The Birth of Stars (Clouds ,Nebulae Star Births And Deaths)
Star formation at the present epoch
The birth of a star is a rare, slow event; all but a very few of the stars visible to the naked eye have existed longer than mankind. It is therefore first necessary to consider the evidence that new stars are being formed in the Galaxy at the present time.
The energy which a main-sequence star radiates into space is generated by the conversion of hydrogen to helium at its centre. By comparing the rate at which energy is being emitted to the mass of hydrogen fuel in such a star we can estimate its potential lifetime. It is found that the maul-sequence lifetime of a star depends strongly on its mass; low-mass stars (such as type M) are small, cool, and long-lived; however high-mass stars (such as type 0) are large, hot and short-lived. Our Sun is now half-way through its total main-sequence lifetime of 1010 years, but an O star with a mass 30 times greater than the Sun would live for only a few million years. The fact that such bright stars are seen to exist today implies that star formation must have taken place within the last few million years; since our Galaxy is about twelve thousand million years old, it is reasonable to assume that somewhere in the Galaxy the same process is taking place even now. Recent estimates are that the equivalent of several stars the size of the »Sun are born somewhere in the Galaxy each year. At the time when the Galaxy first formed, however, the rate of star formation was many times higher than this.
Stars cannot in general move very far across the Galaxy in a few million years. Any O stars we find must therefore still be fairly close to the places where they were formed. Searches for O stars have shown that they are almost always found in the close vicinity of gas and dust clouds .We may therefore conclude that it is out of such clouds new stars condense. These have also shown that 0 and B stars almost always exist in either associations or open clusters and that most of these clusters and associations are in the spiral arms of our Galaxy. The same appears to be true of new stars of low mass, such as the T Tauri stars, and it is now generally accepted that essentially all stars are formed by the gravitational collapse of interstellar clouds in the spiral arms of our Galaxy, and that all types of stars form in groups rather than singly.