Download Slide 1

Supernovae
Lab 9
Let’s go Supernova!
• Stars which are ≥ 8x massive than our
Sun end their lives in a most spectacular
way; they go supernova!
• First, the star swells into a red supergiant
• Then core yields to gravity and begins
shrinking
• As it shrinks, it grows hotter and denser
Sequence of events
• Then a new series of nuclear reactions begin to
occur....temporarily halting the collapse of the
core
• When the core contains just iron, there is
nothing left to fuse and fusion ceases
• In less than a second, the star begins the final
phase of gravitational collapse
• The core temperature rises to over 100 billion
degrees as the iron atoms are crushed together
Core Collapse!
• The repulsive force between the nuclei
overcomes the force of gravity
• This combination, a process called "electron
capture", creates a neutron and releases a
neutrino
• The neutrinos escape from the core, carrying
away energy and further accelerating the
collapse, which proceeds in milliseconds as the
core detaches from the outer layers of the star
• This is called core collapse!
• So the core compresses, but then recoils
Ka-boom!!!
• The energy of the recoil is transferred to the envelope of
the star, which then explodes and produces a shock
wave
• As the shock wave travels to the star's outer layers, the
material is heated, fusing to form new elements and
radioactive isotopes
• The shock then propels the matter out into space
• The material that is exploded away from the star is now
known as a supernova remnant
• All that remains of the original star is a small, superdense core composed almost entirely of neutrons -- a
neutron star
• If the original star was very massive (≥15 or more times
the mass of our Sun), even the neutrons cannot survive
the core collapse...and a black hole forms
Implosion to Explosion!
• Even neutrons sometimes fail depending
on the mass of the star's core. When the
collapse is abruptly stopped by the
neutrons, matter bounces off the hard iron
core, thus turning the implosion into an
explosion: ka-BOOM!!!
Remnant of
Kepler's
Supernova,
SN 1604
Temps of Fusion
• Min temp required for the fusion of Hydrogen is 5 million
degrees
• More protons in nuclei require higher temperatures - to
fuse Carbon requires a temperature of about 1 billion
degrees
• Lighter elements release energy when they fuse and
heavier elements release energy when they split
• As iron "ash" begins to accumulate in the core of the
star, gravity pulls more and more mass into the area of
fusion, which, in turn, goes through all of the steps of
fusion: Hydrogen → helium by the proton chain, He→
carbon by the triple α process, C and He combine into
oxygen, O fuses into neon, Ne into magnesium, Mg into
silicon and Si into iron
The Crab
Nebula is an
expanding
cloud of gas
created by
the 1054
supernova
Animation of a supernova
• http://heasarc.gsfc.nasa.gov/docs/snr.html
Types of Supernovae
• Two distinct types of supernovae -- those
which occur for a single massive star and
those which occur because of mass
transfer onto a white dwarf in a binary
system
• Difference between the two types lies only
in what gets the process started toward
the explosion
Supernovae Classification based
on H
• The presence or absence of a line from
hydrogen
• If a supernova's spectrum contains a
hydrogen line, it is classified Type II,
otherwise it is Type I
A White Dwarf Goes
Thermonuclear – Type I
• A white dwarf star in a binary star system will draw
material off its companion star if they are close to each
other
• Once the in-falling matter from the companion star cause
the white dwarf to approach a mass of 1.4 times that of
the Sun (the Chandrasekhar limit), the pressure at the
center increases so C and O nuclei to start to fuse
uncontrollably
• This results in a thermonuclear detonation of the entire
star
• Nothing is left behind, except whatever elements were
left over from the white dwarf or forged in the supernova
blast
• Among the new elements is radioactive nickel, which
liberates huge amounts of energy, including visible light
Type II
• A much larger star, however, has enough gravity
needed to create T and P to cause the C in the
core to fuse once the star contracts
• The cores of these massive stars become
layered like onions as progressively heavier
atomic nuclei build up at the center
• An outermost layer of H gas, sinks down on a
layer of H fusing into He, the He sinks down into
a layer of He fusing into C, and the C sinks down
to fuse into heavier elements
Type II contd
• These stars go through progressive stages
where the core will shrink, atomic nuclei which
were previously unfusable start fusing then the
core springs back into equilibrium with gravity
• This causes them to be irregular variables each new burst of fusion pushes elements out of
the fusing core into the "stellar envelope“
• This dims the star, and causes gravity to pull
mass back into the fusing core and begin the
cycle over again
Type II subdivisions
• Type II supernovae can be further classified based on
the shape of their light curves into Type II-P and Type II-L
• Type II-P reach a "plateau" in their light curve while II-Ls
have a "linear" decrease in their light curve ("linear" in
magnitude versus time, or exponential in luminosity
versus time)
• This is due to differences in the envelope of the stars.
• II-Ps have a large H envelope that traps energy released
in the form of gamma rays and releases it slowly, while
II-Ls are believed to have much smaller envelopes
converting less of the γ ray energy into visible light
Subdivisions
•
•
Subdivisions according to the presence of other lines and the shape of its
light curve
Type I
- No hydrogen Balmer lines
– Type Ia
- Si II line at 615.0 nm
– Type Ib
- He I line at 587.6 nm
– Type Ic
– Weak or no He lines
•
Type II
- Has hydrogen Balmer lines
– Type II-P
-Plateau
– Type II-L
- Linear
Type Ia
• Observations of Type Ia supernovae
reveal a picture of the cosmic environment
in the way that the width of tree ring
growth indicates the Earth's climatic
environment over time
• Unlike the other types of supernovae,
Type Ia supernovae are generally found in
all types of galaxies, including ellipticals.
Hypernovae
• There has been some speculation that some
exceptionally large stars may instead produce a
"hypernova" when they die
• Here, the core of a very massive star collapses
directly into a black hole and 2 extremely
energetic jets of plasma are emitted from its
rotational poles at nearly light speed.
• These jets emit intense γ rays
Are We Made of Stardust?
• Yes! (recycled, that is)
• The common elements are made through
nuclear fusion in the stable cores of stars
• Supernovae are not stable, so they can
make heavy elements beyond iron that
require more energy to form.
• These are the elements that make up
stars, planets and everything on Earth -including ourselves