Stars
Big Bang occurred some 13.7 billion years ago when universe emerged from an enormously dense and hot state. The Big Bang is a consequence of the observed Hubble's law velocities of distant galaxies that when taken together with the cosmological principle implies that space is expanding. Detailed observations of the morphology and distribution of galaxies and quasars provide strong evidence for the Big Bang. A combination of observations and theory suggest that the first quasars and galaxies formed about a billion years after the Big Bang, and since then larger structures have been forming, such as galaxy clusters and superclusters. Populations of stars have been aging and evolving, so that distant galaxies (which are observed as they were in the early Universe) appear very different from nearby galaxies (observed in a more recent state).
A star is a massive body of plasma in outer space that is currently producing or has produced energy through nuclear fusion. Unlike a planet, from which most light is reflected, a star emits light because of its intense heat. The very first stars to form after the Big Bang may have been larger, up to 300 solar masses or more, due to the complete absence of elements heavier than lithium in their composition. This generation of supermassive star is long extinct, however, and currently only theoretical.
Astronomers estimate that there are at least 70 sextillion (7×1022) stars in the known universe. That is 70 000 000 000 000 000 000 000, or 230 billion times as many as the 300 billion in our own Milky Way.
Birth:
Star formation occurs in molecular clouds, large regions of high density in the interstellar medium. Star formation begins with gravitational instability inside those clouds, often triggered by shockwaves from supernovae or collision of two galaxies..............
