If the cloud is dense enough its own gravity will cause it to contract. Just as a compressed gas heats up so does a contracting cloud of gas.

The time it takes for a star to contract down to the size of a star is short. The protostar phase lasts for only about a few hundred thousand to a few million years.

As the contraction continues density increases, which causes gravity to increase, which leads to more contraction, which heats up the protostar to greater temperatures.

The newborn star will have a luminosity and surface temperature now that will change very little over the course of its lifetime. So it stays put on the H-R diagram for a good long while.

There comes a time when all of the available hydrogen in the core is used up and has been converted to helium. The core is then made almost entirely of helium. The nuclear reactions in the core stop, and the core cools.

This causes gravity to take over again (after quite a long respite) it contracts the star which will heat it again. A shell of hydrogen gas surrounding the core will then begin to heat up and reach sufficient temperature and density to start fusing hydrogen to helium.

This shell of burning hydrogen heats up the envelope of the star which increases the outward pressure which expands the envelope. The surface cools and the Star bloats up to become a Red Giant. The surface temperature is low but the star is huge and thus has a large Luminosity. Earth is fried!

The Planetary Nebula and winds from the star will cause the star to loose mass, revealing the hot Carbon/Oxygen core. In the end all that is left behind is just this white-hot cinder (the core of the Red Giant) that is composed of mostly Carbon and Oxygen, with a little hydrogen and helium still mixed in. This is called a White Dwarf.

The White Dwarf will have a mass less than 1.4M, even if the star had 4M when it was born. It's size is about that of Earth (R ~ 6000 km). So it is incredibly dense! If you brought 1 cm³ of white dwarf matter to the surface of Earth it would weight as much as a 1-ton truck.

The electrons in the star are moving very fast, and they are prevented from slowing down (cooling down, losing energy) by a law in quantum mechanics known as the "Pauli exclusion principle". This says that no two particles may share the same quantum states. Once the lower energy states get filled (become degenerate) the remaining electrons must maintain their fast speeds and hence high temperature and pressure. This pressure provides the hydrostatic balance needed to keep gravity at bay.

The atomic nuclei (positive ions) in the star are not degenerate and are able to lose energy (cool). The white dwarf shines as the atomic nuclei cool. There are no nuclear reactions.

After about 10¹⁰ years, the nuclei cool down and the luminosity of the star becomes very low. The star is now what is sometimes called a "black dwarf". The Universe may not be old enough yet for any black dwarfs to exist. The star will still be supported by the electron degeneracy pressure and the size will remain the same.

Most stars are in binary systems and Mass exchange can occur. Two stars in orbit about each other have a "sphere of influence" about them in which their own gravity dominates, but outside that sphere the gravity of the other star becomes important. In essence stars have Roche limits just like those we talked about with planets and the formation of rings. We call these spheres of influence, Roche Lobes.

When one star bloats up into a Red Giant it can fill its Roche Lobe and the matter on the surface can be more gravitationally attracted to the companion star. Matter Flows on to the companion which in turn can now evolve faster due to the increased mass.

If the smaller companion star is a white dwarf (a long since dead star) and the other stars begins to fill its Roche Lobe (i.e., become a Red Giant), matter will flow onto the White Dwarf's surface.

It doesn't fall directly onto the surface but misses and spirals down onto it through an accretion disk.

This explosion lifts some of the white dwarf surface away and brightens the star considerably for a short time. We witness a Nova (Latin for "new star").

The star brightens by a factor of 100 - 10⁶ in a few days or weeks; then fades over months or years. The process can recur.

Type I: No Hydrogen Lines visible in Spectrum, seen in all kinds of galaxies
Type II: Hydrogen Lines Visible, seen usually in the arms of Spiral galaxies only (where there are a lot of massive stars).

There are other differences as well (e.g., shape of light curve).

Previously "normal" star suddenly (~few days) becomes much more luminous (~10¹⁰L)! Rivals entire galaxy in brightness for a few weeks! Fades over months to years.

These stars have very short main sequence lives. A 15M stars lives for only 10⁷ years before turning into a Red Giant.

When the star first runs out of hydrogen to fuse in its core it will behave similarly to lower mass red giants. It will first begin fusion of hydrogen in a shell around the core and the core will heat and fuse helium into carbon. There will also be carbon and helium fusion into oxygen. The star's envelope also bloats out to very large sizes (Supergiants).

The star has enough mass that when the helium supply in the core is exhausted it will shrink further and heat sufficiently to begin fusion of carbon and oxygen into higher elements. Another shell of hydrogen burning will form, and beneath it a shell of helium burning will form.

The supergiant's core will fuse very heavy elements from carbon and oxygen all the way up to Iron.

The star takes on an "onion" like structure with shells of different elements fusing into heavier elements. This all happens over a few million years tops.

Iron is the most tightly bound of all atomic nuclei. Trying to fuse it to heavier elements will cost the star energy. (Endothermic reaction). The star cannot do this and so there will be no more nuclear reactions in the core. It continues to shrink and becomes degenerate (like a white dwarf star) The mass of the core continues to increase as matter from the fusion shells above it falls down.

p⁺ + e^- --> n + : Neutrons are formed!

With no pressure to stop them any longer the outer layers begin to freefall toward the core.

Outer layers bounce of the core and BANG! (actually the neutrinos do most of the work).

• Initially was a 20M star (but a blue supergiant, not red - probably because LMC deficient in heavy elements to cool the surface).
• Neutrinos detected! Explosion mechanism was core collapse + rebound. Neutron star created at least temporarily.
• Total energy (emitted mostly in 1 second): comparable to energy emitted in one second by all normal stars in the observable Universe!
• >99% of energy was in neutrinos, <1% was in the energy of the ejected material, < 0.01% was in visible light!

Supernovae are the most powerful explosions in the Universe (with the possible exception of gamma ray bursters)

Nucleosysnthesis of heavy elements

Heavy elements are made in stars prior to and during explosions. They are then dispersed into interstellar space where they are incorporated into new gas clouds.

From those gas clouds new stars form enriched in heavy elements --> Planets, life.

Cosmic Rays: high energy particles are shot out from the expanding shock wave.

Supernovas occur 1-2 time per century in any given galaxy. The last one seen in the Milky Way was in 1604 (Kepler). Gas and dust hides many from our view.

Neutron Stars were predicted in the 1930s but not observed until 1967 as Pulsars (Jocelyn Bell): Extremely regular bursts of radio waves (e.g., P = 1.3373011 seconds).

So far hundreds have been found in the plane of the Milky Way Galaxy: Pulsars are highly magnetized and very quickly spinning neutron stars.

A rapid pulsar (P = 0.033 sec) sits in the middle of the Crab Supernova Remnant.

NOTE: Every pulsar is a neutron star, but not every neutron star is a pulsar. Might not be "on" or might b pointing away from us.

Gravity becomes so strong at the "surface" that not even light can escape: Black Hole.