Life Cycle of Stars

Question 1

what determines the life cycle path a star goes towards

Answer 1

the mass of the star

Question 2

where do massive stars and low mass stars end up

Answer 2

• massive stars end up as neutron stars or black holes

- low mass stars end up as white dwarfs

Question 3

what is the simple version for the life cycle of a low mass star

Answer 3

• protostar
• main sequence star
• red giant
• planetary nebula
• white dwarf

Question 4

what is the simple version for the life cycle of a massive star

Answer 4

• protostar (bigger)
• blue supergiant
• red supergiant
• type 2 supernova
• neutron star or black hole

Question 5

how does a protostar become a main sequence star

Answer 5

• nuclear fusion of hydrogen in the core of the protostar causes it to expand and garner more mass from its surroundings
• the increase of mass increases the strength of its gravity trying to compress the star
• this happens until a delicate balance between the outward thermal expansion of fusion and inward compression of gravity is reached

Question 6

how does a main sequence star become a red giant

Answer 6

• the star starts running low on hydrogen, but the amount of energy its produced causes it to expand
• the expansion of the outer layers causes them to cool, making them red and increasing the stars size

Question 7

how does a red giant become a planetary nebula

Answer 7

• when most of the hydrogen is used helium will start to fuse
• this process can cause an explosion that throws the outer layers of the star into space
• this forms a planetary nebula around the star

Question 8

how does a planetary nebula become a white dwarf

Answer 8

• the star runs out of fuel so thermal expansion decreases
• therefore gravity to take over which contracts it to a much smaller size
• this heats up the core significantly, making it a white dwarf

Question 9

practically, what will happen to the white dwarf after it has reached this stage

Answer 9

it will gradually run out of the energy it has and eventually become cool

Question 10

what is it called during this after-stage

Answer 10

• a red dwarf

- then eventually a black dwarf

Question 11

why are black dwarfs still considered to be theoretical however

Answer 11

• it takes a white dwarf longer than the current age of the universe to cool this much
• so there hasnt been time for any to actually develop yet

Question 12

what condition needs to be met for a protostar in order for it to go down the neutron star / black hole route

Answer 12

it needs to have more than 4 times the mass of our sun at that stage

Question 13

what is formed from protostars this large

Answer 13

blue supergiants

Question 14

because the core temperature of blue supergiants is significantly higher than main sequence stars, how does their fusion of hydrogen differ

Answer 14

• the hydrogen nuclei fuse at a much quicker rate

- so the fuel burns out very quickly in bigger stars

Question 15

what further occurs due to these high temperatures however and what stage would the star be at this point

Answer 15

• the fusion of larger nuclei
• they could fuse up to iron
• during this process of fusing heavier elements it would be at the red supergiant stage

Question 16

what happens to the balance between thermal expansion and gravity when the star has fused up to iron and why

Answer 16

• at iron nuclear fusion can no longer occur
• so the star stops producing energy and therefore a thermal expansion pressure
• this causes the balance between thermal expansion and gravity to dramatically shift in gravitys favor

Question 17

what two factors contribute to the sudden dramatic shift in balance

Answer 17

• the fact that nuclear fusion stops altogether at iron

- and the large gravity formed from the very large mass

Question 18

what happens due to this shift

Answer 18

• the star undergoes a massive collapse
• the sudden increase in density produces a huge burst of energy that almost bounces the collapse back out
• forming an explosion called a type 2 supernova

Question 19

what is so incredible about supernovae

Answer 19

• they are the most immense bursts of energy we have ever witnessed in the universe
• giving them a brightness at about 10 billion times the luminosity of our sun

Question 20

why have scientists concluded that supernovae had to have occurred in the past in order for us to have elements heavier than iron in the universe

Answer 20

• supernovae explosions are the only entities we know of that have so much energy
• that they can cause nuclear reactions to occur that produce elements above iron
• the natural occurrence of heavier elements means supernovae had to have been doing it from the start

Question 21

what is the product that these supernovae leave behind

Answer 21

• a neutron star
• or a black hole
• both are the core of the star

Question 22

if these two arent formed, what else could happen

Answer 22

the entire star may be completely shattered

Question 23

what is a neutron star, other than one of the possible conclusions of the life of a large mass star

Answer 23

• it is a small and very dense core
• composed of densely packed neutrons
• acting as the nucleons within the nucleus of the atom

Question 24

what is a black hole, other than one of the possible conclusions of the life of a large mass star

Answer 24

• a region of space-time
• in which the gravity is so strong that it prevents anything from escaping
• including EM radiation

Question 25

do neutron stars and black holes show up in the hertzsprung-russell diagram and why

Answer 25

• no
• because we cant measure their temperatures of luminosity with any redeemable amount of accuracy
• a black hole literally doesnt emit EM radiation