Novae and Supernovae - Nova (means new) – A star that dramatically increases in brightness in a short period of time. It can increase by a factor of 10,000.

A nova is actually a white dwarf star undergoing surface explosions that result in rapid, temporary increases in luminosity.

From Earth we can observe two or three novae each year.

Recurrent novae - stars that “go nova” repeatedly. Why?? White dwarfs are stars that are at the end of their stellar evolution; they eventually cool to a black mass in space, if it is an isolated star.

But if it is part of a binary system, and the distance is small enough, the dwarf’s gravity can pull matter (H and He) from its main sequence companion.

A stream of gaseous matter moves from the companion to the dwarf. As material builds up, this gas becomes hotter and denser. Eventually, it reaches 10 7 K, and H-fusion to He occurs.

This is a sudden, brief, extremely luminous explosion as fuel is burned and the rest of the material is thrown off into space. The dwarf then expands, cools, and dims.

The original stream of matter to the white dwarf is pulled into orbit around the equator of the star and forms an accretion disk.

Friction causes the gas to spiral slowly down to the surface, while heating up. The accretion disk gives off X-rays which can be observed.

The point where the matter strikes the accretion disk may form a “hot spot” which can cause light fluctuations.

Supernovae Low mass stars (less than four solar masses) never become hot enough to fuse carbon in the core. A high mass star can fuse H, He, and C, O plus larger elements.

This “burning” rate accelerates over time and the core temperature increases. This is a highly unstable situation.

Fusion in a star occurs in layers. The outermost layer is fusion of hydrogen. Inside hydrogen is a layer of helium fusion followed successively by C, O, Ne, Mg, Si, and finally iron at the core.

As each element is burned to depletion, the core contracts, heats up, and starts to fuse the ash of the previous burning stage. A new inner core forms and repeats the process.

A 20 solar mass star burns Hydrogen for 10 million years, Helium for 1 million years, Carbon for 1000 years, Oxygen for 1 year, Silicon for 1 week, and Iron for less than 1 day.

Once the inner core begins to change to iron, the massive star is in trouble. Fusion involving iron does not produce energy. Iron is like a “fire extinguisher” to the core.

Large quantities of iron shut down the core production of heat. This condition is very unstable. The gravitational pull overwhelms the pressure of the hot gas and the star implodes.

Two things help intensify this implosion: 1) The core temperature rises to 10 billion K, the photon energy is now high enough to split iron into individual protons and neutrons. This is called the photodisintegration of heavy elements in the core.

In less than one second, the collapsing core undoes all of the fusion that has occurred in the past 10 million years. But this splitting of iron requires lots of energy (it is the reverse of the fusion processes in the past). This tends to cool the core and reduces the pressure.

2) The core is all now composed of all simple particles: electrons, protons, neutrons, and photons at high densities. As the core continues to shrink, the protons and electrons are crushed into neutrons and neutrinos.

p + e = n + neutrinos This is called the neutronization of the core. Most of the neutrinos escape into space, carrying energy. The net result is protons and electrons leave the nucleus.

All that is left is neutrons; and the core collapses until all the neutrons come into contact. The star is like a huge atomic nucleus with a density of g/cm 3.

The collapse of the star is slowed by the neutron degeneracy pressure. But by the time the collapse is stopped the star has overshot the equilibrium and may have a density of or g/cm 3 before it begins to re-expand.

The time from the start of the collapse to the “rebound” is about one second. This rebound is an enormously energetic shock wave, at high speed, blasting all the outer layers of the star -including the heavy elements outside the inner core - into space.

One star may outshine the rest of its galaxy for days due to this high-mass star explosion. This is called a supernova.

The exploding star is called a supernova’s progenitor. Novae and supernovae can appear very similar. The difference was not detected until the 1920’s.

A supernova is 1 million times brighter than a nova. The same star can be a nova many times, a star can be a supernova only once.

The material leftover after a supernova is called the supernova remnant. This remnant is usually in the form of hot gases and is called a nebula. It is often in the form of rings.