At birth, most stars in our current galaxy look chemically the same: They are roughly 71% hydrogen, 27% helium, and some 2% heavier elements collectively labeled metals. Deep within their interiors, the pressure, density, and temperature are sufficiently high to achieve nuclear fusion of hydrogen into helium. For most of a star’s life, that and subsequent fusion reactions will generate the internal pressure force needed to counterbalance gravity and keep the star from collapsing on itself.
How brightly and how long a star burns, however, can vary greatly depending on its birth mass. Stars similar to the Sun, say, with a mass less than or equal to twice the solar mass M⊙, live billions of years but undergo only two nuclear cycles—hydrogen burning and helium burning—before dying quietly as carbon- and oxygen-rich white dwarfs.
At the other extreme, stars with mass greater than 8 M⊙ live just...
