[4097] Physics 316: Extragalactic Astronomy and Cosmology (Summary of Lectures)

This is supplemental material for [4097] PHY 316 given in 2002 (& will not be the subject of exams unless the background is in red).

Very Brief Overview of Supernovae Classification

Supernovae are classified into several types, based primarily on the shape of their light curves and the elements observed in their spectra. All classes share the characteristics
  • that the core of the star collapses (rapidly)
    (nuclear reactions are unable to proceed at a rate fast enough to conteract gravitational collapse).
    This collapse (& associated release of energy) results in
    • a shock wave that expels the stellar "atmosphere" (at few x 103 km/s)
    • a huge increase in luminosity of the star systm (often equivalent to 109 Lsun)
      • followed by a "slow" decline lasting weeks, months, years
    The ejected material forms a Supernova Remant (SNR) which can remain illuminated (visible) for thousands of years. The core of the star becomes a
    • Neutron star,
    • Black hole
    depending upon its mass.
The classification schemes used are (of course) historicial - but turn out to still be useful. Here are the various main types:
  • Type II
    • Hydrogen & heavier elements seen in the spectrum
    • Due to the collapse of a degenerate Fe core in a star with an initial mass > 8 or 9 Msun
    • Rate is one every 44 yr in our Galaxy
    • Examples:
      • SN1054 ("The Crab Nebula")
        documented by Chinese astronomers
      • SN1987A (in the LMC)
    • Subdivided into two classes based on the shape of their light curve
      • Type II-L - Light decline relatively smoothly
      • Type II-P - Light curves exhibit a "plateau" 1 to 3 months after the peak due to the radioactive decay of 56Ni (t1/2 = 6.1 days) produced by the shock front as it passed through the stellar material
        • decay of other species also seen
          • 57Co (t1/2= 271 days)
          • 22Na (t1/2= 2.6 yr)
          • 44Ti (t1/2= 47 yr)
  • Type I
    • Supernovae that do NOT show prominent Hydrogen in their spectra
      (all the hydrogen has been "burnt")
    • One every 36 years in the Milky Way
    • Examples:
      • SN1572 ("The Tycho SNR")
      • SN1604 ("The Kepler SNR")
    • Subdivided into three main classes
      • Type Ic have weak He lines in their spectra
      • Type Ib have strong He lines in their spectra
      • Type Ia have strong SiII (615nm) lines
Types Ib & Ic
  • Appear to occur only in Spirals
  • Occur near regions of recent star formation
  • Thought to be due to the core collapse of short-lived massive stars (similar to Type II SNe)
Type Ia
  • Occur in all types of galaxies
  • Thought to be due to the "explosion" of a White Dwarf (WD) in a close binary system with a Carbon-Oxygen (C-O) core
    • Most stars are actually binary systems (two stars orbiting their center of mass) In any binary, system, clearly mattter follows its gravitational destiny. The matter is the outer atmosphere of the normal star is strongly influenced of the gravity from both the normal star itself and that of WD. It will be more attracted to either the "normal star", or the (in this case) WD star. In one location the gravitational fields will be equal. What determines what happens to the matter (& under what circumstances) at these points is (again) beyond the scope of this course.
    However under some circumstances matter (ie. mass) can be transferred to the WD
  • The additional mass desposited in this way will disrupt the equilibrium of the WD
    • When the mass of the WD reaches 1.3 Msun "Carbon burning" starts at the center. The increase in temperature does not produce an expansion of the degnerate core that would slow the reaction rate. The front of vigourous Carbon burning therefore moves towards the surface.
      [Carbon deflagration for those who have taken the Stellar course]
    • The subsequent expansion produces a cooling that eventually dampens the nuclear burning, leaving a mixture of partially processed intermediate-mass elements surrounding a Nickel-iron core. The energy release completely disrupts the star resulting in a Type Ia SN
  • The outer layers are expelled at 104 km/s