Nova means new in latin. In astronomy a nova is a star that undergoes a rapid and dramatic brightening.
During a nova outburst a star increases its brightness by 50,000 times or even as much as 106.
During a nova outburst a star will increase its brightness to 105 times that of the sun.
This means that in a few hundred days a nova pumps out as much energy as the sun does for more than 105 years.
There are about 100 nova per year
in our galaxy.
A clue to what occurs in a nova outburst comes from its "light curve" which is a plot of its brightness vs time.
Most nova light curves have the same general shape even if the magnitude of the brightening changes.
The spectra also help. They show stellar-like absorption lines and then, later, emission lines like those in HII regions appear.
Later the Nova will also show rings of material expanding away from the star similar to a planetary nebula.
During the outburst the star can
blow off as much as 10-4 of a solar mass.
The model we currently believe works for nova involves two stars orbiting each other: a binary system.
In this case one star is a red giant, the other is already a white dwarf.
When the red giant expands it gets so big that it fills its "Roche" lobe. This is the surface around the star which demarcates where its gravitational influence ends and where the white dwarf's begins.
Matter from the red giant which is near the roche lobe can flow over to the white dwarf.
Angular momentum conservation again
forces an accretion disk to form.
As material piles up on the surface of the white dwarf the temperature rises (the gas is degenerate) until POW!
A thermonuclear runaway.
A nova outburst
The disk is cleared away and the
whole process starts over again.
A Supernova is a stellar explosion that generates 10 billion times the Sun's luminosity in a very short time.
Supernovae are extraordinary events and have been recorded for millennia. In 1064 the supernova which produced the crab nebula was recorded both by Chinese and Native American astronomers.
It was a supernova that Tycho Brahe saw in 1572. (Kepler also saw one in 1604).
These were the last ones visible in our galaxy!
Supernova are rare: they occur about once every 60 or 40 years depending on the type (there are two types).
On Feb 24 1987 astronomer's dreams came true when a supernova went off in the Large Magellanic Cloud, a small satellite galaxy which orbits our milky way.
This event - called SN1987A or SN87A
was a milestone in modern astronomy
From the form of their light curves, supernovae appear to come in two forms: Type I and Type II.
Type I exhibit a sharp maximum and then have a gradual decline.
Type II have a broader peak at maximum and then decline more quickly.
The spectra of type II supernova
do not show lines of H which means that they come from highly evolved objects.
The basic models for Type I and Type II are:
Type I (puzzle?) - again a binary
model (red giant and white dwarf) where the accreted mass is so high it
pushes the white dwarf over the chandrashekar limit. The core collapses
igniting the carbon in one big burst. The star blows itself to bits and
no remnant remains.
Type II - a massive star burns all its fuel up until iron and collapses.
Lets look at the sequence of burning
12C + 12C -> 24Mg + + energy
12C + 4He -> 16O + energy
16O +16O -> 32S + energy
After this 4He nuclei are added making a succession of heavier elements. Many of these are unstable and decay into Iron (56Fe).
Iron can not fuse with other nuclei
and give up energy.
When iron is created in the core its all over.
The massive outer layers of the star begin to free fall in towards the core.
In less than 1 second neutronization occurs in the core as the density increases.
When the density in this "proto-neutron star" becomes as great as that in nuclear matter the core becomes incompressible. You can't squeeze it any more.
The infalling material hits the core and "bounces" off. A shock wave is sent heading outward into the material falling in!.
The shock wave turns this material
around and propels it outward.
This is the optical supernova. It takes about 1 day to become visible. Why?
The core is also full of neutrinos produced during the neutronization. For a few seconds these can be trapped in the core (they actually help push the shock wave).
Then they zip right out. These carry most of the energy of the supernova!
In SN87A we caught a few (about 20) of the neutrinos. The timing was perfect and confirmed many of the fundamental ideas surrounding supernova theory.
When SN87A went off it lit up the surrounding region and guess what, their was a bipolar outflow there as well.
This was left over from the evolution
of the star before it went supernova. Red Giant (slow wind) -> Blue Giant
(fast wind)
In general Type II supernova leave neutron stars behind.
Can a supernova produce black holes? (Answer not clear)
The fact that there was a blast means a bounce occurred so something was there to bounce against. A black hole would not do this.
The most massive stars should swallow everything.
It may be that exotic particles (Bose-Einstien condensates of strange particles) might soften a neutron star up and allow it collapse.
No neutron star has been found yet
in SN87A.
Elements heavier than Fe are produced
by either slow or rapid neutron capture followed by radioactive decay.
In this way all the elements in the universe are built up from stars and
supernova.
In the wave of supernova a powerful blastwave goes sweeping into the ISM.
The speeds in SNR can be as high as 10,000 km/s.
The shock wave produces the glowing
filaments of gas
