1) As the universe expanded, it cooled and uncharged atoms formed
2) The force of gravity pulled matter into stars and galaxies
Most scientists believe that the Universe was created by a Big Bang about 13 billion years ago. At first the universe was a hot glowing ball of radiation. In the first few minutes the nuclei of the lightest elements formed. As it expanded, over millions of years, its temperature fell. Uncharged atoms were formed.
Eventually dust and gas were pulled together by gravitational attraction to form stars. The resulting intense heat started off nuclear fusion reactions in the stars, so they began to emit visible light and other radiation.
Gravitational force attracts more dust and matter around stars and this may have formed planets.
Very large groups of stars are called galaxies. Our sun is one of the many billions of stars in the Milky Way galaxies. The universe contains billions of galaxies.
LIFE HISTORY OF A STAR
1) Low mass star: Protostar ---> Main sequence star ---> Red giant ---> White dwarf ---> Cold (black) dwarf
2) High mass star, after the white dwarf stage: White dwarf ---> Supernova ---> Neutron star ---> Black hole (if there is sufficient mass)
Stars form from clouds of dust and gas.
Gravitational forces make the clouds become increasingly dense, forming a prorostar.
As a protostar becomes denser, it gets hotter. If it becomes hot enough, the nuclei of hydrogen atoms and other light elements start to fuse…