E.5.2 Fusion and Stellar Evolution

fusion

each small nucleus has a positive charge so they will repel, to make the nuclei come close enough together that the strong force attracts them, they must be thrown together at high velocities.

• thus the matter must be heated up to extremely hot temperatures, almost core of sun or thrown in a particle accelerator

fusion reaction produces a lot of energy per unit mass of fuel

stellar stability

nuclear fusion causes radiation pressure to maintain equilibrium which counteracts the gravitational force from collapsing

Hertzsprung- Russell diagram

scatterplot of stars showing the relationship between star’s absolute magnitudes or luminosities and their stellar classifications or effective temperatures

what is the life cycle of a star

stellar nebula - massive star - red supergiant- supernova - neutron star or black hole

stellar nebula - average star - red giant - planetary nebula - white dwarf

how do stars start their life

in giant molecular clouds called nebulae, if gas is compressed by the shock wave from an exploding star or collision between clouds, gravity overcomes the thermal pressure, and clouds begin to collapse from different-sized stars

as they collapse they emit IR radiation.

gravity keeps collapsing the protostar until the center becomes dense and hot and hydrogen nuclei start to fuse to make helium. This releases energy, as mass of products is less, which mass is converted to energy

the outwards radiation pressure counteracts inwards gravitational pressure. Now main sequence star

nebulae

contain clouds of gas and dust left over after the formation of a galaxy, made up of mainly hydrogen.

temp about 10K, the reason why molecules present

clouds are held together by gravity but kept from collapsing from pressure of molecules moving in random motion.

protostar

a new star

fusion in core of star

hydrogen to helium

sun sized star from red giant turns into what

white dwarf

luminiousty of big stars to main sequence stars

stays constant as they are getting smaller but hotter

small stars to main sequence stars

the core of a small star never gets that hot, so it gets less and less bright as it contracts

main sequence

all main sequence stars have a core that is undergoing fusion from hydrogen to helium. this radiates energy, that causes a pressure that prevents the star from collapsing under the force of gravity

short characteristic of red giant and how are they formes

as rate of fusion slows down, this heats the outer layer, causing them to expand and chnaging the main sequence star into a red giant

a cool star that gives out a lot of energy must be very big. the luminosity 100 times greater than sun. if they are same temp as sun, they must have an area 100 times greater, thus radius 10 times bigger

short characteristic of white dwarf and how are they formed

small hot star, hotter than the sun, but only the size of the earth. they have low luminosity

core is compressed by gravitational force, to a point where electrons can not get closer, thus not decreasing in size but increaisng in temp. the outer layer blows away. the core no longer producing energy, not getting any smaller

large stars

with stars of a mass over 4m, the pressure generated in core is enough to enable carbon and oxygen to fuse into larger elements such as neon and magnesium the

for stars of 8m process continuous until iron is formed - no more energy production, leading to type II superonva and remainder of core is neutron star or black hole

large stars don’t stay long on main sequence, since they are luminous so they use their fuel up faster

type II supernova

from an 8M star iron is fused, and fusing iron with other elements to produce larger nuclei will not liberate energy as it is on top of the binding energy curve

as the core runs out of nuclear energy, the core collapses, increasing the temperature. this allows iron to fuse again, this causes electrons to combine with protons to form neutrons

• this continues until core-only neutrons , and there is a collapse between the core and outer layer causing explosion leaving everything but the core (core is called neutron star or black hole)

neutron star or black hole

the core remained from type II supernova

black hole double size of a neutrino star (the core of the star must be more than 3 soalr masses to be black hole)

variable star

changing luminosity , so position on HR diagram not constant. This is due to the change in size of star. As it gets bigger luminosity increases, the variation is called Cepheid variable

arcsecond

how a star’s distance is calculated, the angle from the sun to the earth on one side,

1 arc second = 1/ parsec

What is the evolution of a low mass star

starts at bottom right from main sequence to red giant phase to white dwarf to black star

short characteristics of supergiants

a very big cool star, luminiosity 10⁶ greater than the sun, and radii up to 1000 greater

how to know how long a star will stay on main sequence

how long a star is on the main sequence is dependent on how much hydrogen the star contains and rate of it being used up

when does a core start collapsing

when the rate of fusion gets less, increase in core temperature.

hydrostatic equilibrium

gravitational force equal to nuclear fusion pressure