126 9. STELLAR EVOLUTION
us, a particular star stays roughly in the same part of the main sequence for most of its
life. After that, rapid changes ensue, and the star’s luminosity and temperature no longer place
it on the main sequence; we take up those changes later.
9.4.1 MASS AND THE MAIN SEQUENCE
If individual stars spend most of their lives near only one part of the main sequence, why is it
a long band, with many combinations of luminosity and temperature, rather than only a small
region? e answer is that when different individual stars form, they begin their lives with a
combination of temperature and luminosity that places them at different locations along the
main sequence. e key physical attribute that decides where on the main sequence a star will
spend 90% of its visible life is mass:
Stars that form with a relatively large initial mass end up on the upper main sequence,
while those that form with a relatively small initial mass end up on the lower main
sequence.
And so the main sequence is a progression not only of luminosity and temperature, it is
also a sequence of mass. e most luminous and hottest main sequence stars have masses of
roughly 50 M
@
, while the least luminous and coolest have masses of only about 0.1 M
@
.
9.4.2 MAIN SEQUENCE LIFETIME
Whatever is the specific “fuel” for the source of a star’s luminosity, one would expect that a star
of greater mass has more of that fuel. From this isolated observation, one might expect that stars
of greater mass have a longer main sequence lifetime; it should take longer for them to run out of
fuel simply because they have more of it.
But luminosity is equally important, and it turns out to have a far larger effect. e lumi-
nosity is the rate at which the star emits energy. And so a star with a higher luminosity needs
proportionally more fuel to emit light for the same amount of time. Put another way, for a given
amount of fuel to burn, higher luminosity stars burn it faster, and so should have shorter lifetimes.
But we have seen that the higher-mass main sequence stars also have higher luminosities.
So which is the bigger effect? e answer is clear if we note that a star 50 times the mass of
the Sun (50 times as much fuel to burn) has a luminosity one million times greater (it burns that
fuel 1 million times faster). And so the high-luminosity stars on the upper main sequence have
much shorter main sequence lifetimes.
To summarize:
A star high on the main sequence is hotter, much more luminous, more massive,
larger, and has a much shorter lifetimes than a star low on the main sequence.
9.5. EVOLUTIONARY TRACK OF THE SUN 127
9.5 EVOLUTIONARY TRACK OF THE SUN
When a star runs out of its main power source, it changes rapidly. We can follow these changes,
plotting both temperature and luminosity on the H-R diagram as time passes, to make a curving
evolutionary track.
Of course, we can’t use a telescope to literally gather such data for an individual star; for
most stars, we would have to watch it for billions of years. What we observe for a particular
star is simply a point on the H-R diagram, representing what the star is like today (or in the
relatively recent past, depending on the look-back time of the star). But if we understand the
physics of why stars are the way they are, we can use that understanding to calculate what such
a theoretical evolutionary track of the star would look like.
Figure 9.5 shows the theoretical evolutionary track for a star like the Sun, which lies
roughly in the middle of the main sequence. What do I mean by “like the Sun?” It means a
theoretical calculation for a star that had the same chemical composition and, most importantly,
the same mass as the Sun when it first became a main sequence star. We call this the initial mass
of the star.
Mass is the key factor that decides the particular changes a star goes through; composi-
tion is an important but much-smaller secondary factor. For most stars, the mass stays roughly
constant throughout its visible lifetime. We shall see that this is not at all true for the final stages
of the life of most stars, and so evolutionary tracks often do not include those final stages. But
with these caveats aside, the theoretical track of a star is mostly determined by its initial mass.
Figure 9.6 shows a more schematic evolutionary track of the Sun, and the black line labeled
“zero age main sequence” is more affectionately known as the ZAMS (pronounced “zams”). It
represents the theoretical locations on the H-R digram for stars of all masses, just as they arrive
at the main sequence, and it occupies the left-lower edge of the observed broad band of the main
sequence. is represents the temperature and luminosity of a star, given its mass, when it first
becomes stable, fusing hydrogen to helium in its core.
We shall explore in more detail why stars shine in Chapter 15. But the simple answer is
that main sequence stars fuse hydrogen into helium in their cores; this is the source of energy that
allows them to support themselves against their own self gravity, and also to emit an enormous
amount of light over millions or even billions of years.
Upper-main-sequence stars, with their enormous luminosities, fuse that hydrogen rapidly,
and so they run out of it relatively quickly. For the Sun, in the middle of the main sequence, this
core hydrogen burning phase lasts about 9 billion years. During that time it changes very little
on the outside, moving only slightly to the upper right, almost perpendicular to the ZAMS.
But the highest luminosity main sequence stars run out of this hydrogen in their cores in only a
million years.
After the hydrogen runs out in the core, the star changes rapidly, swelling to enormous
size to become a red giant, and moving far to the upper right on the H-R diagram. A star like
the Sun spends about a billion years in this phase, only one tenth its main-sequence lifetime. So
128 9. STELLAR EVOLUTION
Evolution of a 1 M⊙ Star
Main sequence
Subgiant
branch
Red
giant
branch
Red
clump
Instability strip
Horizontal
branch
Planetary nebula
and white dwarf
Asymptotic
giant
branch
Post - AGB
Temperature (K)
Luminosity (L⊙)
10
4
10
3
10
2
10
1
1
10
4
10
3
10
2
10
1
1
3000400050006000700080009000
3000400050006000700080009000
Figure 9.5: Calculated evolutionary track of the Sun, from the moment it begins to undergo
hydrogen fusion to the end of its fusion-powered visible life. (Graphic by Lithopsian—Own
work, CC BY-SA 4.0.)
catching a solar-type star in the act of being a red giant is less likely than finding it as a main
sequence star.
e rest of the changes are far more rapid still, as shown on the diagram. Very briefly, a
star like the Sun will become nearly as large as a red supergiant like Betelgeuse, in a phase called
the asymptotic giant branch (AGB). Such AGB stars are rare, because any individual star spends
very little time (relatively) in this phase.
After the AGB phase, a star like the Sun will become unstable and gently eject its outer
layers, losing much of its mass. e exposed core will eventually contract to a white dwarf, about
the size of Earth. e white dwarf will then gradually cool off, becoming dimmer until it is no
longer visible in telescopes.
9.5. EVOLUTIONARY TRACK OF THE SUN 129
Temperature [K]
Luminosity (L⊙)
Evolution of the Sun
from main sequence to end of fusion
10,000
1,000
100
10
1
0.1
8,000 7,000 6,000 5,000 4,000
Zero Age Main
Sequence
Core hydrogen burning
(9 billion years)
Shell hydrogen burning
Red Giant
(1 billion years)
Core helium ignition
(flash)
Core helium burning
Horizontal branch
(100 million years)
Helium shell burning
Asymptotic Giant Branch
(<1 million years)
Fusion ends
Planetary nebula
Towards white dwarf
Figure 9.6: e evolutionary track of a star like the Sun, showing the zero age main sequence
(ZAMS). e specific path shown is only schematic, and not from precise calculations as in
Figure 9.5. (Graphic by Szczureq—Own work, CC BY-SA 4.0.)
e expanding outer layers are briefly illuminated by ultraviolet light from the contracting,
extremely hot core. And this causes the gas to glow, forming a planetary nebula. Such an object
has absolutely nothing to do with planets, except in a roundabout way. ey were first identified
as a type of object by William Herschel. eir superficial resemblance in a telescope to the planet
Uranus (which Hershel discovered) led him to coin the phrase. See Figure 9.7 for one of the
most famous examples, the Ring Nebula. It is visible in a small telescope as a faint smokey ring.
Planetary nebulae, although tiny compared to the vast Orion nebula, usually have beautiful
symmetric shapes. eir full glory can be seen only if photographed with large high-resolution
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