Contraction stops - heat from fusion
Contraction stops - heat from fusion
M.S. is longest stage in lifetime
Time in M.S. inversely proportional to (mass) of star:
| Mass | Class | Time in M.S. |
| 30 | O | 2 million yrs. |
| 10 | B | 20 million yrs. |
| 1 | G | 10 billion yrs. |
| 0.2 | M | 200 billion yrs. |
Exponential Relationship
MASS OF STARS
Can use Newton's Laws for eclipsing binaries if period
can be measured, but is difficult (m1 + m2) P2
= r3
If main sequence star - mass - luminosity relation
More massive - greater luminosity "
Energy Transport:
Convection in core only
Core Reactions:
Mainly Proton-Proton chain (P-P Chain) & also CNO
cycle
Both: H - He 92% P-P
8% CNO for Sun
Evolution of Sun-like Stars
Hydrogen in core used up
He core shrinks and heats up
Fusion of hydrogen continues in shell surrounding
contracting core
Core degenerate matter (free electrons)
Meanwhile, outer layers expand greatly - lst red giant stage
Convective outer layers
Shell source contributes to heating of core
Helium core - relatively small
Helium core continues to shrink and heat up until helium ignites
in Helium flash - sudden rapid ignition of core - Triple
alpha reaction: 3He - Carbon
Degeneracy is destroyed - normal matter restored in core
Outer layers retract - hotter - horizontal branch on H-R
Finally He in core exhausted leaving carbon (from Trip. alpha)
He fusion continues in shell
Hydrogen shell may also exist
Star expands and cools - second red giant stage (
1 million yrs.)
Stellar Evolution: 5-10 solar mass
Middle - Upper Main Sequence
Gradually increases luminosity while on m.s. Uses up hydrogen
in core 100,000,000 yrs. Core contracts
Hydrogen shell ignites, star expands to R.G. Core not degenerate
- temp. too high
Triple - alpha (helium fusion) begins w/o helium flash Core temp.
increases - alpha-capture reactions:
Triple alpha - Carbon, Carbon + Helium nucleus (alpha particle)
- oxygen, oxy + alpha - neon. . . . etc. Buildup of heavy elements
in core.
Eventually core is exhausted
Fusion continuous in helium shell - 2nd R.G. stage
Meanwhile, mass lost due to dense stellar winds
Reduce number of alpha-capture reaction stages and overall lifetime
Can end up as white dwarf if has lost enough mass - 1. 4 solar
masses or less Chandrasekhar's Limit
White dwarf will have dif ferent. composition than one f rom I
s.m. star
Contain more heavy elements, oxygen, neon, etc.
If insufficient mass lost, sudden infalling when reactions cease
- Supernova explosion
- neutron star (2-3 s.m. left) or black hole
Massive Stars > 10 s.m
Evolution occurs quickly
Do not become more luminous (move upward on H-R) because are losing
mass due to strong winds
Alpha-captive reactions progress all the way to iron (can go no
further)
Meanwhile, winds can strip envelope to reveal portion of core
- Wolfe-Rayet star: hot star with strong winds apparent, emission
lines
Carbon, nitrogen, oxygen seen-emission lines (from triple alpha
and alpha-capture)
Supernova explosion
Alpha capture reactions progress to iron then stop
Core then collapses rapidly
Outer layers crash inward - massive rebound explosion
or Supernova - driven by flood of neutrinos from rapid nuclear
reactions
Stellar Remnants
Form of remnant depends on mass remaining after red
giant
1.4 s.m. or less - white dwarf
Larger stars - neutron star or black hole:
Neutron star: >1.44 sm, <2-3 sm
Black hole: >2-3 sm
White dwarfs: faint stars showing few very broad
spectral absorption lines
Zeeman effect (doubling of lines due to magnetic
field)
Extreme contraction of W.D. can greatly intensify
magnetic field
Spectral lines shifted toward longer (red) wavelengths
gravitational redshift, as predicted by relativity theory
Solitary white dwarf takes up to several billion
years to completely cool
Outer skin nearly opaque gas - blocks radiation of
thermal energy
Nova Explosions:
White dwarf average 60,000 x brightness increase
Dwarf novae - scaled-down novae, also recurrent
Sudden flare-ups of novae due to degenerate gas present
in W.D. - runaway explosion as in helium flash
Fuel for nova explosion must come from outside source
- other star in binary system
Lines widened due to extreme pressure in atmosphere, and
Recurrent novae - same star goes nova more than once (perhaps
decades apart)
