Life cycle of stars

Question 1

How is a main sequence star formed?

Answer 1

• Begins with a cloud of dust and gas, known as a nebula
• Overtime, the force of gravity pulls the dust and gas of the nebula together, forming a protostar
• Protostar continues to grow, which causes the force of gravity to continue to increase
• Gravity squeezes the protostar, making it more dense
• This also causes particles to collide and increase the temperature of the protostar
• When the temperature is high enough, hydrogen nuclei start fusing and forming helium nuclei (binary fusion)
• Binary fusion gives out lots of energ. At this point, the protostar has become a mains sequence star

Question 2

Why is the main sequence star considered stable?

Answer 2

• The outward pressure due to binary fusion is prefectly balanced by the inward pressure of gravity

Question 3

What happens to the main sequence star after a long stable period?

Answer 3

• After a long stable period, the star runs out of hydrogen and can no longer undergo nuclear fusion
• At this point, the force of gravity makes the star contract into a small ball
• The small ball is so hot and dense that nuclear fusion starts again, causing the star to expand
• Nucear fusion forms heavier elements than helium (up to iron)

Question 4

What happens to the main sequence star if it is a small / medium star?

Answer 4

• The main sequence star expands to form a red giant
• After a while, the red giant becomes unstable and expels its outer layers of dust and gas
• This leaves behind a hot, dense and solid core that doesn’t do anymore nuclear fusion. We call this a white dwarf
• Over time, the white dwarf emits all of its energy and can no longer emit light. This is now called a black dwarf

Question 5

What happens to the main sequence star if it is a really big star?

Answer 5

• The main sequence star expands to form a red super giant
• The red super giant undergoes more nuclear fusion and passes through several cycles of contraction and expansion
• After a while, the red super giant explodes in a supernova, creating elements even heavier than iron
• If the star was big, it will condense into a neutron star
• If the star was massive, it might collapse in on itself and form a black hole