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  • The Universe or the Cosmos, as perceived,consists of millions of Galaxies. A Galaxies is a huge congregation of stars held togather by the forse of gravity.
  • Edwin Hubble in 1924 frist demonstrated existence og galaxies beyond milky way. He proved that thease galaxies are flying away from each other and that the farther they are, faster they fly.This means that the Universe is expanding like a balloon that is being blown up.
  • Our galaxy is Milky Way or the Akash Ganga. It is a spiral in shape.It consists of over the 100 billion stars rotating and revolving about its center. Nearest galaxy to ours is Andromeda.
  • The Big Bang Theory evaluates that 15 billon years ago,cosmic matter (Universe) was in an extreamly compressed state, from which expansion started by primordial explosion. This explosion broke up super -dense ball and cast its fragments far out into space, where they are still traveling at thousand of miles per second.It is from this speeding fragments of matter that our galaxies and star has not halted the speed of expansion. And, as it happens in all explosion, the farthest pieces are flying in the fastest.

Measurement unit of space are:

  1. Light year: It is the distance covered by light in one year vaccum at a speed of 3x10^5 km/s.
  2. Astronomical unit: It is the mean distance between the earth and the sun. One light year is equal to 60,000 A.U
  3. Persec: It represented the distance at which the mean radiusof earth's orbit subtends an angel of the second of an arc.It is equal to 3.26 ligth year.

  • Stars are self-luminous bodies that acvount for 98 percent of the material in the galaxy.The rest 2 percent consists of interestellar of galactic gas and dust in a attenuated from.Star forming matter is richer, in hydrogen and helium.
  • A star's colour indicates the temperature of its surface. Blue colour denotes maximum temperature. Then comes yellow,then red,  etc.
  • If the star is sun's size, it becomes a White Dwarf. There central density can reach up to 10 degree grams/ cubic cm.
  • Stars having mass greater than three times that of the sun, because of there great gravitational power,have contracted so much that they have devoloped super density of 1016 grams per cubic cm.It is so dense the nothing, not even light,can escape from its gravity and hence called 'Black Hole'
  • Brightest star outside of solar system is Sirius,also called Dog Star.
  • Closeter star of solar system is Proxima Centauri (4.2 light years away). Then come Alpha Centauri (4.3 light years away) and Barnard's Star (5.9 light year away)

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