Stars are born in huge clouds of low ____ gas and dust called ____. The temperatures in these clouds are very ____ - typically between 10 K and 20 K. Higher temperatures would cause the particles to have too much kinetic energy to stay together. Gravitational forces then start to act pulling the matter towards the centre causing further increase in ____ attraction and even more matter is pulled in. The gravitational ____ energy lost by this matter is converted to ____ energy which is expressed through the ____ energy of the particles which are mostly ____ nuclei. Hydrogen nuclei are ____ charged and so ____ each other. Eventually they have sufficient kinetic energy to overcome this ____ repulsion and nuclear ____ begins. Stars spend most of their life on the ____ of the Hertzsprung-Russell diagram where stars generate thermal energy from the fusion of hydrogen to ____ in their ____. During this phase of their life, the outward thermal ____ counteracts the inward ____ force preventing the star from collapsing further. A balance has been reached. Stars on the main sequence vary in colour, ____ and surface ____. Where a star is on the main sequence depends on its ____. More massive stars are hotter as they have a greater source of gravitational ____ energy. They also have higher luminosities and are more ____ in colour. Smaller mass stars are redder and have ____ temperatures and luminosities. The mass of a star determines how long it spends on the main sequence. More massive stars burn through their fuel more quickly and evolve sooner than lower mass stars. What happens as a star comes to the end of its life is entirely dependent on its ____. Stars with a mass less than about 8 solar masses evolve to become ____ before their outer layers are cast off into space as a planetary ____. Despite the name, this has nothing to do with planets! The ____ (bit that is left behind) at the centre is called a ____. These are very hot, but have a low ____ because they are very ____. A typical white dwarf is about the same mass as the Sun, but is about the same size as the Earth. They are very ____! A red giant forms when a star has used up all its hydrogen by fusion to ____. At this point the star moves off the main sequence on the ____ diagram. The core can't support itself against gravitational collapse and gets very hot. Hydrogen in a shell around the core is heated until ____ occurs there. So fusion is now happening in a ____ around the core and not in the ____. The increasingly hot core pushes the outer layers of the star outwards. The star's surface temperature ____ as it expands and cools. This is now a red giant - its lower surface temperature leads to it emitting ____ wavelength light, but it has a very high luminosity due to its enormous ____: When the Sun becomes a red giant it will engulf the Earth's orbit. When the core of red supergiant collapses, fusion of helium can occur to produce other elements up to ____. A star larger than about 8 solar masses evolves to become a ____ star. When these die, they do so in a more violent way. When the fuel runs out and there is insufficient thermal pressure to prevent gravitational ____, a ____ explosion occurs. A supernova explosion is so bright it can briefly outshine an entire galaxy of stars! If the mass of the original star is ____ solar masses, the remnant will be a neutron star. If the mass of the original star is ____ solar masses, the remnant is a ____. Neutron stars are incredibly ____: a typical neutron star of 1.4 solar masses would be about 11 km in diameter. A black hole is even more extreme where the ____ exceeds the ____ of light. Elements heavier than iron are formed during supernovae.

Stellar Evolution

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