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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?
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).
Birth of a star
• A star is born when its core can finally
generate energy via nuclear fusion.
• This energy prevents the core from
collapsing.
• This is done by radiation pressure and gas
pressure (they counteract gravity).
• But to keep this up requires the constant
generation of energy in the core.
Main Sequence Star
• Now that the star is born, what will it do?
Main Sequence Star
• Now that the star is born, what will it do?
• Well, not much.
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 will happen?
• What happens when the red dwarf runs
out of Hydrogen to burn?
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
• 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.