Transcript 12-stellar-evolution

Stellar Evolution
Chapters 12 and 13
Topics
• Humble beginnings
– cloud
– core
– pre-main-sequence star
• Fusion
– main sequence star
– brown dwarf
• Life on the main sequence
• Retirement
– low mass stars (<10 solar masses)
– high mass stars (>10 solar masses)
H-R Diagram
Mass related to luminosity
• For binary stars, that we
can reliably measure their
masses and luminosities,
graph luminosity vs. mass
• HUGE changes in
luminosity correspond to
small changes in mass -power relationship!
• L ~ M4 for main sequence
stars
So how do stars grow?
How do we know?
• Develop computer models and theories
based on physics
• Compare observations with predictions
• Although changes to stars generally occur
over large time scales, there are enough
stars that we occasionally see a change
occur (like novae and supernovae)
“Oh, honey, let’s have a baby...”
• Cloud of dust and gas
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–
–
–
–
mostly gas
lots of hydrogen
diameter 10,000Ds.s.
density <~ 1000 atoms/cm3
in equilibrium
Milky Way
Milky Way in infrared (COBE)
Emission Nebulae
M20: Trifid Nebula (900 pc)
Barnard 68: Dark Nebula
Horsehead Nebula in Orion
Eagle Nebula in M16
“Pickles and Lamaze”
• Internal temperature and
pressure increases
– A shock wave likely produced by a
• Gravitational Collapse
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–
–
–
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nearby nova or supernova disturbs
the cloud.
The cloud is no longer in
equilibrium.
Local regions of higher density.
Some of the dust and gas get close
enough to each other that the
gravitational force is significant
enough that they collide and begin
to clump.
dense cores form
these cores are protostars
– loss of gravitational energy
results in a gain of kinetic energy
and thermal energy
– temperature and pressure at the
core increases
– rate of collapse slows down
– continues to contract although at
a slower rate
– pre-main-sequence star
• fusion
“The water breaks!
A star is born!”
– as the star contracts, the
temperature and pressure at
the core increase
– high temperature allows fusion
to take place
– most common type of fusion at
this stage is the proton-proton
chain; six hydrogen atoms
yield one helium and two
hydrogen atoms
– mass is transformed into
energy (E=mc2)
• equilibrium
– temperature and pressure
increase in the core
– the outward pressure
balances the inward
gravitational force
– star is in hydrostatic
equilibrium
– main-sequence star
“Fat stars die young”
• A greater mass star requires a greater
pressure to achieve equilibrium.
• Greater mass stars are thus hotter.
• M - L relationship!
• The more massive stars “burn” energy (i.e.
convert hydrogen to helium) at a much
higher rate.
• More massive stars die younger.
“When the birth goes wrong”
• What if the temperature of the star is not high
enough for fusion to begin?
– miscarriage: brown dwarf
– brown dwarfs are different from planets in how they
form
– they have approximately the same mass of large Jovian
planets (gas giants)
– hard to detect; looking for lithium is one way
– we define a brown dwarf as having mass 10-80 Jupiter
masses
Adolescence to adulthood
• The star is on the main sequence.
• It continues to convert mass to energy by
the process of fusion.
• The more massive stars will “burn out”
sooner.
• So which stars on the H-R diagram are
younger?
H-R Diagram
Two retirement plans
• what happens next depends on the star’s mass
• low mass stars (~<10 solar masses when on the
main sequence)
– red giant
– planetary nebula
– white dwarf (<1.4 solar masses)
• high mass stars (~>10 solar masses when on the
main sequence)
– red giant
– Type II supernova
– neutron star or black hole
Low mass stars
• evolve from main sequence stars to red giants as it
exhausts its hydrogen supply in its core for fusion
and subsequently cools
• as it cools, its outer layers expand to form a
planetary nebula
• its core contracts until reaching equilibrium
• the core is so small and so dense that electrons
cannot be packed closer together
• it is a white dwarf; a corpse
• stable for M<1.4 solar massses (the
Chandrasekhar limit)
High mass stars
• If at the end of a star’s life, the mass of its core is
greater than 1.4 solar masses, the pressure due to
the “electron gas” is not great enough to balance
gravitation.
• It undergoes further collapse until it reaches a new
equilibrium where the pressure of a neutron gas is
great enough to counteract gravitation.
• It is a neutron star.
• For M > 2 or 3 solar masses, even a neutron gas
cannot withstand the gravitational forces.
• For these masses, it becomes a black hole.
Novae
• A binary system of a white dwarf and red giant.
• The high gravitational force of the white dwarf
attracts loosely held matter from the outer surface
of the giant.
• As the matter accretes onto the white dwarf, its
temperature increases.
• When fusion begins, the outer layer of the dwarf
explodes.
• Process can be repeated over and over.
• Luminosity can be 10 or 100 times the luminosity
of the Sun.
Supernovae
• Type I
– a white dwarf increases enough mass to exceed 1.4
solar masses
– the entire star and core explode
– nothing is left
• Type II
–
–
–
–
death of a massive star (blue or red giant)
core rapidly collapses, mass exceeds 1.4 solar masses
explosion
birth of a neutron star (or pulsar)
Crab Nebula - supernova remnant from 1054 A.D.
SN 1987A
Summary
• Gravitation births stars in clouds
• Gravitation kills massive stars through in
supernovae explosions.
• Fusion generates heavier elements.
• Supernovae expel dust and gas back into the
interstellar medium, only to form stars
again.