The vast surrounding space in which stars,planets,satellites as well as our Earth and all the objects on it is called Universe.
Nobody knows how big our Universe is or it has any limits.All the heavenly bodies exist in it.The billions of stars exist in the Universe are not distributed uniformly in space.These stars occur in the form of cluster of billions of stars called Galaxies.The Universe is made up of over 90% hydrogen.On the scale of atoms most of these atoms are formed into gas stars.Among the various theories on the origin of Universe including the steady state and the pulsating theories,the one most widely accepted has been the Big Bang theories.
The Universe comprises of galaxies that are huge,congregation of stars held together by the forces of gravity.Galaxies occur in three structural forms : Spiral, Elliptical, and Irregular.
Each galaxy may contain as many as 1000 million stars.It is believed that Earth’s own galaxy known as Milky Way alone contains as many as 100000 million stars.
UNIT OF MEASUREMENT
LIGHT YEAR A light year is unit of distance.It is the distance that light can travels in one year,i.e.,(9.46✕1012 km).
1 light year =300000✕365✕24✕60✕60 km.
PARSEC It is another unit of measurement used in astronomy.It is equal to 3.26 light year.Parsec is one of the oldest methods for measuring the distances to stars.
ASTRONOMICAL UNIT (AU) This unit is used to measure distances within the solar system.The astronomical unit is the average distance between the Sun and Earth,i.e., 149597870 km.
Galaxies are giant assemblies of stars,planets,gases and dust,they are not distributed uniformly in space.These stars occur in huge bunches or cluster.They are so big that they have sometimes been called “Island Universe”.
Galaxies are found in different shape and size.
Ours Nearest Galaxy is Andromeda.
Our Earth lies in spiral type of galaxy called Milky Way.It consist of over 100 billion stars.
BIG BANG THEORY
The Big Bang Theory was postulated in 1950 and 1960,it explains the origin of Universe.
It says that all the matter in the Universe was originally a concentrated lump called primeval atom.
Big Bang was an explosion that occurred 15 billion years ago,leading to the formation of galaxies of stars and other heavenly bodies.
In the solar system the planet to the Sun in Mercury and the planet farthest from the Sun is Neptune.
Pluto is now considered as a dwarf planet,earlier it used to be ninth planet of the solar system.
The size of the solar system has been estimated to at about 105 AU.
The solar system is dominated by the Sun which accounts for almost 99.9% of the matter in the whole solar system.
The Sun is the only source of all energy in the solar system. The planets does not produce light of their own. Almost all the energy of the solar system is derived from the Sun.
All planets except Uranus and Venus rotate around Sun counter clockwise.
These are clusters of stars organised into distinct shapes and figures and named accordingly.Hydra is the largest constellation.
Stars are the heavenly bodies like the Sun that are extremely hot and have light of the own.
Our Sun is a star.
For seeing brightest star is Sirius,also called Dog star and closest to Earth is Proxima Centauri.
Stars are made up of hydrogen gas,helium and dust.
The stars are born,grow old and finally die.
Stars die due to a great supernova explosion and left neutron star which is called Black hole.
LIFE CYCLE OF STAR
Proto Star→Red Giant→Nova and Supernova→White Dwarf Way→Neutron Star→Pulsars→Black Holes.
This is the result from the collapse and compaction under gravity,of a star of mass greater than three times that of the Sun,at the end of its life cycle.
Nothing ,not even light can escape from its gravity.
S Chandrashekhar had predicted an upper limit to the mass of stars,which they either explode or form a black hole. This is known as Chandrashekhar limit.