Question

How does a star's life begin?

Answer 1

Stars are considered as hot bodies of gas and start their life cycle as clouds of gas and dust known as nebulae and undergoes different stages in life cycle and finally at it eventually cools dim and stops shining known as Black dwarf(Dead star).

Complete solution:
A star begins its life cycle in the form of a large cloud of gas, and the temperature inside is good enough to form molecules in it, in this process atoms of light elements squeezed in enough pressure so the nuclei can undergo fusion and the fusion reaction begins. The majority of gas is consisting of hydrogen and helium, sometimes of different elements as well.

As the region will begin to heat up it forms a stage known as Protostars and the central temperature reaches around 15 million centigrade nuclear reactions of hydrogen and helium can start and it starts releasing energy and starts shining and we can call it a Main sequence star.

Now, the hydrogen fuel that powers the nuclear reactions within star will begin to run out and the star will enter into a new stage and begin to expand, cool and become red giants and for massive star they will undergo into Red Supergiant now the outer layers drift away from the core and star will experience its final phase in the form of Planetary Nebula or Supernova, now from the remaining core the final stage begins and average star like sun will be converted into White Dwarf and supernova can transform into Neutron star or can contract to become Black Hole.

Note:
Stars can vary in size from 500 times smaller or thousands of times larger than sun and in surface temperature can be from 5000 degrees Celsius to over 50000 degrees Celsius, sun has a surface temperature of around 5,500 degrees. The exact lifetime of star depends very much on its size, larger stars can burn their fuels rapidly and for smaller stars it is considered as a slow process and can last for several billion years.