Astronomy and cosmology

Question 1

Nuclear fusion

Answer 1

Two nuclei joining together and releasing energy from a change in binding energy

Question 2

nebulae

Answer 2

areas of dust and gas

Question 3

formation of stars

Answer 3

● Stars are formed from nebulae - areas of dust and gas - over time the gravitational force brings them together.
● As the mass increases the gravitational collapse is accelerated, the gravitational energy is converted to thermal
energy. The temperature increases until the gas is hot enough to glow, forming a protostar.
● The pressure and temperature continues to increase until they are high enough for nuclear fusion to occur and
produce helium
● The star is now formed and is main sequence, it is stable as the gravitational force inward balances the radiation
pressure outwards.
● Once the gas runs out the star will stop being main sequence and what happens at this point is dependent on the
star’s mass.

Question 4

formation of low mass stars

Answer 4

Once most of the hydrogen has been used up, nuclear fusion stops. This means that there is no longer any
radiation pressure preventing collapse
● The net force inwards leads to the core contracting and outer layers expand out and cover a larger volume than
before - they cool and become a red giant
● As the core continues to collapse, the temperature becomes high enough for helium fusion - heavy elements like
oxygen and carbon are made
● Once all the helium runs out, the fusion stops as the red giant is not hot enough for fusion of heavier elements. The
outer layers of gas are ejected and form planetary nebulae
● The core continues to collapse and heats up - it forms a white dwarf but eventually cools down.