Transcript Stars and Light

The
Sun and
the Life
of Stars
Orion Nebula
NEBULA:
A large ball
of dust and
gas.
All the mass in
the nebula will
be pulled in
toward the
centre.
The density of
the forming
star (called a
protostar) will
increase.
• Since these nebula are massive (~1030 kg), the
gravitational force has a strong pull towards
its centre on all its matter.
• This results in immense pressure in the core.
• The high pressure results in high temperature
(16 million oC), which allows nuclear fusion
to occur.
Nuclear Fusion
• Fusion results when two nuclei collide at very
high speeds to “fuse” into a new nuclei.
• The speeds have to be sufficient to overcome
the strong electrical repulsion from the
positive nuclei. (This is why the temperature
must be high.)
Nuclear Fusion
• The simplest fusion is
between H nuclei.
• Essentially, the H nuclei
fuse together (in stages),
to create a helium nuclei
• Along with He, energy
in the form of light is
produced (along with
invisible particles called
neutrinos).
Nuclear Fusion
• The light produced is very high
energy  rays. If this light made
it to Earth, we would be
“fried”.
• So what happens?
• The light goes through a series
of collisions with atoms in the
radiative zone, which we call a
random walk.
• During each collision the light
loses energy, eventually
becoming UV and visible light.
Nuclear Fusion
• Similar processes can occur to create larger
nuclei, but this would require…
• …higher temperatures, which means…
• …higher pressure in the core, which means…
• …more mass is needed, which means…
• …a larger star!
• But… this can happen to our star too!
Nuclear Fusion
• During the productive phase
of its existence, the gravity
pushing towards the centre of
a star is balanced with the
outward pressure from fusion.
• Once the fusion stops, (H runs
out) gravity will force the sun
to collapse, which will
increase the temperature so
He can fuse (to form carbon).
• When it does this, the outer
layers “explode” and it
becomes a Red Giant star.
• The outer layers of the red giant, are now
too far away to be held tightly by the star’s
gravity, so they drift away.
• What is left collapses inward to become a
small, hot dense white dwarf
• This will eventually cool and fade.
Questions
• 1) How do you think this process
would be different for a) larger stars?
b) smaller stars?
• 2) Our sun will be “productive” for
about 10 billion years. How do you
think this compares to a) larger stars?
b) smaller stars?
Answers:
• Smaller stars have a smaller gravitational
force, therefore…
– Less acceleration, so it takes longer to form
– Less pressure, so the internal temperature is
less, so fusion proceeds less rapidly, so the star
lasts longer (even though it has less “fuel to
burn”
• Other than that, the process remains the
same.
Larger Stars…
-If a star has more than 8 solar masses (rare), it
becomes a red supergiant instead of a red giant.
-When the core collapses inward, the outer layers
explode outward in what is called a supernova
APOD: January 24, 1997 - Supernova 1987a
Fireball Resolved
-The core continues to collapse, becoming a very
dense neutron star
-If the star is more than 30 solar masses (very
rare), when it collapses it becomes a black hole
which is so dense nothing can escape its
gravity once it gets too close.