To be a bona fide star, it must spontaneously fuse hydrogen atoms to form helium, releasing enormous amounts of energy. This energy stabilizes the star's core, such that it stops contracting. This whole process can take about 40 million years.

A star is considered to be 'born' once nuclear fusion reactions begin at its centre. Initially hydrogen is converted to helium deep inside the star. A star that is converting hydrogen to helium is called a main sequence star.

Original Q.: “What is the birth of a star called?” “Nova,” which means “new.” Not “supernova,” which is the death of a star when it partially collapses and explodes the outer layers into space. I think of the two processes as similar events happening in opposite directions.

Stars are giant balls of hot gas – mostly hydrogen, with some helium and small amounts of other elements. Every star has its own life cycle, ranging from a few million to trillions of years, and its properties change as it ages.