Astrophysics - Stellar Evolution

Question 1

how are stars classified?

Answer 1

according to their colours

Question 2

what does a star’s colour relate to?

Answer 2

the star’s surface temperature: blue as the hottest and red as the coldest

Question 3

what is the evolution of a star with a similar mass to the sun?

Answer 3

• nebula
• main sequence
• red giant
• white dwarf

Question 4

what happens when a star is a nebula?

Answer 4

• stars form from large clouds of dust and gas particles (nebulae) that are drawn together by gravitational forces over millions of years
• as the particles get closer the temperature and pressure becomes so large that nuclear fusion of hydrogen nuclei to helium nuclei occurs
• this releases enormous amounts of energy in the form of heat and light.

Question 5

what happens when a star is a main sequence?

Answer 5

• fusion produces forces that make the star expand outwards, but the gravitational force is always pulling the particles within the star inwards
• when these two opposing forces become balanced a star is stable and called a main sequence star
• it should stay this way for millions of years, at a constant size and temperature

Question 6

what happens when a star is a red giant?

Answer 6

• eventually, hydrogen fusion stops as the star runs out of fuel
• gravitational force is now bigger than the outward fusion force which causes the star to collapse inwards and compress
• this causes it to heat up to even higher temperatures so that fusion of helium nuclei begins
• the increased power output causes the star to expand greatly
• the surface area is so large that it is cooler than before, so its colour changes to red and the star is called a red giant

Question 7

what happens when a star is a white dwarf?

Answer 7

• eventually, fusion stops when the star runs out of helium nuclei and the gravitational force causes the star to collapse inwards and compress again
• this heats it so it changes colour to emit white light
• the star is squashed so greatly by the gravitational force to become a small and very dense white dwarf
• a white dwarf eventually cools down and changes colour as it does so, eventually becoming black

Question 8

what is the evolution of a star with a larger mass to the sun?

Answer 8

• nebula
• main sequence
• red supergiant
• supernova
• neutron star/black hole

Question 9

what happens when a red supergiant becomes a supernova?

Answer 9

• after the stable period, a giant star expands into a red supergiant (it produces all the elements up to iron during nuclear fusion)
• when it finally runs out of nuclei to fuse it collapses due to the gravitational force, and then explodes into a supernova

Question 10

what happens when a supernova becomes a neutron star?

Answer 10

• the supernova throws dust and gas back into space and so another nebula is formed
• a dense core remains (a neutron star)
• if its mass is large enough it can compress further to become a black hole