Stars form as a cloud of gas collapses. Gravity packs the gas closer and closer together. This raises the temperature and eventually the gas is hot enough for nuclear reactions to start. The proton-proton chain converts H to He. The star is now on the main sequence of the H-R diagram. The H burning takes place in the core initially. The fraction of H in the core was initially 75% but in the sun, this is now decreased to 30% or so as He has been created.

The Sun was originally a little bit smaller than it is today. This is because He contains less particles than H so gravity can compress the core. This raises T which generates a higher pressure out from the core. This causes the outer layers to expand slightly (core and outer layers behave quite separately in stellar evolution). H burning starts to occur in the shells out of the core which are closer to the surface which also creates and outward pressure, causing the sun to expand.

More massive stars actually burn their H faster than smaller stars - live fast, die young (leave a good looking corpse) is the way I remember it. t propto 1/M^2.5 .

Once the H in the core runs out the stars enter their next phase - the Red Giant phase. Nuclear reactions stop. Pressure decreases a bit and gravity can contract the core further, increasing the temperature. The increased temperature causes the reactions in the shell to burn faster and faster and exert an outward pressure. The converted He falls onto the core, increasing the gravity and T. The core of a 1Msun star compresses to 1/3 of it's size at this stage. The outer layers feel the outward pressure and expand drastically as the core contracts. The expansion cools the outer layers and T drops, hence star appears redder. The luminosity of these objects is high though cos of large radius.

This is what a star looks like in the first three stages. In very low mass stars M < 0.7 Msun, gravity is never strong enough to raise T higher than 10^8K required for He to start burning. We'll come back to this case.

Stars with mass in the range 1-2 Msun can raise T high enough to start He burning however the He starts burning in a He flash. These low mass stars are so compact when the Temperature gets to 10^8 K the gravity is balanced by something called electron degeneracy pressure - the electrons are resisting being pushed together. When He starts to burn the T is free to rise dramatically and a lot of He is burnt very quickly. Eventually T gets high enough that it is more important than the electron degeneracy pressure, the core expands slightly and He burns as normal. The He flash is not actually seen as a flash of light - most of the energy is used to heat the core and stop the degenerate state. The outer layers absorb any of the heat that is released. As the core expands, T drops and the outer layers contract as less pressure outwards so the radius decreases. As the radius decreases, the surface temperature increases. Moves on H-R diagram.

For a star with mass > 2 Msun, the gravity is high enough to raise T and start He burning with out the core becoming degenerate. No helium flash.

He burns into C and O via a process called the triple alpha reaction.

Globular clusters are useful for testing stellar models. All stars formed at same time, range of sizes though. All at same d so get relative L's.

When we look at globular clusters, we find part of the main sequence is missing.

This is because the stars have evolved off the main sequence. The turn off position reveals their ages - the more of the main sequence that survives, the younger the globular cluster.

As an aside, there are also some special stars that come up later that have special places on the HR diagram. These are RR Lyrae's, Cepheids and Mira variables. Their brightness changes on well known timescales.

They are found on the instability strip. They have a P-L relation - if you period can imply their peak luminosity and hence get distance - standard candles.

Cepheid light curves.

Back to stellar evolution: As said, as the He starts burning, core expands and cools slightly. The shell therefore releases energy more gradually so L goes down. Surface T increases though as outer layers contract a bit. Moves across the horizontal branch. Eventually the He in the core is used up though (roughly lasts in this phase for 20% of the main sequence time). Shells are still burning H and He. The products of this (C and O) fall onto the core and it contacts. T increases but this time for all low mass stars (M < 4Msun or so), electron degeneracy pressure is important and gravity can never raise T high enough to start C burning. This is where the star effectively dies. The core contracts and T is increased, This causes the shell burning to happen faster which causes the outer layers to expand. Surface T goes down but R increases rapidly so L increases up the asymptotic giant branch.

The sun will become so swollen at this stage that mars will be eaten by it. The core though is so compact that it is not much larger than the earth!

Again globular clusters help provide our model evidence as we see the asymptotic giant branch.

The next stage for low mass stars (and we'll include the ones where He burning never started again) is the planetary nebular phase. This has nothing to do with planets. H and He (or just H) are burning in the shell. They create thermal pulses outwards. This causes the outer layers of non-burning H and He to expand outwards. This goes on for a while but eventually the outer layers are so expanded that they no longer feel the gravitational pull of the core and they are ejected off from the star. 40% of the stars original mass can be lost here. We see 20,000-50,000 PN in our galaxy. We can use the doppler shift to work out how fast they are expanding.

Their projection can give rise to cool looking objects. Eventually they become so diffuse you cannot see them. 5Msun of H and He (and bits of C and O also escape) are put back into the interstellar medium each year via this process.

What are then left with is the core. This is an object known as a white dwarf. The first one to be detected was Sirius neighbour - the little splodge.

The WD is not burning anymore. Gravity is supported by electron degeneracy pressure. They are very dense - 10^19 kg/m^3, the size of the earth and they cool down passively with time. They have an unusual mass-radius relation - the more mass, the smaller the radius. There is an upper limit known as the Chandrasekhar limit.

Different mass WD's cool differently with time.

High mass stars (M>4Msun or so) are very different. Gravity is high enough to raise T to start C burning before the core becomes degenerate. T is high enough to burn C, O, Ne, Si etc. The star goes back and forth on the HR diagram, becomes very large and a lot of the outer layers are ejected off from the star.

This is the 'onion' skin diagram. All elements up to iron can be burnt. Iron is a special cases though. Burning iron is an endothermic reaction rather than an exothermic one i.e. it requires energy to fuse iron into something else rather than releasing energy. This is because the positively charged protons in the core of the iron atom resist further protons being added to them. This is the death of high mass stars. Core at high T. The final reaction generates neutrinos that escape and take energy with them. The core contracts, increasing T. The photons in the core are very energetic. They smash into the iron atoms and break them up - photodisintegration. Gravity then contracts core further, forcing protons and electrons to combine: P + e = n + neutrinos. Again the neutrinos escape. After 0.25s the core < 20 km in diameter and density 4x10^14 kg/m^3. Neutron degeneracy pressure kicks in - however it is too slow. Imagine driving and suddenly seeing a wall. If you are too late applying the breaks - you slam into it and bounce back. That is what happens here. The contracting core feels the wall of neutron degeneracy pressure and bounces back. This sends a shock wave through the star which blasts the outer layers off in an event called a Type II supernova. 96% of the mass is released back into the ISM and all the heavy elements in the universe are formed here - we are made of star dust. The cores create something called a Neutron star or a Black Hole (depending on initial mass and mass remaining).

The stages.

A supernova 1987a.

Lightcurve of 1987a

Other types of supernova occur. If a WD is in a binary system then mass from the neighbour star as it enters the red giant phase can be transfered onto it via a process called accretion. This pushed the mass of the WD over the Chandrasekhar limit and the star explodes. This is a type Ia supernova.

Type Ia and

Type II are the important types here.

Type Ia are believed to be standard candles - their peak luminosity is understood.

Lots of weird reactions occur in the blasted off layers. See lovely supernova remnants.

Heavy elements (not H, He, Li, Be, C, O but all the others) created here

Summary of stellar evolution