Stars+and+stellar+evolution


 * Red Dwarfs**

Red dwarf stars are main sequence stars of relatively low mass and therefore low temperature (surface temperature in the region of 2 500K. The have a high proportion of hydrogen fuel for their mass and (relatviely) slowly covert this to helium through nuclear fusion. They have lifetimes of trillions of years (long lived).


 * Red Giants**

As the main sequence stars get close to converting all their hydrogen to helium, the radiation pressure will reduce as the fusion process slows down and the start will collapse under the force of gravity. This increased the core temperature until it is high enough to ignite helium fusion into heavier elements. This increased radiation pressure will cause the start to expand to be much larger than before. Our Sun is expected to expend to encompass the Earth at this stage of its life. As the surface temperature is lower these stars are typically red in colour and are called red giants. However their size means that they are relatively bright, so they appear in the top right hand corner of the Hertzsprung-Russell (H-R) diagram.


 * White Dwarfs**

In smaller red giants the helium will be fused into Carbon and Oxygen. If the mass of the start is not sufficient to fuse Carbon the star will start to cool. The radiation pressure will however be strong enough for the start to shed its outer layers as a stream of charged particles (stellar wind) or through pulsations. If the remnant has a mass of <=1.4M sun (the Chandrasekhar limit) this results in a planetary nebula (named due to its appearance as a large planet to a low powered telescope) around a hot ball of Carbon. This hot ball is called a White Dwarf. Over time it cools and eventually becomes a black dwarf.


 * Neutron stars and pulsars**

In larger red giants the fusion process will continue, creating larger elements until iron is generated. Over this time they will become even larger (a super red giant) until the fusion process ceases and the star collapses. The violence of this collapse cases a rebound known as a **supernova**. In the supernova the heavier elements (more massive than iron) are created (through nuclear fusion + energy, i.e. this fusion process requires the addition of energy, rather than seeing energy given out, as happens for lighter fusion processes.

If, after a supernova, the mass of the remenant is more than 1.4M sun but less than about 3M sun then the star loses a lot of its energy via a burst of neutrinos, and the remnant will be a **neutron star**. Above this mass and the remnant will be a **black hole**. In a neutron star the core will consist of neutrons that are backed together so each neutron is closer to its neighbour than an atomic radius, so they are incredibly dense (the are prevented from collapsing further by quantum degeneracy (i.e that the Pauli exclusion principle prevents any two (fermionic) particles sharing the same place and quantum state simultaneously).

Many neutron stars are known as pulsars as the charged particles (e.g. protons) cause the star to spin rapidly and the resulting magnetic field propels the charged particles out is a stream, like a lighthouse beam. We can see this emission of cosmic rays as a regular pulsing of the star.


 * Black Holes**

A black hole is a neutron star this is sufficiently massive that its gravitational field warps space-time so much that even light cannot escape (from within the event horizon). However we can detect this objects by the effect on the surrounding space e.g. the bending of light around the black hole (gravitational lensing). Also, the existence of black holes in binary systems can be inferred by the movement of the companion star.