Outline

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Outline

• Novae (detonations on the surface of a star)
• Supernovae (detonations of a star)
• The Mystery of Gamma Ray Bursts (GRBs)
• Sifting through afterglows for clues

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Stellar Explosions
Novae
White dwarf in close binary system
WD's tidal force stretches out companion, until
parts of outer envelope spill onto WD. Surface
gets hotter and denser. Eventually, a burst of
fusion. Binary brightens by 10'000's! Some gas
expelled into space. Whole cycle may repeat
every few decades gt recurrent novae.
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Nova V838Mon with Hubble, May Dec 2002
4.2 pc
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Death of a High-Mass Star
M gt 8 MSun Iron core Iron fusion doesn't
produce energy (actually requires energy) gt core
collapses in lt 1 sec.
T 1010 K, radiation disrupts nuclei, p
e gt n neutrino
Collapses until neutrons come into contact.
Rebounds outward, violent shock ejects rest of
star gt A Core-collapse or Type II Supernova
Such supernovae occur roughly every 50 years in
Milky Way.
Ejection speeds 1000's to 10,000's of
km/sec! (see DEMO) Remnant is a neutron star
or black hole.
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Binding Energy per nucleon
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Example Supernova 1998bw
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Cassiopeia A Supernova Remnant
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A Carbon-Detonation or Type Ia Supernova
Despite novae, mass continues to build up on
White Dwarf.
If mass grows to 1.4 MSun (the "Chandrasekhar
limit"), gravity overwhelms the Pauli exclusion
pressure supporting the WD, so it contracts and
heats up. This starts carbon fusion everywhere
at once. Tremendous energy makes star explode.
No core remnant.
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Supernova 1987A in the Large Magellanic Cloud
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SN 1987A is evolving fast!
1998
1994
Light from supernova hitting ring of gas,
probably a shell from earlier mass loss event.
Expanding debris from star. Speed almost 3000
km/sec!
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A Young Supernova
SN 1993J Rupen et al.
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In 1000 years, the exploded debris might look
something like this
Crab Nebula debris from a stellar explosion
observed in 1054 AD.
2 pc
Or in 10,000 years
Vela Nebula debris from a stellar explosion in
about 9000 BC.
50 pc
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Remember, core collapse (Type II) and
carbon-detonation (Type I) supernovae have very
different origins
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Supernova light curves
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Making the Elements
Universe initially all H (ps and es). Some He
made soon after Big Bang before stars, galaxies
formed. All the rest made in stars, and returned
to ISM by supernovae.
Solar System formed from such "enriched" gas 4.6
billion years ago. As Milky Way ages, the
abundances of elements compared to H in gas and
new stars are increasing due to fusion and
supernovae.
Elements up to iron (56Fe, 26 p 30 n in
nucleus) produced by steady fusion (less abundant
elements we didnt discuss, like Cl, Na, made in
reactions that arent important energy makers).
Heavier elements (such as lead, gold, copper,
silver, etc.) by "neutron capture" in core, even
heavier ones (uranium, plutonium, etc.) in
supernova itself.
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Clicker Question
What is the remnant left over from a Type Ia
(carbon detonation) supernova? A a white dwarf
expanding shell B a neutron star expanding
shell C a black hole expanding shell D no
remnant, just the expanding shell
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Clicker Question
What is the heaviest element produced by stellar
nucleosynthesis in the core of a massive star?
A Hydrogen B Carbon C Iron D Uranium
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Clicker Question
All of the following atoms have a total of 4
nucleons (protons or neutrons). Which of the
following has the smallest mass? A 4 hydrogen
atoms B 2 deuterium atoms C 1 tritium atom
and 1 hydrogen atom D 1 Helium atom E None of
the above, they all have the same total mass
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Final States of a Star
1. White Dwarf If initial star mass lt 8
MSun or so. (and remember Maximum WD mass is
1.4 MSun , radius is about that of the
Earth) 2. Neutron Star If initial mass gt
8 MSun and lt 25 MSun . 3. Black Hole If
initial mass gt 25 MSun .
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Pulsars
Discovery of LGM1 by Jocelyn Bell and Tony Hewish
(Cambridge) in 1967. Nobel Prize to Hewish in
1974. Pulse periods observed from 0.001 sec to
10 seconds - DEMO Explanation "beamed"
radiation from rapidly spinning neutron
star. Usually neutron stars are pulsars for 107
years after supernova.
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The Crab Pulsar
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Neutron Stars
Leftover core from Type II supernova - a tightly
packed ball of neutrons.
Diameter 20 km only! Mass 1.4 - 3(?) MSun
Density 1014 g / cm3 ! Surface gravity 1012
higher Escape velocity 0.6c Rotation rate few
to many times per second!!! Magnetic field
1012 x Earth's!
A neutron star over the Sandias?
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An Isolated Neutron Star
T 2 million K Size 30 km
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The Lighthouse model of a pulsar
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Pulsars are incredibly accurate clocks!
Example period of the first discovered
"millisecond pulsar" is P
0.00155780644887275 sec It is slowing down at a
rate of 1.051054 x 10 -19
sec/sec The slowing-down rate is slowing down at
a rate of 0.98 x 10 -31
/sec
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Multi-wavelength observations of Pulsars
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Pulsar Exotica
Binary pulsars two pulsars in orbit around each
other. Einstein predicted that binary orbits
should "decay", i.e. the masses would spiral in
towards each other, losing energy through
"gravitational radiation". Confirmed by binary
pulsar.
Curve prediction of decaying orbit. Points
measurements.
year
Planets around pulsars A pulsar was found in
1992 to have three planets! Masses about 3
MEarth, 1 MEarth, and 1 MMoon !
Millisecond pulsars periods of 1 to a few msec.
Probably accreted matter from a binary companion
that made it spin faster. Gamma-ray Bursts
some pulsars produce bursts of gamma-rays, called
Soft Gamma-Ray Repeaters or SGRs
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Time history of the 4 confirmed SGRs
Woods Thompson 2004
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Soft Gamma-Ray Repeaters

• Eiso a few1044 erg in gamma-rays

Where does this energy come from?
X-ray image
- Accretion? No sign of a disk - Rotation?
Not enough energy available - Magnetic fields?
Yes
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Clicker Question
What is our basic model for a pulsar? A a
rotating white dwarf B a rotating neutron
star C a rotating black hole D an oscillating
star
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Clicker Question
What is the diameter of a 2 Msun neutron star?
A 20 km B 2000 km C 14,000 km (size of the
Earth) D 1,400,000 km (size of the Sun)
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An early gamma ray-burst
Vela satellite
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A Gamma Ray Burst Sampler
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Great debate 1967-1997
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Bepposax Satellite
GRBM 40-600 keV WFC 2-30 keV NFI 2-10 keV
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X-Ray Afterglow from GRB 971214
t6.5 hrs
t12.5 hrs
t54 hrs