Supernova

1
Supernova

• Physical Astronomy
• Professor Lee Carkner
• Lecture 17

2
High Mass Stars

• Very high mass stars (Mgt8Msun) are rare and also
  can have extreme properties
• Have rapid, dramatic post main sequence evolution
• Go supernova
• producing very heavy elements

3
Luminous Blue Variables

• T 15000-30000 K, L 106 Lsun
• Occupy an instability zone on the HR diagram
• What accounts for mass loss and variability?
• Pulsation

4
Wolf-Rayet Stars

• Hot, bright stars with rapid rotation and mass
  loss
• Have strong emissions lines of different elements
• WN He and N
• WC He and C
• WO O
• from C He burning

5
High Mass Evolution

• Once high mass stars are off the main sequence,
  they have different post MS stages than low mass
  stars
• Then a Wolfe-Rayet stage
• Ends life with supernova

6
Types of Supernovae

• Nova just means new star
• A supernova is a very bright nova
• Accretion onto white dwarf causing core collapse
  (Type Ia)
• Core collapse of high mass star (Type Ib, Ic, and
  II)

7
Classification

• Supernovae classified by spectral features and
  light curve
• Must be from stars that have lost their outer
  layers
• Type Ib have strong He
• Type II have strong H
• Type II-L have a more rapid drop off

8
Core Collapse

• Excluding the Type Ia, we can refer to the rest
  as core collapse supernova
• Generates about 1046 J of energy
• mostly in the form of neutrinos

9
The Core

• High mass stars burn many elements and have
  onion-like shells of material
• As the fusion products move towards iron, the
  energy released per nuclei decreases
• Iron cant be burned, so star cant stay in HSE

10
Core Processes

• The iron burning core is so hot the photons have
  enough energy to destroy nuclei
• T and P are also high enough that the protons can
  fuse with electrons, producing neutrons
• Cores ability to support outer layers drops
  rapidly
• Removing electrons decreases electron degeneracy
  pressure

11
Explosion

• We can try to model supernova on computers
• hard to do
• This causes the collapse to rebound and send a
  shock wave out
• A neutrinosphere forms behind the shock
• Blasts atmosphere away and produces 1036 W of
  radiation
• about billion suns

12
Supernova 1987A

• Located in Milky Way satellite the Large
  Magellanic Cloud
• Progenitor was 12th magnitude blue supergiant

13
End Products

• If initial star was smaller than about 35 Msun,
  core will form a neutron star
• Remains of atmosphere and shock wave form
  supernova remnant
• Remnant can also collide with ISM causing
  emission and triggering star formation

14
Radioactive Decay

• The decay of these isotopes add energy to the
  supernova and effect the shape of the light curve
• Most important reaction is 56Ni decaying to 56Co
  and then 56Fe
• With half life of 6.1 days and 77.7 days

15
Half-Life

• How much energy is released by decay?
• Decay goes as
• N(t) N0e-lt
• Where N0 is the initial number of atoms and N is
  the number at some time t
• l (ln 2)/t1/2
• where t1/2 is the half-life (time for ½ of atoms
  to decay)

16
Light Curve Slope

• The magnitude of the supernova is proportional to
  l
• dMbol/dt 1.086l
• For example supernova 1987A has a light curve
  after 200 days well matched by 56Co and 57Co

17
Elemental Abundances

• H and He are the most plentiful
• Li, Be, B underabundant
• Some elements (C, O, Ne, ) are abundant because
  the are created in post-main sequence stars
• Fe created in supernovae cores

18
Next Time

• Test 2
• Same structure as test 1
• 8 multiple choice, 4 problems
• For Friday
• Read 16.1-16.5
• Homework 16.1b,c,d, 16.4