Stellar Evolution
Stages in the Life of a Star
This set of notes by Nick Strobel covers: stellar evolution and stellar
remnants. Most of these notes will be in outline form to aid in
distinguishing various concepts. As a way to condense the text a bit, I'll
often use phrases instead of complete sentences. I have italicized the
vocabulary terms.
- Stages and timescales depend on mass (little bit of composition
dependence). Massive stars evolve quicker than light stars. Main sequence
lifetime = fuel/consumption rate
Mass/Luminosity. [Note: the symbol
means
``proportional to''. I'll use proportions rather than exact
equations since I don't want to write a bunch of symbols that are constants of
nature. Ex: instead of writing the equation for an angle =
,
I'd write angle
.]
Since we find that
luminosity increases with mass as: L 
,
we have Mass/Luminosity 
MS lifetime. If use Mass in terms of solar
masses, then the lifetime is relative to solar lifetimes (one solar lifetime =
years).
- The basic scheme is: Gas Cloud
Main Sequence Red Giant
(Planetary Nebula or Supernova) Remnant. Details:
- Giant Molecular Cloud--large dense gas cloud (with dust) that is
cold! 100,000's to few million solar masses of material that has fragments of
10's to 100's solar masses that start collapsing for some reason (shock waves,
cool enough for gravity to take over, etc.).
- Protostar--gas clump collapsing and heating up in center as it
collapses. Gravitational energy being converted to heat. Lots of Infrared and
Microwave radiation produced. Gets hot enough to glow red (2000-3000 K), but
gas/dust cocoon blocks visible light. IR and Microwave can pass through dust.
- T-Tauri--starlike object visible to outside. Strong winds eject
lots of material from young star (preferentially along rotational axes). Cocoon
gas/dust blown away. Star starts fusion.
- Main Sequence--star is stable
because of Hydrostatic Equilibrium.
Fusing Hydrogen to Helium in core. Stars spends about 90% lifetime as
main sequence.
- Subgiant, Red Giant, Supergiant--Run out of core fusion fuel.
Hydrostatic equilibrium upset. Core shrinks. Fusion in shell around core stars.
Fusion is very rapid. Luminosity (energy output) increases
gas envelope surrounding the core puffs out. At the bloated out surface, the energy
is spread out over a larger area so each square centimeter will be cooler
red color. Red
giants can eject a lot of mass through ``winds''.
Note: A Red Giant may be large in terms of linear size, but it is less
massive than the main sequence star it came from!
- Core Fusion--core has shrunk enough to create high enough temperatures
to start Helium (or heavier element) fusion. In low mass stars the onset of
Helium fusion can be very rapid, producing a burst of energy-helium
flash. Eventually it settles down. Fusion is releasing more energy than
main sequence stage, so star is bigger, but stable!
- Red Giant, Supergiant--core fuel runs out again. If massive enough,
repeat stage 5. Number of times to do stages 5 --> 7 cycle depends on
mass. Stellar nucleosynthesis of heavy elements.
Interplay of gravity and nuclear fusion.
- Planetary Nebula or Supernova--
outer layers ejected as core shrinks to most compact state. Low mass stars
(
during main sequence) will go the planetary nebula route; high mass stars
(
during main sequence)
will go the explosive supernova route. Supernova explosion happens
because the core has formed a very stiff neutron star and the infalling outer
layers rebound off it (analogy: drop a basketball with a tennis ball on top of
it; see the tennis ball really bounce off the basketball when the basketball
hits the floor).
- Remnant--low mass core (
)
shrinks to white dwarf. Electrons prevent further collapse. Size about
that of Earth. Outer layers are planetary nebula. Higher mass core
(
)
shrinks to
neutron star. Supernova happens when neutron star is created. Neutrons
prevent further collapse. Size about that of a large city. Highest mass core
shrinks to a point. On the way to total collapse it may momentarily create a
neutron star and
the resulting supernova rebound explosion. Gravity finally wins. Nothing holds
it up. Space so warped around the object that it effectively leaves our space.
Black hole! Event horizon radius =
km, where the core mass is relative to the Sun. Details about the
remnants are given in the Stellar Remnants section below.
- Stellar Nucleosynthesis--creating heavier elements from lighter
elements in stars. Heavy elements (heavier than Helium) made in cores of stars
(up to Iron). Elements heavier than Iron are made in supernova explosion. Lowest
mass stars can only synthesize Helium. Stars around the mass of our Sun can
synthesize Helium and Carbon. Massive stars
(
)
can synthesize
Helium, Carbon, Oxygen, Neon, Magnesium, Silicon, Sulfur, Argon, Calcium,
Titanium, Chromium, Iron.
- Cluster color-magnitude diagrams change with age. Main Sequence
Turnoff-mass at that point tells you age of cluster. Assume that all stars in
cluster form at about the same time. Stars slightly heavier than turnoff have
already evolved away from main sequence.
First I'll give a definition of ``degenerate matter'' as used in astronomy.
Then I'll discuss the different types of core remnants.
What I'll call ``remnants'' in this section is the compressed core. The
spewed out gases in supernova explosions and planetary nebula are also sometimes
referred to as ``supernova remnant'' or ``planetary remnant'', but I'll be
focussing on the remaining core left after the spectacular mass loss has
occurred. It is important to remember that what happens to the core depends on
the mass of the core, rather than the original mass of the main sequence
star from which it came, because the only thing left for gravity to really
compress is the core.
- Degenerate matter--very dense matter in a state where the
pressure no longer depends on temperature due to quantum mechanical effects.
Rules: a) Certain permitted energies in a given space; b) Only 2 electrons (for
white dwarfs) or 2 neutrons (for neutron stars) can share the same energy level
in a given volume at one time; c) Closest spacing depends inversely on mass of
particle (electrons further apart than neutrons and protons). Features:
- Resist compression. Electrons or neutrons
locked into place because all
the lower energy shells are filled up and the only way they can move is to
absorb enough energy to get to upper energy shells (hard to do!). To
compress a degenerate gas means to change the degenerate particle motions. But
that requires A LOT of energy. Degenerate particles have no ``elbow room'' and
their jostling against each other strongly resists compression. Gas like
hardened steel!
- Pressure only depends on speed of degenerate particles NOT the temperature. To change speed of degenerate particles requires A LOT
of energy.
Adding heat only causes the non-degenerate particles to move faster, but the
degenerate ones supplying the pressure are unaffected.
- White Dwarfs--if core mass < 1.4 solar masses. Electrons are
degenerate. Mass of Sun compressed to size of Earth. Higher mass compressed to
smaller radius! Neutrons and protons have room to move around freely. Density
g/cm
(one sugarcube > 1 car!). White dwarf cools off from
initial formation temperature of about 100,000 K. Form as outer layers of
red giant star puff out to make planetary nebula.
- Neutron Stars--1.4 < core mass < 3 solar masses.
Compression so great that protons fuse with electrons to form neutrons.
Neutrons are degenerate. About 30 km across! Density
g/cm
(one sugarcube = mass of humanity!). Formed in supernova explosion.
- Pulsars--rapidly rotating neutron stars with STRONG magnetic
fields (
times Sun's). Light flashes with period of milliseconds
at start and lengthening to periods of about 4 minutes over time. Why would
neutron stars be fast rotators? Conservation of angular momentum (remember
spinning ice skater)! Slowly rotating Red Giant star has same angular momentum
as tiny, fast rotating neutron star.
Rotating neutron star only thing that could create that
frequency of pulses. Lighthouse model-strong magnetic field creates
electric field making charged particles (mostly electrons) flow out of the
magnetic poles. As the charged particles spiral around the magnetic field lines,
they produce a non-thermal radiation (synchrotron) beam at the magnetic
poles. If the beam sweeps past Earth, we see a flash of light.
- Radiation production robs energy from pulsar so pulsar rotation slows down (angular momentum does slowly decrease).
- Every now and then, neutron star suddenly shrinks by about 1 mm. Spin
rate suddenly increases to conserve angular momentum. See glitch in pulse rate.
- Black Holes--core mass > 3 solar masses. Gravity finally
wins, compressing core to mathematical point at center. Formed in supernova
explosion. Surface gravity so strong that nothing can escape (not even light!)
within a certain distance from mass point. Boundary is called the event
horizon (or Schwarzchild radius)-no messages of events happening within
radius (= 3 * core mass [in solar masses] km) make it to the outside. Use
Einstein's General Relativity to describe gravitational effects. Gravitational
Redshift & Time Dilation. Signatures of a black hole:
- For binary, look at how the black hole moves visible companion around. Use
Kepler's 3rd law to find sum of masses. Guess mass of visible companion
find unseen
companion's (black hole's) mass.
- For binary, look for X-rays produced in hot accretion
disk--material pulled off visible companion spirals onto black hole. Disk
formed.
Friction increases inward causing increasing temperature as approach the event
horizon-produce wide spectrum of radiation. To make rapidly varying X-rays,
unseen companion must be small!
last update 29 Aug 95
Nick Strobel --
Email:
strobel@astro.washington.edu
(206) 543-1979
University of Washington
Astronomy
Box 351580
Seattle, WA 98195-1580