space - lifecycle of stars

Question 1

how are stars around the same size as the sun formed?

Answer 1

• in a main sequence star, hydrogen nuclei fuse together to make larger nuclei such as helium
• at some stage hydrogen begins to run out. at this point outward force due to fusion energy is less than the inward force due to gravity . causing star to collapse inward which causes temperature to increase
• now helium nuclei fuse together to create heavier elements
• star expands to form a red giant. at some point, red giant will stop fusing helium. at this stage, will shrink and form white dwarf. due to no longer carrying out fusion, it will cool down and eventually form a black dwarf that releases no energy

Question 2

when are elements heavier than iron produced?

Answer 2

only produced in a supernova

Question 3

how are stars much bigger than the sun formed?

Answer 3

• once stars much bigger than the sun run out of hydrogen, they expand and turn into red super giants leaving the main sequence stage
• helium nuclei fuse together to produce heavier elements
• red super giant will stop carrying out nuclear fusion. at this stage, star explodes (supernova). temp of supernova is high enough to produce elements heavier than iron.
• when supernova explodes, these elements are distributed throughout the universe
• after supernova, remains of star can either form a neutron star which consists of neutrons densely packed together or it can form a black hole. this has such a large gravity that not even light can escape