Last time: looked at proton-proton chain to
convert Hydrogen into Helium, releases energy.
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Understanding Stars: Hydrostatic Equilibrium, Fusion, and Life Cycles, Study notes of Chemistry
The process of hydrostatic equilibrium in stars, the role of temperature in fusion, and the life cycles of stars, including the conversion of hydrogen into helium and the eventual death of a star. The document also touches upon the measurement of mass and luminosity, and the impact of mass and temperature on a star's lifetime.
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Last time: looked at proton-proton chain to
convert Hydrogen into Helium, releases energy.
Last time: looked at proton-proton chain to
convert Hydrogen into Helium, releases energy.
Fusion rate ~ Temperature
i.e. the hotter it is, the more the core will fuse.
Last time: looked at proton-proton chain to
convert Hydrogen into Helium, releases energy.
Fusion rate ~ Temperature
i.e. the hotter it is, the more the core will fuse.
Even hotter temperatures: you can start fusing heavier
elements. This is NOT happening in the sun now.
E = mc
2
Stars exist in a state of hydrostatic equilibrium for
most of their lives.
This balances the inward force of gravity with the outward pressure of very hot gasses.
Stars exist in a state of hydrostatic equilibrium for
most of their lives.
This balances the inward force of gravity with the outward pressure of very hot gasses.
Decline in core temperature causes
fusion rate to drop, so core contracts
and heats up.
Rise in core temperature causes
fusion rate to rise, so core
expands and cools down.
What happens to the sun as it burns through its hydrogen?
The sun started out with some
Helium when it was born, ~10% of
the sun by mass, and that helium
was spread throughout the sun.
R
When the sun was born.
fractional
composition
Distance from the Sun’s center.
1
0
hydrogen
helium
What happens to the sun as it burns through its hydrogen?
The core of the sun is where fusion happens
and suddenly most of the core is actually
made up of helium, as the hydrogen is
consumed, still most of the sun is still made of
hydrogen, but towards the outter layers.
R
After 10 billion years.
fractional
composition
Distance from the Sun’s center.
1
0
hydrogen
helium
When the sun is burning hydrogen via fusion
as usual, it is a main sequence star.
main sequence
The temperature of the
star determines the rate
of nuclear fusion.
colder stars,
slower burners
hotter stars,
much faster
burners
If we have mass
constraints on stars
(giving their hydrogen
mass = nuclear fusion
fuel) and their
luminosity, we can
estimate their
lifetimes.
main sequence
Mass and Lifetimes of Main Sequence Stars
2 ⇥ 10
4
L
4 ⇥ 10
3
L
0. 1 M
10 M
How do we measure
mass again?
How do we measure
luminosity again?
F =
L
4 ⇡r
2
main sequence
Mass and Lifetimes of Main Sequence Stars
2 ⇥ 10
4
L
4 ⇥ 10
3
L
0. 1 M
10 M
How do we measure
mass again?
How do we measure
luminosity again?
F =
L
4 ⇡r
2
main sequence
Mass and Lifetimes of Main Sequence Stars
2 ⇥ 10
4
L
4 ⇥ 10
3
L
0. 1 M
10 M
Life expectancy of the sun: 10 billion years.
Life expectancy of blue, massive star: 10 M
Life expectancy of red dwarf star:
0. 1 M
Burning fuel 10
4
times faster, 10 times more fuel, lifetime is 1/1000 that of the sun.
main sequence
Mass and Lifetimes of Main Sequence Stars
2 ⇥ 10
4
L
4 ⇥ 10
3
L
0. 1 M
10 M
Life expectancy of the sun: 10 billion years.
Life expectancy of blue, massive star: 10 M
Life expectancy of red dwarf star:
0. 1 M
Burning fuel 10
4
times faster, 10 times more fuel, lifetime is 1/1000 that of the sun.
10 million years
main sequence
Mass and Lifetimes of Main Sequence Stars
2 ⇥ 10
4
L
4 ⇥ 10
3
L
0. 1 M
10 M
Life expectancy of the sun: 10 billion years.
Life expectancy of blue, massive star: 10 M
Life expectancy of red dwarf star:
0. 1 M
Burning fuel 10
4
times faster, 10 times more fuel, lifetime is 1/1000 that of the sun.
Burning fuel 10
3
times slower, 1/10th the fuel, lifetime is 100 that of the sun.
10 million years
1 trillion years
Eventually, the sun runs out of its fuel, like all stars.
What happens then??