Download Last time: looked at proton-proton chain to convert Hydrogen into

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.
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 = mc2
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.
Thinking about hydrostatic equilibrium (gravity vs. pressure):
Fusion rate ~ Temperature
i.e. the hotter it is, the more the core will fuse.
What would happen if the core temperature of the sun dropped a
little bit? DISCUSS.
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?
When the sun was born.
fractional
composition
1
helium
0.5
hydrogen
0
Distance from the Sun’s center.
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
What happens to the sun as it burns through its hydrogen?
After 5 billion years
fractional
composition
1
helium
0.5
hydrogen
0
Distance from the Sun’s center.
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
What happens to the sun as it burns through its hydrogen?
After 10 billion years.
fractional
composition
1
helium
0.5
hydrogen
0
Distance from the Sun’s center.
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
When the sun is burning hydrogen via fusion
as usual, it is a main sequence star.
The temperature of the
star determines the rate
of nuclear fusion.
m
ain
hotter stars,
much faster
burners
se
qu
en
ce
colder stars,
slower burners
If we have mass
constraints on stars
(giving their hydrogen
mass = nuclear fusion
fuel) and their
luminosity, we can
estimate their
lifetimes.
Mass and Lifetimes of Main Sequence Stars
2 ⇥ 104 L
10 M
m
ain
se
qu
en
4 ⇥ 10
3
L
How do we measure
luminosity again?
ce
0.1 M
How do we measure
mass again?
Mass and Lifetimes of Main Sequence Stars
2 ⇥ 104 L
10 M
m
ain
se
qu
en
4 ⇥ 10
3
L
How do we measure
luminosity again?
L
F =
4⇡r2
ce
0.1 M
How do we measure
mass again?
Mass and Lifetimes of Main Sequence Stars
2 ⇥ 104 L
10 M
m
ain
se
qu
en
4 ⇥ 10
3
L
How do we measure
luminosity again?
L
F =
4⇡r2
ce
0.1 M
How do we measure
mass again?
Mass and Lifetimes of Main Sequence Stars
2 ⇥ 104 L
10 M
m
ain
se
qu
en
4 ⇥ 10
3
L
ce
0.1 M
Life expectancy of the sun: 10 billion years.
Life expectancy of blue, massive 10 M star:
Life expectancy of 0.1 M red dwarf star:
Mass and Lifetimes of Main Sequence Stars
2 ⇥ 104 L
10 M
m
ain
se
qu
en
4 ⇥ 10
3
L
ce
0.1 M
Life expectancy of the sun: 10 billion years.
Life expectancy of blue, massive 10 M star:
Burning fuel 104 times faster, 10 times more fuel, lifetime is 1/1000 that of the sun.
Life expectancy of 0.1 M red dwarf star:
Mass and Lifetimes of Main Sequence Stars
2 ⇥ 104 L
10 M
m
ain
se
qu
en
4 ⇥ 10
3
L
ce
0.1 M
Life expectancy of the sun: 10 billion years.
Life expectancy of blue, massive 10 M star: 10 million years
Burning fuel 104 times faster, 10 times more fuel, lifetime is 1/1000 that of the sun.
Life expectancy of 0.1 M red dwarf star:
Mass and Lifetimes of Main Sequence Stars
2 ⇥ 104 L
10 M
m
ain
se
qu
en
4 ⇥ 10
3
L
ce
0.1 M
Life expectancy of the sun: 10 billion years.
Life expectancy of blue, massive 10 M star: 10 million years
Burning fuel 104 times faster, 10 times more fuel, lifetime is 1/1000 that of the sun.
Life expectancy of 0.1 M red dwarf star:
Burning fuel 103 times slower, 1/10th the fuel, lifetime is 100 that of the sun.
Mass and Lifetimes of Main Sequence Stars
2 ⇥ 104 L
10 M
m
ain
se
qu
en
4 ⇥ 10
3
L
ce
0.1 M
Life expectancy of the sun: 10 billion years.
Life expectancy of blue, massive 10 M star: 10 million years
Burning fuel 104 times faster, 10 times more fuel, lifetime is 1/1000 that of the sun.
Life expectancy of 0.1 M red dwarf star: 1 trillion years
Burning fuel 103 times slower, 1/10th the fuel, lifetime is 100 that of the sun.
Eventually, the sun runs out of its fuel, like all stars.
What happens then??
Eventually, the sun runs out of its fuel, like all stars.
What happens then??
Stars Part 3: life & death.
Stars Part 3: life & death.
Where a star sits on the HR
diagram is determined by
its mass,
its age.
Stars on the main
sequence are burning
through their main supply
of hydrogen via the p-p
chain (and when really massive, the
CNO cycle).
We learned how to compute
lifetimes of stars on the main
sequence knowing their
masses and luminosities.
supergiants
m
ain
ts
n
ia
se
g
qu
e
nc
e
wh
ite
dw
ar f
s
How does a star get there? And what
happens when that fuel runs out?
Where a star sits on the HR
diagram is determined by
its mass,
its age.
Stars on the main
sequence are burning
through their main supply
of hydrogen via the p-p
chain (and when really massive, the
CNO cycle).
We learned how to compute
lifetimes of stars on the main
sequence knowing their
masses and luminosities.
supergiants
m
ain
ts
n
ia
se
g
qu
e
nc
e
wh
ite
dw
ar f
s
Early life! How are stars born?
Collapse of molecular gas clouds…
Conservation of Angular Momentum propels gas into fast
rotating disks from which solar systems form.
Collapse of molecular gas clouds…
Conservation of Angular Momentum propels gas into fast
rotating disks from which solar systems form.
Along with the “core” which becomes a star, a disk forms…
What do star-forming clouds look like?
As a star is born, it accretes mass from its disk and heats up.
Fusion has started, and the star is adding mass…
Young stars which are still accreting material are called T-Tauri Stars.
Because mass is piling on, they
sometimes have explosive outbursts.
Young stars which are still accreting material are called T-Tauri Stars.
Because mass is piling on, they
sometimes have explosive outbursts.
Eventually the dust and debris in the protoplanetary disk is cleared out,
potentially leaving planets & other debris behind. The star is no longer
adding mass and has landed on the main sequence, where it will sit for
most of its lifetime, until it runs out of fuel…
The birth of stars.
Why do young stars form disks?
once there is an overdense region of matter it has a
run-away effect and everything collapses down into a
disk due to gravity.
the angular momentum of the original star forming
(b)
cloud builds with time forming a disk
the angular momentum of the star forming cloud is
(c)
constant with time and so as things collapse they start
to spin and through friction form a disk,
(d) trick question; not all stars have disks when they form.
(a)
The birth of stars.
Why do young stars form disks?
once there is an overdense region of matter it has a
run-away effect and everything collapses down into a
disk due to gravity.
the angular momentum of the original star forming
(b)
cloud builds with time forming a disk
(c) the angular momentum of the star forming cloud is
constant with time and so as things collapse they
start to spin and through friction form a disk,
(d) trick question; not all stars have disks when they form.
(a)
The birth of stars.