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Phase of Ultraviolet Radiation was followed in the very 1st One Billion Year of the Big Bang by the formation of Stars and Galaxies.
We know that in Universe 99.9% we have hydrogen in clumps i.e gaseous clumps which might not be uniformly distributed. So non-uniformly distributed gaseous clumps of hydrogen may be denser at some place and less denser at some place and hence wherever it is denser, their it will have its own gravitational attraction. The gravitational attraction of this gaseous clump will start contracting it.
Nebula is basically a cloud of gases. There were many Nebula initially which might be different from each other and it is also possible that inside a bigger Nebula there are many small Nebula, that simply means that clumps of hydrogen may be denser at some places and less denser at some places. So a very big nebula can be considered as a galaxy and within a galaxy there are many small nebula. These small nebula which are denser will start contracting due to its gravitational attraction. When the nebula contract, the temperature increases which might reach upto 10^7 Celsius. When the temperature increases to such high degree, fusion of hydrogen will start inside nebula and helium will be formed and in this process it releases very high amount of energy in form of light. This light we see as a light of Star. Star has no hard surface, it is only compiled of gases.
The initial stars were much bigger in size than stars which we see now and they also had much higher hydrogen content. They consist no bigger element but only Hydrogen and these stars were generally at the center of their respective galaxies.
Oldest stars does not have planetary system around them because of primarily being composed of Hydrogen. These stars were 1000 times bigger than our Sun. When these stars are exhausted, they exploded releasing Nebula of higher elements.
The newer stars formed from the remains of the older stars have heavier elements. They are relatively smaller in size and will lead to formation of planets or exo-planets(planets beyond galaxy).
Galaxy: Galaxy are formed by gravitational attraction of initial gaseous matter formed in universe. They compose of large number of stars and had an expanse of 80 thousand to 1.5 lac light years.
Nebula: It is a cloud of gases and dust which can lead to formation of stars in Galaxies along with other celestial bodies. It is also formed from a death of star.
Star: It is a localized nebula which under its own gravity contracts so as to attain temperature high enough for thermonuclear reaction due to which visible light radiation is emitted which makes it visible to us as a star.
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