Transcript Type II

SUPERNOVAE
The Brightest Lights
in the Universe
By Michael Davis
So, exactly what is a supernova anyway?
Main Entry: su·per·no·va
Pronunciation: "sü-p&r-'nO-v&
Function: noun
Etymology: New Latin
1 : the explosion of a very large star in which the star
may reach a maximum intrinsic luminosity one billion
times that of the sun
2 : one that explodes into prominence or popularity;
also : SUPERSTAR
Merriam-Webster Online Dictionary
Supernovae come in two main types. Astronomers
have cleverly named them Type I and Type II
supernovae.
What observable difference caused astronomers to
divide supernovae into two groups?
•Type I: supernovae DON’T HAVE hydrogen
absorption lines in their spectrum.
•Type II: supernovae HAVE hydrogen absorption lines
in their spectrum.
This is a small but important difference. More on this
later.
Type I supernovae are further subdivided into three
types that astronomers have cleverly named types Ia,
Ib, and Ic.
What observable differences led astronomers to
subdivide type I supernovae into three types?
•Type Ia: no hydrogen lines, no helium lines, strong
silicon lines.
•Type Ib: no hydrogen lines, strong helium lines.
•Type Ic: no hydrogen lines, no helium lines, no
silicon lines.
So including Type II, there a total of four types of
supernovae recognized by astronomers.
But Wait! There’s more. It turns out that Types Ib and
Ic are actually just special cases of Type II
supernovae. So in reality, there are really only two
types of supernovae after all. More on this later.
Supernovae were first classified into four types based
on their spectra alone before astronomers actually
understood the mechanisms that produce them.
Technology and theory have evolved since then, and
we now (think) we know what causes all supernovae,
and we now know that there are really only two types.
The reason that there are two different types of
supernovae is because there are two different
mechanisms that lead to supernovae explosions.
Before we can understand the two different
mechanisms that lead to supernova explosions, we
first need to understand how stars work. We need to
know how different mass stars live, and how they die
in order to understand supernovae. So lets review how
stars work.
A star is just a big ball of Hydrogen gas. All stars start
out made of essentially the same stuff. Mostly just
Hydrogen gas with a little Helium and traces of other
elements thrown in.
The only thing that separates one star from another
and determines how long it will live and how it will die
is its mass.
•Low mass stars live a very long time and die quietly
without producing supernovae.
•Middle mass stars live shorter lives and may, or may
not end in Type Ia supernovae.
•High mass stars live very short lives that almost
always end in Type II Supernovae.
No matter their mass, all stars initially produce their
energy through the same mechanism; Nuclear Fusion
of Hydrogen into Helium.
Low mass stars, < 0.3 Sol, never make it beyond the
Hydrogen burning phase and never produce
supernovae.
Heavier stars, 0.3 – 3 Sol, move on to the next stages
of nuclear fusion. Helium “ash” builds up until
sufficient heat and pressure exists to initiate Helium
burning which produces Carbon and Oxygen.
Once Helium burning begins, the star swells into a red
giant and begins pulsating. Over time, much of the
star’s atmosphere is blown off into space by these
pulsations forming a planetary nebula and leaving
behind the naked Carbon/Oxygen core of the star as a
white dwarf.
The white dwarf left behind at the end of the planetary
nebula phase has a mass somewhere between 1/3
and 1.4 solar masses and is made almost entirely of
Carbon and Oxygen. A newly born white dwarf is quite
hot. So hot that it radiates most of its energy in the xray and ultra-violet regions of the spectrum. Over
hundreds of billions of years the white dwarf cools and
will eventually become a black dwarf.
White dwarves contain the mass of an entire star in a
sphere only about the size of the Earth. The density
and surface gravity are extremely high. The density is
so high that white dwarves are made of degenerate
matter.
Degenerate matter can’t support itself above 1.4 solar
masses.
White dwarves are quite common. The universe is full
of them. Globular clusters are full of them, as are
galactic halos.
Probably the best known and probably closest white dwarf to
Earth is Sirius B, only 8.6 LY away.
Since white dwarves are so common, it’s obvious that
some can be found in close binary systems. When
the second star in the system enters the red giant
phase, it’s possible that the white dwarf can skim off
material and increase in mass.
Now we have the potential for a supernova explosion.
The degenerate matter of the white dwarf can’t
support itself if its mass exceeds 1.4 solar masses.
As the white dwarf skims mass away from its
companion it gets closer and closer to this limit. Once
the limit is exceeded, run-away nuclear fusion of
Carbon and Oxygen into Iron begins. The reaction is
so violent that the white dwarf is totally destroyed in a
colossal explosion. This is a type Ia supernova
explosion.
Type Ia supernovae are the most powerful type of
supernova explosion. Also, since all type Ia
supernovae begin with a 1.4 solar mass
Carbon/Oxygen white dwarf, they are all remarkably
uniform in spectrum and absolute brightness.
Since all type Ia supernovae are essentially identical,
they are used by astronomers as “standard candles”
for measuring the distance to distant galaxies, and for
measuring the expansion rate of the universe.
Type Ia supernova 1994D in NGC4526 as seen by HST.
Sn 2006 X in M100 a type Ia supernova
The origin of Type II supernovae is different. They
originate in the death throws of massive stars.
A star with 10 or more times the mass of the sun will
go beyond burning Helium into Carbon and Oxygen.
The heat and pressure in the core of a massive star
will ignite the Carbon and Oxygen “ash” in the core
before it builds up to the critical 1.4 solar mass point.
The Carbon and Oxygen will quietly fuse into Neon
and Magnesium. Then the Neon and Magnesium will
fuse in to Silicon and Sulfur. Eventually the Silicon and
Sulfur will build up to the point where it ignites and
begins fusing into Iron.
The core of a massive star begins to look like the
layers of an onion with all the different shells of fusion.
The “onion” layers in a massive star. (Not to Scale.)
Iron is the end of the road for nuclear fusion within the
star. Iron sits at the very top of the Curve of Binding
Energy. Elements lighter than Iron can be fused into
heavier elements with a release of energy. Elements
heavier than Iron can be fissioned into lighter elements
with a release of energy. However, there is no nuclear
reaction that can get energy out of Iron.
The Iron is truly inert, and simply builds up in the core
of the star.
Since the Iron is not producing any energy to support
itself against gravity, once the critical 1.4 solar
masses of Iron have built up, electron degeneracy
can no longer support the core. The core collapses
into a neutron star.
In a fraction of a second, the roughly 1000 mile
diameter Iron core collapses into a 10 mile diameter
neutron star. Protons and electrons in the atoms are
squeezed together until they merge to form neutrons
and neutrinos. The neutrinos escape the core
collapse taking away most of the energy of the
collapse with them. The neutrons left behind aren’t
compressible, and so the collapse stops.
The newly formed neutron star is actually overcompressed. The inertia of the collapse has
compressed the ball of neutrons to a higher density
than it wants to be, so it rebounds in a tiny fraction of
a second. The rest of the massive star is trying to fall
into the void left by collapsing Iron core. The
rebounding neutron star slams into in-falling material
and drives it back outward with tremendous force.
The core rebound creates a shock wave that begins
moving outward through the star tearing it apart.
Neutrinos boiling out of the collapsed core (about
10^58 in only a few seconds) impart extra energy to
the shock wave, accelerating it outward with even
more energy.
When the shock wave reaches the surface of the star it
explodes as a Type II supernova. The outer layers of
the star are blown into space at a large fraction of the
speed of light.
So much energy is available that it drives many nuclear
fusion reactions that are not normally possible. Heavy
elements beyond Iron on the Periodic Table are created
in massive quantities and blown out into the interstellar
medium by the force of the explosion, thus enriching
the universe in heavy elements.
Sn 1987 A in the LMC a type II supernova. Before
and after pictures.
“One day in 1987, an astronomer by the name of Ian
Shelton was making observations of the Tarantula
Nebula in the Large Magellanic cloud at Las Campanas
Observatory, and he noticed something strange on his
image. After seeing this, he did something that
astronomers haven't done for some time, he went
outside and looked up.” – rochesterastronomy.org
Sn 1987 A peaked at magnitude 2.7, easily visible to
the naked eye.
Sn 1987 A was the closest and brightest supernova in
400 years. It rapidly became the most studied object in
the sky.
The neutron star left over at the center of a type II
supernova explosion is amazingly dense. It packs at
least 1.4 times the mass of our sun into a sphere only
a few miles across. Everything about neutron stars is
extreme.
•The surface gravity is in the Billions of G’s
•The magnetic field is in the Billions of Gauss
•The spin rate can be up to 38000 RPM
•The temperature is 100 Million degrees K.
Being made almost entirely of neutrons, a neutron star
is essentially one giant atomic nucleus.
As mentioned earlier, types Ib and Ic supernovae are
believed to really be just special cases of type II
supernovae. To review:
•Type Ib: no hydrogen lines, strong helium lines.
•Type Ic: no hydrogen lines, no helium lines, no
silicon lines.
In type Ib supernovae the outer Hydrogen envelope of
the star has been blown away by strong stellar winds
before the explosion
In type Ic supernovae, both the Hydrogen and Helium
layers have been blown away before the explosion.
Otherwise, types Ib and Ic supernovae are the same
as a classic type II supernova.
The debris from supernovae explosions continue
expanding at high speed into space for millions of
years, creating a large, glowing nebula. The initial
glare of the Supernovae will fade, but then often
brighten again months or years later as debris moving
at high speed away from the center of the explosion
slams into clouds of material shed by the progenitor
star earlier in its life, or dense clumps of gas in the
interstellar medium. These collisions energize the gas
and make it glow.
Supernovae also re-brighten due to light echoes. Light
from the initial explosion illuminates clouds of dust in
an expanding sphere as the light of the explosion
moves outward into the universe.
The Veil Nebula is an older supernova remnant.
Over billions of years, the debris from supernovae
explosions enriches the universe in heavy elements.
All elements heavier than Lithium were created inside
stars and spread throughout the universe by
supernovae explosions.
The shock waves from supernovae explosions may
also provide the catalyst needed to cause the
collapse of galactic gas clouds to form new stars.
Those new stars will also be enriched in heavy
elements by the passage shock wave allowing solid
planets like Earth to form in their orbits.
So, as Carl Sagan used to say, “We are all made of
star stuff.”
We wouldn’t exist without supernova.