Origin of Universe
- Geocentric Theory: The theory is proposed by Ptolemy in AD 140, which states that the Earth is the centre of the Universe.
- Heliocentric Theory Proposed by Copernicus in AD 1543, according to which the Sun is the centre of the Universe.
- Solar system is merely a small part of a galaxy.
- In 1925, Edwin P Hubble an American astronomer said that the Universe has a diameter of 2.5 billion light years, and is composed of several galaxies.
Evolution of Universe
- Big Bang Theory: It is the most acclaimed theory, proposed by E George Lamantor in 1960-70.
- The Big Bang Theory proposes that the Universe was once extremely compact, dense and hot.
- Some original event, a cosmic explosion called the big bang, occurred and the Universe has since been expanding and cooling.
- According to this theory, our Universe sprang into existence as “singularity” around 13.7 billion years ago.
- The recent experiments at the Large Hadron Collider (LHC), built by the European Organization for Nuclear Research beneath the Franco-Swiss border near Geneva, Switzerland, aims to recreate conditions just after big bang to have a better understanding of the phenomenon.
- Big bang was an explosion that happened 15 billion years ago, leading to formation of galaxies of stars and other heavenly bodies.
Steady State Theory
- This theory states that number of galaxies in observable Universe is constant and new Galaxies are continuously being created out of empty space, which fill up the gaps caused by those galaxies, which have crossed the boundary of the observable Universe. As a result of it, the overall size and mass of the observable Universe remains constant . Thus, a steady state of the Universe is not disturbed at all.
- The pulsating universe theory, which is more commonly known as the oscillating or cyclic universe theory, posits that the universe goes through regular cycles of expansion and destruction.This theory is credited to Albert Einstein.
- In the cyclic model, the universe begins with a "big bang" and ends with a "big crunch." After the big bang, the universe expands until gravitational forces force it to stop. At this point, the universe contracts until it implode into a singularity. A gravitational singularity is a point where gravitational forces are infinite. After this contraction occurs, the cycle begins again with a new big bang.
- The limitless space consisting of the Solar system, Stars, Galaxies, etc is called the Universe.
Constituents of Solar System
- TheSolar System is the gravitationally bound system comprising the Sun and the objects that orbit it.
Some features of Solar system
- Sun is the centre of the Solar system around which eight planets revolve in elliptical orbits.
- During rotation velocity and distance from the sun keeps on changing.
- The speed is the highest when they are closest to the Sun and slowest when farthest away.
- The gravitational pull of the Sun on the planets controls their motion.
- The Solar system also includes asteroids, comets and meteors.
- System of millions or billions of stars, together with gas and dust, held together by gravitational attraction.
- Milky Way or Akash Ganga is one such Galaxy in which we live. It can be seen on a clear night as a hazy band of white light stretching across the sky.
- Our Sun and other stars, which can be seen by naked eyes belong to this Milky Way, which contains more than a hundred billion stars.
- A star is a scorching shining heavenly body. The Temperature of the star is very high and it emits its own light.
- Sun is the nearest star of earth. Its light reaches the earth in 8-3 minutes.
- On a clear night, we can see about 2500 stars at a time though the total number is much more.
- Stars may occurs single, in pairs,or in clusters.
- Earth’s nearest star after Sun is Alpha Centaury.
The life sycle of a star
- Outlined below are the many steps involved in a stars evolution, from its formation in a nebula, to its death as a white dwarf or neutron star.
- Nebulae are thebirthplaces of stars. A nebula is a cloud of gas (hydrogen) and dust in space. Huge clouds of dust and gas collapse under gravitational forces.
- Huge clouds of dust and gas collapse under gravitational forces, forming protostars. These young stars undergo further collapse, forming main sequence stars.
- Stars expand as they grow old. As the core runs out of hydrogen and then helium, the core contacts and the outer layers expand, cool and become less bright. This is a red giant or a red super giant ( depending on the initial mass of the star). It will eventually collapse and explode.
- Its fate is determined by the original mass of the star. It will become either a black dwarf, neutron star or black hole.
- This is very small, hot star, the last stage in the life cycle of a star like the 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:-
- Type IThese occur in binary star systems in which gas from one star falls on to a white dwarf, causing it to explode.
- Type IIThese 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. They leave behind neutron stars and black holes. Supernovae are thought to be main source of elements heavier than hydrogen and helium.
- 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.
- Neutron stars are very dense. Typical stars having a mass of three times the Sun but a diameter of only 20 km.
- If its mass is 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.
- Black holes are believed to form from massive stars at the end of their life times.
- The gravitational pull in a black hole is so great that nothing can escape from it, not even light.
- The density of matter in a black hole cannot be measured. Black holes distort the space around them, and can often suck neighbouring matter into them including stars.