Transcript Goal: To understand the lifetime of a star and how

Goal: To understand the lifetime
of a star and how the mass of a
star determines its lifetime
Objectives:
1)
To learn what defines a Main sequence star
2)
To understand why Energy is important for a star
3)
To examine the Cores or stars
4)
To understand what determines the Lifetime of a
star
5)
To see when the Beginning of the end is going to
occur
During break: Why does fusion create energy?
To prevent collapse
• Remember when we looked at the core of
the sun that we saw that the sun held itself
up with a combination of gas pressure and
radiation pressure (light has energy)
• This was called “Hydrostatic Equilibrium”
Proton – Proton Chain
• Short answer: method by which a star
converts protons (Hydrogen nuclei) to
Helium nuclei (the electrons in the core of
a star fly around on their own).
Proton – Proton Chain
• However it is a lot more complicated that I
have made it seem.
• After all, how do we take 4 protons and
make a helium atom when a helium atom
has 2 protons and 2 neutrons?
Why don’t the atoms in this room
fuse together?
Repulsion
• In the cores of stars all the nuclei have +
charges.
• + charges repel other + charges.
• So, they won’t attract and fuse by
accident.
• So, what do we need to be able to do it?
Energy
• It takes energy to overcome this repulsive
force.
• Much like it takes energy to get up the
stairs.
• The energy they have is measured by their
temperature
Step one
• We take 2 protons in the core to the sun
and try to slam them together.
• They get closer and closer.
• Here come the fireworks!
• And!
Step one
• We take 2 protons in the core to the sun
and try to slam them together.
• They get closer and closer.
• Here come the fireworks!
• Nothing happens….
Quantum Mechanics!
• No, I will not do a lecture on Quantum.
• Just 1 basic principal: there is uncertainty
in the position of each proton.
• In laymen’s terms that means that a proton
is not just in a specific position, but has a
small probability at being in a nearby
position.
So,
• When 2 protons start to get close, there is
a small probability they will actually be in
the same spot.
• This is called quantum tunneling –
basically tunneling through the repulsive
barrier.
• This allows us to have fusion!
However,
• The probability of this tunneling is very small,
and it depends very highly on how close they
get.
• This means that how rapidly you fuse protons
depends very highly on the temperature (and
also on the density squared).
• Fusion in the proton – proton chain (sometimes
call p-p chain) relies on temperature to the
FORTH power!
Step 1 concluded
• So, eventually we get 2 protons to collide.
• What do we get?
• No, we don’t get a Helium atom with 2 protons
and no neutrons. Those don’t exist.
• Another difficulty in the fusion process is that
you turn 2 protons into deuterium (which is
hydrogen with a neutron in it) + stuff.
• So, that means a proton has to convert to a
neutron. That is hard to do.
Step 2
• It would be easy to say 2 deuterium go to
1 helium.
• It would give you 2 protons and 2
neutrons.
• But, sadly, it does not work that way.
• Reason, there just is not enough
deuterium.
Instead
• Deuterium fuses with what is the most
common thing around, a proton.
• This creates Helium 3 (Helium which has a
weight of 3; 2 from the 2 protons and the
last from 1 neutron).
Step 3a
• 3a occurs 69% of the time in our sun.
• In time you will get some amount of
Helium 3.
• If 2 of these fuse, then you get a Helium 4
and 2 protons.
Step 3b
• 31% step 3b occurs instead.
• In this case a Helium 3 fuses with a
Helium 4 creating Beryllium 7.
• The Beryllium 7 combines with an electron
(converts a proton into a neutron) to create
Lithium 7.
• The Lithium 7 fuses with a proton to create
2 Helium 4 atoms.
Carbon – Nitrogen – Oxygen Cycle
• While the sun utilizes the p-p chain. Other
stars use this (called hereafter the CNO
cycle).
• Instead of fusing protons and protons we
now fuse protons to carbon.
• Larger atom to fuse makes it a LOT harder
Charges
• Protons have 1 atomic charge.
• Carbon has 6 (6 protons).
• Therefore, it takes more energy, which
means higher temperatures.
• This method depends on temperature to
the 20TH power!!!
Why does fusion create energy?
• 4 protons have more mass than 1 Helium
atom.
• So, when you fuse protons into helium,
you loose mass.
• Mass is a form of energy.
• Once again, energy is always conserved!
• So, you gain energy (in forms of photons
and neutrinos).
Other than the stuff our sun does
now
• Stars on the main sequence slowly burn
their fuel.
• While the do get a little brighter with time
(10-50% over their lifetime), their outer
temperature, radius, and brightness all
stay approximately the same (well within a
small range anyway).
Core
• Now lets examine different sizes of stars.
• Stars come in all sizes from 200 times the
mass of our sun to 1% the mass of our
sun.
Smallest stars
• The smallest stars are called Brown
Dwarfs.
• These stars are between 1-8% of the
mass of our sun and about the size of
Jupiter.
• These stars are too small to fuse
Hydrogen.
• Instead they fuse Deuterium into Helium.
Red Dwarfs
• Next up the stellar ladder are Red Dwarfs.
• Red dwarfs are 8-40% the mass of the
sun.
• Unlike the sun, the Red Dwarfs do not
have a Radiative Zone (a zone where
matter does not move through).
• In fact, the entire star is convective (like a
boiling pan of water).
• So, eventually, it will burn all the Hydrogen
in the star to Helium.
continued
• Red Dwarfs are very dim compared to the
sun.
• What does that tell you about the energy
generated at the core of a Red Dwarf?
• A) there is less of it
• B) it takes longer to get to the surface
• C) the energy has a harder time escaping
from the star
• D) tells you nothing
What does this tell you about the
expected lifetime of a Red Dwarf?
•
•
•
•
A) It is longer than our sun
B) It is the same as our sun
C) It is shorter than our sun
D) Tells us nothing about its expected
lifetime.
Yellow/Orange Dwarfs
• This is just a silly way of saying stars like
our sun.
• So, starts like our sun.
• They have Radiative Zones which
separate the core from the rest of the star
(much like our Stratosphere keeps clouds
in the Troposphere).
• The core is about 10% of the mass of the
sun.
Larger Main Sequence Stars
•
•
•
•
•
Here we have Blue stars.
Blue stars are always big.
They are very hot.
Their cores are very hot.
That means that even though they are bigger,
they use up their fuel a lot faster.
• So, they don’t live very long.
• A star stays on the main sequence for about:
10 Billion years / (its Mass in solar masses)2
• So, a star 10 time the mass of our sun will only
be on the main sequence for 100 million years –
they don’t live long.
Properties of stars
• Temperature: bigger star means higher temps
both on surface and in the core.
• Lifetimes: Bigger stars have shorter lives.
• Color: Big main sequence stars are blue.
Medium ones yellow/orange/white. Small ones
are red.
• Brightness: Bigger means much brighter (Mass
cubed).
• Size: More massive stars have bigger sizes (by
factor of mass).
• Density: Oddly, bigger stars have LOWER
densities! The biggest stars have an average
density of our air.
Concept question
• If a star is fusing Helium into something
else in its core then is it considered a Main
Sequence Star?
• Suppose a star uses up all its Hydrogen in
its core so only does fusion of Hydrogen to
Helium in a shell outside of the core. Is it
considered a Main Sequence Star?
However
• No matter what the size of star, with the
exception of the Brown Dwarf, all fuse
hydrogen into helium in the core (using
either p-p chain or CNO cycle).
• Eventually each of them will run out of
fuel.
• What happens next? Well, stay tuned. It
all depends on the size of the star.
Conclusion
• Fusion is really hard even in the cores of stars
• Fusion depends on Quantum Mechanics (or
Quantum tunneling) and very highly dependant
on temperature
• Stars don’t change much on the main sequence
over the course of their lifetime.
• Stars come in a wide range of masses (0.01 to
200 solar masses).
• Different massed stars have slightly different
attributes, but all do the same thing – fuse
protons into Helium.