Page 382 - 9780077418427.pdf

/Users/user-f465/Desktop
tiL12214_ch14_351-376.indd Page 359 9/3/10 6:16 PM user-f465
tiL12214_ch14_351-376.indd Page 359 9/3/10 6:16 PM user-f465 /Users/user-f465/Desktop

happens to it when it uses up the hydrogen in the core depend slower rate. The overall life span on the main sequence ranges
on the mass of the star. Of course, no one has observed a star’s from millions of years for O-type stars to trillions of years for
life cycle over billions of years. The life cycle of a star is a theo- M-type stars. An average one-solar-mass star will last about
retical outcome based on what is known about nuclear reac- 10 billion years.
tions. The predicted outcomes seem to agree with observations
of stars today, with different groups of stars that can be plotted Red Giant Stage
on the H-R diagram. Thus, the groups of stars on the diagram—
The next stage in the theoretical life of a star begins when the
main sequence, red giants, and white dwarfs, for example—are
hydrogen in the core has been fused into helium. With fewer hy-
understood to be stars in various stages of their lives.
drogen fusion reactions, less energy is released and less outward
balancing pressure is produced, so the star begins to collapse.
Protostar Stage The collapse heats the core, which now is composed primarily
The first stage in the theoretical model of the life cycle of a star is of helium, and the surrounding shell where hydrogen still ex-
the formation of the protostar. As gravity pulls the gas of a pro- ists. The increased temperature causes the hydrogen in the shell
tostar together, the density, pressure, and temperature increase to undergo fusion, and the increased release of energy causes
from the surface down to the center. Eventually, the conditions the outer layers of the star to expand. With an increased surface
are right for nuclear fusion reactions to begin in the core, which area, the amount of radiation emitted per unit area is less, and
requires a temperature of 10 million kelvins. The initial fusion the star acquires the properties of a brilliant red giant. Its po-
reaction essentially combines four hydrogen nuclei to form a sition on the H-R diagram changes since it now has different
helium nucleus with the release of much energy. This energy luminosity and temperature properties. (The star has not physi-
heats the core beyond the temperature reached by gravitational cally moved. The changing properties move its temperature-
contraction, eventually to 16 million kelvins. Since the star is a luminosity data point, not the star, to a new position.)
gas, the increased temperature expands the volume of the star.
The outward pressure of expansion balances the inward pres- Back Toward Main Sequence
sure from gravitational collapse, and the star settles down to a
balanced condition of calmly converting hydrogen to helium in After about 500 million years as a red giant, the star now has a
the core, radiating the energy released into space (Figure 14.9). surface temperature of about 4,000 kelvins compared to its main
The theoretical time elapsed from the initial formation and col- sequence surface temperature of 6,000 kelvins. The radius of the
lapse of the protostar to the main sequence is about 50 million red giant is now 1,000 times greater, a distance that will engulf
years for a star of a solar mass (a star with the mass of our Sun). Earth when the Sun reaches this stage, assuming Earth is in the
same position as today. Even though the surface temperature has
decreased from the expansion, the helium core is continually
Main Sequence Stage heating and eventually reaches a temperature of 100 million kel-
Where the star is located on the main sequence and what hap- vins, the critical temperature necessary for the helium nuclei to
pens to it next depend only on how massive it is. The more undergo fusion to produce carbon. The red giant now has he-
massive stars have higher core temperatures and use up their lium fusion reactions in the core and hydrogen fusion reactions
hydrogen more rapidly as they shine at higher surface tem- in a shell around the core. This changes the radius, the surface
peratures (O-type stars). Less massive stars shine at lower temperature, and the luminosity, with the overall result depend-
surface temperatures (M-type stars) as they use their fuel at a ing on the composition of the star. In general, the radius and
luminosity decrease when this stage is reached, moving the star
back toward the main sequence (Figure 14.10).

Beginning of the End for Less Massive Stars
Inward force After millions of years of helium fusion reactions, the core is
of gravity gradually converted to a carbon core, and helium fusion begins
in the shell surrounding the core. The core reactions decrease
as the star now has a helium fusing shell surrounded by a sec-
Core
ond hydrogen fusing shell. This releases additional energy, and
Outward force the star again expands to a red giant for the second time. A star
of expansion from the size of the Sun or less massive may cool enough at this point
energy
that nuclei at the surface become neutral atoms rather than a
plasma. As neutral atoms, they can absorb radiant energy com-
ing from within the star, heating the outer layers. Changes in
temperature produce changes in pressure, which change the
balance among the temperature, pressure, and the internal
FIGURE 14.9 A star becomes stable when the outward forces
of expansion from the energy released in nuclear fusion reactions energy generation rate. The star begins to expand outward
balance the inward forces of gravity. from heating. The expanded gases are cooled by the expansion

14-9 CHAPTER 14 The Universe 359