4.8.1.2 The Life Cycle of a Star

Question 1

What does a star go through?

Answer 1

a life-cycle

Question 2

What is a star’s life-cycle determined by?

Answer 2

the life cycle of star is determined by the size of the star

Question 3

Diagram of the life-cycle of stars the size of the sun and of stars much bigger than the sun:

Answer 3

Question 4

Life-cycle of stars the size of the Sun:

Answer 4

• star begins life as a cloud of dust and gas (nebula)
• gravity causes the cloud of dust and gas to collapse
• as particles move faster and the temp rises to millions of degrees C - this early stage is a protostar
• if temp gets high enough then hydrogen nuclei join together to make larger nuclei e.g. helium nuclei - nuclear fusion
• now a main sequence star
• nuclear fusion releases huge amount of E and stars can stay in the main sequence for a v. long time
• at some stage the hydrogen is the star begins tor run out so not as much nuclear fusion can take place
• at this point outward force due to nuclear fusion energy is less that the inward force due to gravity and this causes the star to collapse inwards
• collapse of the star causes temperature to increase
• helium nuclei fuse together to form heavier elements (but cannot make any elements heavier than iron) and the star expands to form a red giant
• at some point, the red giant stops fusing helium and then the star shrinks and collapses to form a white dwarf
• as the white dwarf is no longer carrying out fusion it gradually cools down
• eventually the white dwarf stops releasing any energy and it cools to form a black dwarf

Question 5

Life-cycle of stars much bigger than the Sun:

Answer 5

• star begins life as a cloud of dust and gas (nebula)
• gravity causes the cloud of dust and gas to collapse
• as particles move faster and the temp rises to millions of degrees C - this early stage is a protostar
• if temp gets high enough then hydrogen nuclei join together to make larger nuclei e.g. helium nuclei - nuclear fusion
• now a main sequence star
• nuclear fusion releases huge amount of E and stars can stay in the main sequence for a v. long time
• at some stage the hydrogen is the star begins tor run out so not as much nuclear fusion can take place
• at this point outward force due to nuclear fusion energy is less that the inward force due to gravity and this causes the star to collapse inwards
• star leaves the main sequence stage and expands into a red super giant
• helium nuclei fuse together to produce heavier elements
• at some point the red super giant stops carrying out nuclear fusion and then at this stage the star explodes - supernova
• the temperature of a supernova is high enough to produce elements heavier than iron
• when the supernova explodes these elements are distributed throughout the universe
• after the supernova the remains of the star can either form:
  
  • a neutron star - consists of neutrons densely packed together
  • black hole - such a large gravity that not even light can escape

Question 6

What do fusion processes in stars produce?

Answer 6

fusion processes in stars produce all of the naturally occurring elements

Question 7

Why is a direct sequence star stable?

Answer 7

Gravitational collapse in equilibrium with expansion due to fusion energy