The Lives and Deaths of Stars

• Spacetime Wrinkles  (from NCSA's Expo/Science & Industry)
• Strobel on General Relativity
• Brian "Supernova" Schmidt's Public Lecture

Gravity vs. Pressure

• The life of a star is a battle between:
• Gravity: wants to crush the star into a black hole
• Pressure: wants to blow the star to bits
• When pressure and gravity balance, we have a stable star like the Sun
• Most stars are stable for millions or billions of years, and then
• end up as a "white dwarf" star
• with M ~ 1 solar mass
• radius ~ and Earth radius
• end up as a "neutron star"
• with M ~ 1.4 solar mass
• radius ~ 10 km  (almost a black hole!)
• most of star blown to bits
• black hole
• complete disruption (blown to bits!)

Gravity

• gravity gets much stronger as star shrinks
• compact stars need much greater pressure to support themselves.

Sources of Pressure

• Heat
• radiates away
• that's why the Sun heats the Earth
• since energy is conserved - source of heat must eventually run out
• gravity
• When a star shrinks, it heats up.
• gravitational "potential" energy is converted to heat
• gravity can power the Sun for 10 million years
• Nuclear energy
• This is the power source for most stars
• if M < 0.08 M_sun, core doesn't get hot enough to have nuclear reactions
• most of the energy comes from burning Hydrogen to Helium ( 4H -> ⁴He )
• the Sun will burn for about 10 billion years until the fuel runs out.
• 5 billion years and counting....
• heavier elements can also burn if the star is massive enough
• Iron = nuclear ashes
• energy must be added to change iron to another element
• Degeneracy Pressure
• the only type of pressure that works at T = 0 K
• quantum mechanical
• a gas has discrete states (or energy levels) for electrons just like an atom does
• There are many more states than in an atom
• so the discrete nature is not obvious, except that
• the states can fill up
• because only 1 electron is allowed in each state
• this is called a degenerate gas
• This happens when the gas is very dense - squeezed into a small volume

• the low velocity (or low energy) states fill up first
• as more electrons are added they can only go in high velocity (or high energy) states
• high velocity electrons => lots of pressure
• But, what about the ultimate speed limit = c
• the electrons can't go faster than the speed of light
• degeneracy pressure does not increase fast enough with density when v ~ c
• there is an upper mass limit for a degenerate star M_ch = 1.4 M_sun
• Stars supported by electron degeneracy pressure are called white dwarfs
• the maximum mass of a white dwarf is 1.4 M_sun
• typical radius ~ Earth radius
• Stars supported by neutron degeneracy pressure are called neutron stars
• the maximum mass of a neutron star is 2-3 M_sun
• typical radius ~ 10 km (almost a black hole)
• an object dropping from a large distance to the surface will hit at ~ 0.3c
• The idea of a maximum mass to a stable star (at low temperature) was initially very unpopular
• without quantum mechanics maximum mass would be 0
• gravitational collapse for M > 3 M_sun  => black hole!
• Here's a summary:

• note also the funny  mass - radius relation  for degenerate stars

The Lives of Stars

• stars like the Sun
• burns Hydrogen and Helium
• the Earth gets fried in one of the "red giant" phases

• blows off a planetary nebula like:
•  the Ring and Helix Nebulae
•  the Cat's Eye nebula
•  the Hourglass Nebula
• note the central white dwarf stars
• ends up as a Carbon-Oxygen white dwarf
• this is the fate of stars between 0.6 and 8 M_sun
• Here are some older, cooler  white dwarfs
• low mass stars
• similar to Sun - like stars but no Helium burning
• ends up a Helium white dwarf
• burn slowly - dim stars that live a long time
•  Big Stars  - Life in the Fast Lane!
• M > 8 M_sun
• Much brighter than solar - type stars
• they burn all their nuclear fuel in 50 million years or less
• core is hot enough to  burn down to iron
• but  iron can't burn
•  Stellar cross section  as iron core is about to reach 1.4 M_sun
•  image  of star in the Large Magellanic Cloud shortly before iron core mass = 1.4 M_sun
•  image  of the same star a short time later
• the stellar core collapses quickly
• outer layers are blow off as a supernova (type II)
• supernova explosions are the main production mechanism for elements besides Hydrogen and Helium
• here's  SN 1987A  again
• here's what's left of  SN 1054  (the Crab Nebula)
• What happens to the collapsing core?
• either black hole
• or neutron star
• theoretical calculations can't tell for sure
• some cases are definitely neutron stars = pulsars
• first seen as periodic radio "pulses"
• 1 or more pulses per second
• the  Crab Pulsar  pulses at 30 times per sec. See the "on" and "off" states
• this is the pulsar left over from SN 1054
• rapid pulsing and variability imply that object must be small
• a white dwarf can't spin at 30 times per sec.




• Here's the model:
• the pulsar emits radiation in only a narrow beam (related to the pulsar's magnetic field)
• the pulsar is "on" when the beam rotates to point at us.

• Here's  another view

• Hot news:  Magnetars Discovered !
• neutron stars with very strong magnetic field
• rotation of magnetic field creates electron-positron pairs
• What about  black holes ?
• stars above 20 or 30 M_sun  are expected to form black holes
• (but this is not well understood)
• observed compact objects above 3 M_sun have to be black holes
• what else could they be?
• A final type of  Supernova (type Ia)
• occur in binary star systems
• star 1 ends up as a Carbon-Oxygen white dwarf (like the Sun will do)
• star 2 is slightly lower mass and evolves more slowly
• star 2 enters the red giant phase and swells up
• outer layers of star 2 fall onto star 1
• mass of star 1 approaches 1.4 M_sun
• star 1 collapses
• but, it's not made of iron
• Carbon & Oxygen start to burn to iron
• reaction speeds as the star heats up
• pressure can't stop it at first
• star 1 is completely burned to iron leaving nothing but an expanding cloud of iron gas
• a bomb of standard brightness => can be used to estimate distances.