6.14 Life Cycle of Stars Flashcards
What is a cloud of dust and gas called (first stage)
Nebula
How does a nebula form a protostar?
The force due to gravity draws together the gas and dust and they spiral together to form a protostar.
How does a protostar form a main sequence star?
The temperature rises as the gas in the protostar is compressed. When the temperature gets high enough, hydrogen nuclei gain enough energy to undergo nuclear fusion to form helium nuclei. This gives out massive amounts of energy. A star is born
Describe how the main sequence star is maintained
The protostar immediately enters a period of equilibrium - the energy released by nuclear fusion results in an outward pressure that tries to expand the star, but this is balanced by the force due to gravity pulling everything inwards (gravitational collapse). In this stable period it’s called a main sequence star and it typically lasts several billion years.
What will happen when a main sequence star begins to run out of hydrogen?
This means the star won’t be able to do nuclear fusion and so the inward pressure of gravity takes over and contracts the star into a small ball until it’s so hot and dense that nuclear fusion can start up again, causing it to expand again. However, this time, instead of just forming helium the nuclear fusion will form heavier elements including all the elements until Fe on the periodic table.
After a main sequence star loses hydrogen and begins to expand again, how big will the next star be?
What comes next depends on how big the initial star was. If it was a small/medium star like our sun then it will form a red giant, but if it was a very big giant then it will form a red super giant.
Describe what happens when a red giant becomes unstable
After a relatively short time, the red giant becomes unstable and expels its outer layers of dust and gas. This leaves behind a hot, dense and solid core which doesn’t do any nuclear fusion. We call this a white dwarf because it gives off a lot of light and appears white and it is relatively small. Overtime, the white dwarf becomes cooler and darker as it emits all of its energy until finally it transitions into a black dwarf, because it no longer has enough energy to emit light and so appears dark.
Describe what happens after a protostar becomes a red super giant
It stars to shine brightly again as they undergo even more nuclear fusion. After passing through several cycles of expansion and contraction, they eventually explode into a supernova, which forms elements even heavier than iron that get ejected all across the universe. What happens next depends on how big the star was; if it was very big, the supernova will condense into a very dense core called a neutron star however if the star was massive, it might collapse in on itself and become a black hole. The reason they are called black holes is because they’re so dense that their gravity is able to pull in any light that passes nearby.
What is a nebula?
A large cloud of dust and gas
What is a nebula?
A large cloud of dust and gas
What pulls the gas and dust in a nebula together, to form a protostar?
The attraction of gravity
As a protostar grows in size and attracts more particles from the surrounding nebula, what three changes may take place with it?
- The temperature increases
- It becomes more dense
- Nuclear fusion could initiate
After the main sequence, what could a star become?
- A red giant
- A red super giant
What two alternative things might happen to a star after a supernova explosion?
- Condense into a neutron star
- Collapse into a black hole
