in all directions through the space thus galaxies were
formed.
Planet Satellite
It is a heavenly body which revolves around
the sun and rotates on its axis.
It is a heavenly body which revolves around
its planet and rotates on its own axis.
It has a bigger size. It has a smaller size.
There are 9 known planets in the solar
system out of which 8 are classical planets
and one is a dwarf planet.
There are 91 natural satellites known so far
in the solar system.
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