1. A large group of stars is called a galaxy
  2. There are billions of galaxies of different sized regular and irregular shape
  3. Galaxies can be divided into two categories
  4. Normal galaxies
  5. Radio galaxies
  6. Normal galaxies
  7. Theses galaxies emit a comparatively small amount of radio radiations as compared to the total radiations radiated
  8. These galaxies are bright from the center and gradually dim towards the edges
  9. Each normal galaxy contains billions of stars in the form of a band, traveling together in the universe
  10. Depending upon their shapes the normal galaxies may be divided into three groups namely

-elliptical galaxies

-spiral galaxies

-Irregular galaxies

  • About 18%of the galaxies are elliptical,80%of the galaxies are spiral and only 2% of the galaxies are irregular galaxies
  • The irregular galaxies are youngest, spiral galaxies are middle-aged and elliptical galaxies are quite old
  • Radio galaxies
  • These galaxies emit a million times more radio radiations than normal galaxies
  • The radio radiations do not come from the galaxy itself but are believed to be coming from two large radio sources
  • There are 170 billion galaxies in the observable universe
  • The largest galaxies have nearly 400 million stars and our galaxy “the milky way” has about 100 billion stars
  • Stars
  • A star is a fiery luminous heavenly body that has its own light and heat energy
  • Sun is the nearest star to our planet earth and it takes 8.3 minutes for the light to reach the earth from the sun
  • Proxima Centauri is the nearest star is beyond our solar system that is a distance of 4.3 light-years from the earth
  • Stars may exist as a single star but are very few in the universe
  • They may also occur in pairs called binary stars and the rest are multiple stars
  • Alpha Centauri consists of three stars
  • Variable stars are stars that show varying degrees of luminosity
  • Luminosity fluctuates between periods, Delta Sophie is an example
  • Stars of fluctuating luminosity are called cepheid variables
  • Pulsars are variable stars which emit regular pulses of electromagnetic waves of very short duration whereas quarasaras are powerful quasi-stellar sources of radio radiations
  • Stars from when enough dust and gas clump together because of the gravitational force
  • Stars like the sun change into other forms of stars such as the red giants, white dwarfs, neutron stars, and black wholes during their lifetime
  • The fate of a star depends upon how much matter it contains
  • High mass stars are much brighter than low mass stars
  • The sun has enough fuel (hydrogen fuel) to keep it bright for approximately 9 billion years
  • A star is twice as massive as the sun and will burn through its fuel supply in only an 800million yrs.
  • A 10 solar mass star. A star that is 10 times more massive than the sun burns nearly a thousand times brighter and has only a 20 million yr. fuel supply


  • Stars are born in a nebula
  • A nebula is a cloud of gas(hydrogen) and dust in space
  • Huge clouds of dust and gas collapse under gravitational forces
  • These young stars undergo further collapse forming main sequence stars
  • Stars expand as they grew old
  • As the core runs out of hydrogen and then helium the core contracts and the outer layers expand, cool, and become less bright
  • This is a red giant or a red supergiant
  • It will eventually collapse and explode then became either a black dwarf, neutron star, or black hole


Red giant to a white dwarf to a black dwarf

Red supergiant to a supernova to a neutron star/black hole


  • This is a very small, hot star, the last stage in the life cycle of a star like a sun
  • White dwarfs have a mass similar to that of the sun, but only 1% of the sun’s diameter, approximately the diameter of the earth


  • This is the explosive death of a star and often results in the star obtaining the brightness of 100 million suns for a short time

There are two general types of supernova

  1. type I- these occur in a binary star system in which gas from one star falls on to a white dwarf, causing it to explode
  2. type II-these occur in stars ten times or more as massive as the sun, which suffer runaway internal nuclear reactions at the ends of their lives, leading to an explosion


  • These stars are composed mainly of neutrons and are produced when a supernova explodes forcing the protons and electrons to combine to produce a neutron star
  • Typical stars having a mass of three times the sun but a diameter of only 20kms
  • It its mass in any greater its gravity will be so strong that it will shrink further to become a black hole
  • Pulsars are believed to be neutron stars that are spinning very rapidly


  • These are believed to form from massive stars at the end of their live times
  • The density of matter in a black hole cannot be measured
  • The gravitational pull in a black hole is so great that nothing can escape from it not even light not even light
  • It can often engulf neighboring matters including stars and planets etc

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