9.7. UPPER-MAIN-SEQUENCE STARS 131
such star has yet had time to do this! All of the stars on the lower main sequence that have
ever formed are still hissing away, slowly fusing hydrogen to helium, shining dimly compared to
other stars. One such star, Proxima Centauri, is the closest star besides the Sun; yet it is far too
dim to see without a telescope.
9.7 UPPER-MAIN-SEQUENCE STARS
e initial mass of a star decides its eventual fate. And the larger the mass, the faster everything
happens. e most massive stars stay on the main sequence for only a million years or so; the
Sun’s main sequence lifetime is many thousands of times longer than this. But the changes that
upper-main-sequence stars undergo are different in kind as well as degree.
Figure 9.5 shows evolutionary tracks for a variety of stellar masses, from only 0.1 M
@
–
60 M
@
. e figure labels the masses along the ZAMS, where the stars begin their visible lives.
While on the main sequence, all of these stars change very little, drifting roughly perpendicular
to the ZAMS. When they run out of hydrogen in their cores, they all change rapidly; one can see
this rightward edge of the main sequence by the little kink in many of the evolutionary tracks.
e tracks for all of these stars, except for those very low on the main sequence, bring
them to the upper right, either to the giant or supergiant region of the H-R diagram. at is,
after leaving the main sequence they all initially (and relatively quickly) get cooler, larger, and
brighter. But upper-main-sequence stars do this more quickly and to greater extremes.
Notice that the upper-main-sequence stars stay roughly the same luminosity for most
of their visible lives. ey start out hot and bright, and then after leaving the main sequence,
become cooler, but also larger by just about the right amount to stay at roughly the same enor-
mous luminosity. ese very-massive stars are quite rare, because all stages of their lives happen
so quickly. It is hard to catch one in the act of existing! But their enormous luminosities make
them visible at vast distances, and so there are many examples among the common naked-eye
stars.
9.8 STELLAR EXPLOSIONS
Stars on the upper main sequence die not with a whimper, but a bang. After every possible source
of nuclear fusion energy is exhausted, the core of the star undergoes a catastrophic gravitational
collapse, leading to an enormous explosion called a type II supernova. is explosion ejects much
of the outer part of the star at speeds of tens of thousands of kilometers per second. Briefly, a
supernova explosion can emit light equivalent to several billion Suns, making that single star as
bright as a medium-sized galaxy.
ese events are rare in any one galaxy, and in a spiral such as the Milky Way, they occur
mainly in the disk, where dust obstructs our view of all but nearby ones. In 1054 AD, a supernova
was recorded (mostly by Chinese astronomers) that was easily visible in the day time. e Crab
Nebula is seen in telescopes at this same location today; see Figure 9.9.
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