Transcript Stellar Evolution: 33.2

Stellar Evolution
Bret Betz, Nick Jones, Calvin
Schildknecht
Formation of a Star
• Begins with a Nebula- cloud of gas and dust
http://ftp.seds.org/pub/images/hst/M16WF2.jpg
• the density of one region of gas becomes great
enough it begins to contract into a ball under the
force of gravity.
• Gravitational energy is converted into thermal
energy.
• As mass increases in one region the ball of
gases begins to rotate.
Formation of a Star (cont.)
• Temperature and
pressure increase and as
mass accretion begins
(mass falling to center) a
protostar is formed.
• Protostar- infant quasistar which glows in
infrared spectrum and is
on the fringe of the H.-R.
diagram.
Hertzsprung - Russell Diagram
• Hertzsprung - Russell Diagram – graph of
absolute luminosity (J/s) v. surface
temperature (K)
• Main sequence is where 90% of stars fall
on the H.-R. diagram.
Luminosity versus Brightness
• Hertzsprung - Russell Diagram – graph of
absolute luminosity (J/s) v. surface
temperature (K)
• Main sequence is where 90% of stars fall
on the H.-R. diagram.
Fusion in a Newborn Star
• (Back to the protostar)… mass accretion
stops and a “core” is formed.
• When kinetic energy is sufficiently high,
coulomb repulsion that keeps the
hydrogen nuclei apart can be overcome
and nuclear fusion can take place.
• Hydrostatic equilibrium (outward force of
fusion balances gravitational inward force)
occurs—a star is born.
Fusion in Our Sun
• Proton-proton cycle: four protons fuse to form a 42He
nucleus with the release of g rays and neutrinos.
• Requires temperature of 107 K
• When most of the H has been fused into He in the core,
there is no longer sufficient energy to prevent
gravitational forces from causing the core to heat up and
contract.
• The core contracts while outer regions are pushed out
and a red giant is formed.
• http://zebu.uoregon.edu/textbook/images1/pms.
gif
Fusion in Our Sun (cont.)
• When the temperature in a red giant
reaches 108 K (due to the core
contracting) He nuclei begin to undergo
fusion themselves.
• 3 42He  126C
• This is known as nucleosynthesis
• The star moves to the horizontal branch of
the H.-R. diagram.
Fusion and Nucleosynthesis
• Nucleosynthesis – the formation of heavy
nuclei from lighter ones by means of
fusion.
• Cannot fuse nuclei heavier than 5626Fe and
56 Ni…yet.
28
• The star fuses the remaining elements and
cools to become a white dwarf.
What Else Can Happen to a Red
Giant?
• If more massive stars, more elements can be
fused (C-Ne-O-Si–Fe) , red giant becomes
larger.
• Fe core is formed and when the temperature
gets hot enough, photodisintegration (nuclei
break up) occurs, and the core collapses.
• 1s = core the size of earth to 10km radius of
neutrons.
Inverse beta decay- P + e- = n (gravity)
• Outer layers, strike core, rebounds = supernova
explosion.
After Supernova
- Heavier elements are created.
- Large fraction of star is disbursed,
enriching the universe.
- Most known elements are created.
Remnants of Supernova
• After supernova, depending on mass:
• Neutron Star- theoretical object neutrons,
which has large B-field, very high mass.
• Black Hole- (discussed earlier)
Bibliography
• http://zebu.uoregon.edu/textbook/images1/
pms.gif
• http://ftp.seds.org/pub/images/hst/M16WF
2.jpg