| 1) Hints for HW#4:
In problem #1 you are given the power of the supernova,
compared with
that of the sun. You are also given the intensity
of the supernova
compared with that of the sun.
In problem #2 there is a case of motion
of the wave source towards or
away from you (what does that do to the wavelength?)
2) Answer to More Email:
My second question is from class today.
Ok, now supposing a star burns all
it's H fuel, and it begins to contract because
the contracting force of gravity
is greater than the expanding force of pressure.
It seems though, that in some
cases the star would expand and not contract.
Afterall during it's main
sequence which lasts billions of years it's giving
off tremendous amounts of
energy in the form of heat and light. That
energy comes from e=mc^2 correct?
If that is true then the star should be losing
mass during that time. And
wouldn't it also lose mass from solar wind, neutrinos,
and even random gas
ejections. And since the force of gravity
is dependant on the mass of an
object, and the mass is decreasing, it seems
that the star should expand before
it ever reaches the end of the main sequence
and burns out all it's fuel. I
know that you said that stars like our sun expand
slightly during the main
sequence. Is this the reason for it?
To me it seems that with all that energy
given off that the star should expand to much
greater levels.
And my third, and I promise
last question for now anyway, is this: I went
over to my friends mod on the west side last
weekend and flipped on the TV set.
I only got to catch the tail end of the
program, but the commentator was
recapping a story about gamma wave bursts that
come from stars that sterilize
everything for light years. What's the
deal with those?
Answer:
The star loses VERY LITTLE mass through E=mc^2. Look at question #3
on
HW#3, and you will note that only ~10% of Hydrogen give less than 1%
of
their mass throughout the life of the sun. That's less than 0.1% mass
change.
This is also true for solar
wind loss in G2 stars. (Neutrino's mass and energy loss are also
negligible).
So, although gravity on the outer layers of a star is decreased, it
is
decreased by such a small amount that the effects are almost negligible.
There is some effect in the long run though, especially in more massive
stars.
The main reason that stars grow during the life of a star on the main
sequence is simpler: As the hydrogen is depleted in the core the temp
rises as the inner core slightly contracts. That results in HIGHER
fusion
rates, and higher overall temperature in the core. The overall pressure
throughout the star rises, and the star expands.
More on that in Monday |
