ASTR220 Collisions in Space by dffhrtcv3


									Class 17: Properties and
evolution of stars

   Properties of stars
   Hertzsprung-Russell diagram
   Evolution of stars of various mass
       Supernovae and Hypernovae
       White dwarfs and neutron stars
       Black holes
I : Stellar properties

   Stars have various…
   Luminosities
       Feeble star = 0.0001 x Lsun
       Powerful star =50,000 x Lsun
   Color
       Depends on surface temperature of star
       Cool star (RED) = 3000K
       Hot star (BLUE) = 30,000K
Don’t bother copying…

   Stellar temperature/color also gives rise
    to “Spectral Classes”
       O (30,000K)
       B (20,000K)
       A (10,000K)
       F (7,000K)
       G (6,000K) – the sun!
       K (4,000K)
       M (3,000K)
   Knowing luminosity and temperature, we can
    calculate the radius using the “Stephan-
    Boltzmann law”

                        L  4 R2T 4
                        LR T  2   4

   Important points –
       =5.6710-8 W/m2/K4 is a number called the
        Stephan-Boltzmann constant
       Luminosity increases very rapidly with temperature
        (2 temperature gives 16 luminosity) and radius
        (2 radius gives 4 luminosity)
II : The Hertzsprung-Russell
   Very important plot in astronomy
   Plot of Luminosity against temperature
    for stars
   Find that stars fall into definite groups…
III : Evolution of stars

   HR diagram gives clues to stellar evolution

   Main sequence
       Consists of stars living the “normal” part of their
       Producing energy via steady hydrogen burning (I.e.
        hydrogen converting into helium)
       Stars of different mass lie at different points on the
        main sequence
   Eventually, the hydrogen “fuel” runs out and
    the stars starts dying… it then leaves the main
Life of a 0.05Msun “star”

   “Star” forms from rotating collapsing
    gas cloud (see formation of Solar
    System in class 3).
   Core heats up to few million K
   Trace Deuterium burns to form helium
   That’s it…
       Temperature never gets high enough to
        initiate hydrogen burning
       So never really becomes a proper star
   Object becomes a “brown dwarf”
   This is the case up to about 0.08Msun
Evolution of the Sun

   Same beginning… cloud collapses

   This time, core is hot enough to initiate
    hydrogen burning (p-p chain)
   Steady hydrogen burning for 10 billion
    years (5 billion years more to go…)
   Then run out of hydrogen in core
       Nuclear reactions slow then stop
       Core gradually collapses; outer parts of Sun
        puff up tremendously – becomes Red Giant
   Series of explosions (Novae) then blow off
    outer layers of Red Giant
   Produces a “planetary nebula”
   Only the core of the star is left – becomes a
    white dwarf
Evolution of a 10Msun star

   Star forms as before
   H-burning much faster (“CNO cycle”)
       only lasts few million years
       When H is exhausted, core contracts gets
        hot enough for helium-burning (makes
       When He exhausted, core contracts and
        gets hot enough for carbon burning
       And so on… until the core is turned into Iron
        (the most stable element)
   Get shell or “onion”
   No more energy
    available when core
    becomes iron.
   Catastrophic core
       Core turns into
        neutron star
       Rest of star ejected in
        a supernova explosion
  Core-collapse (type-II) supernovae

     Very powerful explosion
         1044 J released as radiation
         100 more released in a neutrino pulse

Neutron stars

   The remnant of the explosion…
       Typical mass of 1.5 Msun
       Typical radius of 10km
       Very dense – same density as atomic nuclei
       made of densely packed neutrons
   Extremely properties
       Very strong gravity on surface
       Very strong magnetic fields on surface
       Can spin very quickly (hundreds of times
        per second)… gives rise to pulsars.

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