Universe: Definition, Facts, Origin, Components

In this article, we are discussing Universe Definition, Facts, Origin, Components, and other important knowledge.


The big bang theory explains the origin of our universe. According to this theory, 15 billion years ago, cosmic matter was in a compressed state from which expansion started by a primordial explosion. The super-dense ball broke to form galaxies, which again broke to form stars and finally stars broke to form planets including earth.

Since the outer space is limitless, conventional units for measuring distances are not suitable. Hence new units as follows are used:

  • Light Year: Distance covered by light in one year in vacuum at a speed of 3×108 m/s. One light year is equal to 9.46 × 1012 kilometers.
  • Astronomical Unit: The Mean distance between the Sun and the Earth (1.49 x 108 km). One light year is equal to 60,000 AU.
  • Cosmic Year: Sun’s period of revolution around the galactic center (250 million years). Also called as ‘galactic year’
  • Parsec: Distance at which the mean radius of the Earth’s orbit subtends an angle of one second of an arc. It is equal to 3.26 light years.

Universe Definition, Facts, Origin, Components


These are the huge congregation of stars that hold together by force of gravity e.g. the Milky Way, Andromeda galaxy, large and small Magellanic cloud, Ursa Minor system, sculptor system, etc. Milky Way or Akashganga is our home galaxy. Our solar system is located in this galaxy.


Stars are self luminous bodies that account for 98 percent of the matter in a galaxy. In the universe, some stars appear small but emit more energy than the other stars of the Milky Way. Such stars are called ‘Quasars’. When the dense galactic nucleus is compressing to form a star, this stage in star formation is called a ‘protostar’ stage. Due to high temperature hydrogen converts to helium and heat and light is emitted. Thus a star is formed. When the hydrogen of a star is depleted, its outer regions swell and redden. This stage of a star is called a ‘Red Giant’. Our sun will turn into a ‘Red Giant’ in 5 billion years. ‘Novae Stars’ are stars whose brightness increases suddenly by 10 to 20 magnitudes due to explosion and then the stars again fade into normal brightness. ‘Super Novae’ are stars whose brightness suddenly increases by more than 20 magnitudes. After the explosion, the dense core of comparatively smaller stars is called the ‘white dwarf’. The dense core of the comparatively larger stars is called the ‘Neutron star’. The neutron star rotates at a high speed emitting radio waves. Such stars are called ‘Pulsar’. ‘Black hole’ stage of the star occurs when the ancient star collapses. Gravity becomes so intense in the hole that nothing escapes, even light.


In the sky at night there are various patterns formed by different groups of stars. These are called constellations. Ursa Major or Big Bear is one such constellation. One of the most easily recognizable constellations is the small bear or Saptarishi (Sapta-seven, Rishi-sages). It is a group of seven stars that forms a part of the large Ursa Major Constellation.

Solar System

The sun along with its eight planets, asteroids and comets comprise the ‘solar system’. The planets are divided into inner or terrestrial planets which have higher densities e.g. Mercury, Venus, Earth and Mars and outer planets which have lower densities e.g. Jupiter, Saturn, Uranus and Neptune.

The Sun

  • The sun is in the center of the solar system.
  • It is made up of extremely hot gases particularly hydrogen.
  • The sun is 109 times bigger than the
  • The sun is about 150 million km away from the earth. The light from the sun reaches earth in about 8 minutes.
  • The glowing surface of the sun is called ‘Photosphere’. Above the ‘Photosphere’ is red coloured ‘Chromosphere’. Beyond the Chromosphere is the ‘Corona’, visible during eclipses.
  • The temperature of the photosphere is about 6000°C and that of the Chromosphere is about 32400°C, and that of the corona about 2,700,000°C. The core of the sun has a temperature about 15 million degrees Kelvin. But that tremendous heat is not felt so much by us because despite being our nearest star, it is far away from us.
  • It takes 250 million years to complete one revolution round its centre. This period is called ‘Cosmic year’.
  • Sun spots’ are dark patches notched on the surface of the sun. They appear dark because they are cooler i.e. they have a temperature of about 1500°C.
  • The ‘Aurora Borealis’ or northern lights are multicoloured lights that sweep across the sky in waves and are visible in the arctic region. The ‘Aurora Australis’ or southern lights are similarly visible near the Antarctica region.
Universe Definition, Facts, Origin, Components

Universe Definition, Facts, Origin, Components

The Moon

  • The moon is the only satellite of the earth.
  • Its size is approximately one-fourth that of the earth. It has a diameter of 3475 km.
  • Its orbit is elliptical. The maximum distance (apogee) of the moon from the earth is 406,000 km and the minimum distance (perigee) is 364,000 km.
  • The moon moves around the earth in about 27 days. It takes exactly the same time to complete one spin. As a result, only one side of the moon is visible to us on the earth.
  • The bright parts of the moon are mountains whereas the dark patches are lowlying plains.


Asteroids are a series of very small planets or fragments of planets lying between the orbit of Mars and that of Jupiter. They number about 45,000. ‘Ceres’ whose length is about 1000km is the largest one. They revolve around the sun in the same way as the planets.

Meteors and Meteorites

The meteors are the remains of comets which are scattered in the interplanetary space of the solar system. On contact with the earth’s atmosphere, they burn due to friction. Those which completely burn out into ash are called meteors or ‘shooting star.’ Those which do not burn completely and strike the earth in the form of rocks are called ‘meteorites’.

Planetary System

There are eight planets in our solar system. They are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Earlier, Pluto was considered as a planet. But recently it has lost this status. All the eight planets of the solar system move around the sun in fixed paths. These paths are elongated. They are called orbits. A new planet 2003 UB 313 has been discovered recently in our solar system. It is bigger than Pluto and farthest from the Sun.

Universe Definition, Facts, Origin, Components

Universe Definition, Facts, Origin, Components


  • Mercury is the smallest and the nearest planet to the Sun.
  • It takes only about 88 days to complete one round along its orbit.
  • It has no atmosphere and no satellite.
  • Its days are scorching hot and nights are frigid.


  • Venus is considered as ‘Earth’s-twin’ because its size and shape are very much similar to that of the earth.
  • It is also called the ‘morning’ or ‘evening star’.
  • It is probably the hottest planet because its atmosphere contains 90-95% of carbon dioxide. The day and night temperatures are almost the same.
  • The atmospheric pressure is 100 times that of the earth.
  • It has no satellite.

 The Earth

  • The earth is the third nearest planet to the Sun.
  • In size, it is the fifth largest planet.
  • It is slightly flattened at the poles. That is why its shape is described as a Geoid.
  • From the outer space, the earth appears blue because its two-thirds surface is covered by water. It is, therefore, called a blue planet.


  • It is marked with dormant volcanoes and deep chasms where once water flowed.
  • It has a thin atmosphere comprising of Nitrogen and Argon.
  • Beneath its atmosphere, Mars is barren, covered with pink soil and boulder. Because of this it is known as ‘red planet’.
  • It has two satellites namely ‘Phobos’ and ‘Demos’.
  • The highest mountain here is Nix Olympia which is three times higher than Mount Everest.
  • Recent explorations have thrown light on the possibility of existence of life here.


  • It is the largest planet of the solar system.
  • Its atmosphere contains hydrogen, helium, methane and ammonia.
  • It contains two and a half times the mass of all the other planets combined.
  • It reflects more than three times the energy it receives from the sun.
  • It has the great red spot which is an enormous eddy in the turbulent cloud cover. It also contains dusty rings and volcanoes.
  • It has 16 satellites like Ganymede, Aayo, Europa, Callisto etc.


  • It is the second largest planet of the solar system.
  • It has a celebrated rings composed of thousands of rippling, spiraling bands of icy rock and dust just 200 feet thick and 270,000 km in diameter.
  • It has 21 known satellites. Among them Titan, Phobe, Tethys and Mimas are important.
  • Its moon, Titan has nitrogen atmosphere and hydrocarbons, the necessity of life but no life exists.


  • It is the only planet that lies on its side. Hence, one pole or the other faces the sun as it orbits.
  • It is one of the coldest planets because of having an average temperature of -223’C.
  • Its atmosphere is made of mainly hydrogen. The landscape is barren and there is frozen methane cloud.
  • There are 9 dark compact rings around the planet and a corkscrew shaped magnetic field.
  • It has 15 satellites; prominent ones are Aerial, Ambrial, Titania, Miranda etc. 6. It rotates north to south.


  • It is the most distant planet from the sun.
  • There are five rings of Neptune. The outer ring seems to be studded with icy moonlets while the inner ring appears narrow and nearly solid.
  • It has 8 satellites like Titron, Merid, N-1, N-2, N-3 etc.
  • Its atmosphere mostly contains hydrocarbon compounds. The atmosphere appear blue, with quickly changing white icy methane clouds often suspended high above an apparent surface.
Universe Definition, Facts, Origin, Components

Pluto from Planet to Plutoid

Pluto, demoted from planet status in 2006, got a consolation prize – it and other dwarf planets like it will be called plutoids. Plutoids are celestial bodies in orbit around the Sun at a distance greater than that of Neptune that have sufficient mass for their hydrostatic equilibrium (nearspherical) shape. The two known plutoids are Pluto and Eris. It is expected that more plutoids will be named as science progresses and new discoveries are made.

Universe Definition, Facts, Origin, Components
Universe Definition, Facts, Origin, Components

 [This post is a part of Geography Notes for UPSC Examination]