Lifecycle of a star - formation A cloud of dust and gas made of 97% hydrogen and 3% helium – nebula Gravity causes these atoms to clump together (the atoms lose potential energy) The loss of p.e. causes an increase in temperature When the temperature is high enough, fusion of Hydrogen occurs and helium is created.

Main sequence Energy is released when hydrogen fusion occurs, this causes an increase in temperature When the temperature is high enough the helium nuclei also begin to fuse releasing more energy.

Its all a question of balance Stars like humans need to stay in balance, they need to balance gas pressure (from burning) and gravity (from mass)

How long will a star stay on the main sequence? This depends on mass A small star uses less fuel to stay in equilibrium so will live . A large star has more fuel to begin with but needs to burn it at a faster rate to stay in equilibrium so will live .

The beginning of the end When the hydrogen runs out, the core contracts and the shell expands (to maintain equilibrium) The temperature increases and helium burning begins The star is now a red giant (small stars) (or a red supergiant for large mass stars)

Heavy stuff As the temperature increases, heavier elements such as carbon begin to fuse. Low mass stars (<1.4SM) shrink, their mass drifts off into space. This is called a planetary nebula. The cloud is blown off leaving the hot, white core. The star is now a white dwarf. The white dwarf will eventually cool to form a black dwarf

What about large stars? They become red supergiants. The core shrinks and heavier elements fuse. Eventually the core of the star is converted to iron. The temperature and gravity increase and eventually the core collapses. The outer layers hit the inner layers and a supernova occurs (a large explosion). Medium mass stars become neutron stars. Large mass stars become black holes.

Hertzsprung-Russel Diagram 1000000 10000 100 1 0.01 0.0001 Relative luminosity (Sun = 1) 2500 40000 20000 5000 O B A F G K M Surface Temp/K Supergiants Main Sequence Giants White Dwarfs