Transcript Stages - A Summary - University of Dayton

Evolutionary Stages
A Summary
Table 20.1 has a very good summary of the stages, time spent at
each stage, Temperatures, densities, and Radii.
STAGE 1
A large volume of a dust/gas nebula begins contracting, falling together
under the influence of the mutual gravitational pull of all the material; as the
cloud gets smaller, fragmentation probably occurs, but the "stellar pieces"
each get smaller and denser, as part of a large cluster of collapsing
fragments. [Note : extended clusters of very young stars are known as
associations and are always found in nebulous regions.]
STAGE 2
As the "cloud fragment" shrinks, the outer "edge" stays cool and much
less dense than the central region, which will get denser and hotter
much faster, although it is still very cold.
STAGE 3
When the central region becomes so dense that its radiation cannot
easily escape, the temperature of the gas/dust mixture increases
dramatically (although the outer "edge" is still very cold); the object's
radiation (from its photosphere) is now detectable and we call such an
"embryonic star" a protostar - a hot gaseous ball, much larger and
brighter than our Sun, that has not yet achieved a balance in its life.
STAGE 4
Even with a central temperature now over 1 million K and a surface a few
thousand Kelvins hot, the inward gravity pull of the slowly collapsing
object is still not quite balanced by the escaping heat generated by the
squeezed gases.
STAGE 5
At an ever-slowing rate, the contraction continues, but at almost
constant temperature, so the size and thus the luminosity decrease,
as the star slowly approaches the stability of the Main Sequence;
this period is called the T Tauri phase, as a vigorous outflowing
"protostellar wind" keeps the surface active.
STAGE 6
Finally the contraction has raised the core temperature to that required to
cause the highly energetic core protons to fuse together in nuclear reactions we can say that "a star is born", although is is not quite exactly stable. The
core temperature is at least 10 million degrees.
STAGE 7
Slight contraction over millions of years finally results in a perfect, continued balance between
outward luminosity and inward gravity. Our star has reached the Zero Age Main Sequence, where it
will stay for over 90% of its life, virtually unchanged externally. [Note : Stars of different masses
experience similar evolutionary tracks on the H-R Diagram, but end up at different points on the
ZAMS; recall that mass => gravity => squeezing => core T => fusion E => luminosity.] Although lowmass stars seem to vastly outnumber their high-mass relatives, a star with too small a mass (<.08
suns) will not have enough "squeeze" in its core to initiate fusion; such objects (termed brown
dwarfs) will be dim and cool and, as they grow older, will only grow dimmer and cooler, ultimately
becoming black dwarfs (see STAGE 14). Astronomers have identified several brown dwarf
candidates, and even have evidence for the presence of Jupiter-like planets in orbit around several
nearby stars. Recently, objects "in between" the two groups have been detected by the extra-solar
planet hunting teams.
A star on the Main Sequence is undergoing (for over 90% of its life) steady core hydrogen burning,
due to its interior structure. While lower-mass M.S. stars (<8 suns, according to your text, although
the exact value is not known) are producing sufficient energy by just using the proton-proton chain
of nuclear reactions, higher-mass stars must primarily rely on the carbon cycle to produce their
greater requirements of energy to balance their greater gravity. The carbon atoms help the hydrogen
atoms to fuse and produce a much greater output of energy, but this causes the star to deplete its
hydrogen supply much faster than the lower-mass stars.
Every star must eventually use up most of its core hydrogen "fuel" and increase its core helium "ash",
until the core energy production diminishes and the star is no longer in equilibrium. As the interior
structure changes, the exterior appearance must also change, and the star is said to evolve "off the
Main Sequence".
STAGE 8
At the point where the
amount of core helium is just
sufficient to reduce the
energy output, the hot core
begins to contract, which
raises the temperature all
through the inside of the
star, and it begins hydrogen
shell-burning, actually
producing more energy than
ever before. The outer
envelope of the star begins
to slowly expand and the
star "leaves" the Main
Sequence region, becoming
larger, brighter, and cooler
as it expands along the
subgiant branch.
STAGE 9
While the star's temperature
doesn't change too much, its
luminosity greatly increases as
more shell hydrogen is fused as
the star's core gets hotter; the
star reaches the red giant
branch of the H-R Diagram; the
small, very dense (termed
degenerate), very hot core is
surrounded by an enormous
"bloated" envelope. As the
temperature of the core
increases, it finally reaches the
ignition temperature of helium
(about 100 million K), and the
core undergoes a "helium
flash".
STAGE 10
The "flash" results in two things : #1
- a hot carbon core begins to form
inside the helium core, and #2 - the
star physically changes, moving to
the hotter, but somewhat dimmer,
horizontal branch of the H-R
Diagram, where it stays for many
years, at a precise place determined
by its mass (sort of a "main
sequence of giant stars"). But the
star continues to change, both
internally and externally, since the
hot carbon core cannot support the
star's mass and soon begins to
contract, increasing the central
temperature and accelerating the
hydrogen- and helium-burning in the
shells -- the star expands even more
and cools as it does so.
STAGE 11
The expansion carries the star up
the asymptotic giant branch into
the red super- giant region; the
star could continue the nuclear
reaction sequence and fuse the
carbon atoms, but its gravity is not
high enough to generate the
temperatures needed (about 600
million K) for this to happen, so it
has essentially reached the end of
its nuclear-burning lifetime, and
death is inevitable.
STAGE 12
As the degenerate carbon core heats
up, the fusion reactions increase in
intensity so much that the outer
envelope continues to swell until it
actually leaves the core - a two-part
object is formed : the central hot
(some have been identified by the
H.S.T. at over 200,000 K)
degenerate core remains in the
center of an expanding cloud of
hydrogen and helium known as a
planetary nebula, which gradually
disperses, recycling its atoms
(mostly hydrogen, but with some
helium and carbon) back into the
nebulae while the core cools and
dims and is finally left all alone...to
die.
STAGE 13
The small, hot, core that remains
is known as a white dwarf. For
stars with masses comparable to
that of the Sun, the composition
will be a mixture of carbon and
oxygen nuclei. It is extremely
dense (degenerate) and
incapable of producing any new
energy output by itself. Thus it
can only slowly cool and fade,
eventually to become ...
STAGE 14
... a cold, dark, dense black dwarf (none of these have probably ever
had time to form - the universe isn't old enough to have allowed this to
happen). This is the expected final end of our Sun and any similar
solitary stars (which includes a high percentage of all stars).
More Massive Stars
M > M‫סּ‬
Very massive stars can go directly to the red giant stage.
They end their life cycle with explosive consequences.
The Final Stages - Summary
End Points of Evolution for Stars of Different Masses
Initial Mass (Solar Masses)
Final State
Less than 0.08
Hydrogen brown dwarf
0.08 – 0.25
Helium white dwarf
0.25 - 8
Carbon-Oxygen white dwarf
8 – 12 (approx)
Neon-Oxygen white dwarf
Greater than 12
supernova
100 Rsun
10 Rsun
1 Rsun
0.1 Rsun