The last couple of lectures we've been discussing
the end game for a medium mass star, like
our sun. We discussed the first red giant
stage, and then the next one is the yellow
giant when the helium in the core is fused
into carbon. Eventually all of that helium
is used up, and the carbon core is formed.
The star is not simply massive enough to squeeze
the carbon core and heat it up so that nuclei
[inaudible 00:42] and carbon can fuse, and
basically, that's the end. What happens next
is that the thermal pressure, and also the
radiation pressure of the central object,
they push on the top layers and move them
out.
They formed so-called planetary nebula, this
spherical shell of gas or material, I should
say, of the star. What is left behind is this
dead, meaning it's not good use again through
fusion any longer, carbon core, with some
odd mixture of oxygen nuclei. That is what
white dwarfs are.
Let's now discuss in more detail remnants
of a medium mass star. They are planetary
nebula. They're composed of the star's material
that is pushed out by the thermal pressure
and radiation pressure during the final red
giant stage. We have this spherical shell
of material that is pushed out by pressure.
It's moving out, so this is planetary nebula.
It's moving out, and what is left behind is
this bit, meaning it's not producing energy
by infusion, carbon core. It forms what is
called white dwarf. Remember I said that it's
possible for a carbon nucleus to fuse with
another helium nucleus and form the nucleus
of Oxygen-16.
These are the remnants of a medium mass star
like our sun. Let me give you a few examples
of planetary nebula. This is the famous Ring
Nebula. We are viewing this edge-on. It looks
as if it was a donut, but it's really a cross-section
of this spherical shell.
Turns out that more light is coming from this
region here, the slice that is perpendicular
to our direction, to the direction which we
are viewing, and therefore, we get more light
from it than from the rest of the shell. We
perceive it as a donut, but it's really a
cross-section of a sphere. I'll explain that
in a moment. Let's look at another one.
This is a helix nebula. The reason they are
called planetary nebula is because when you
view them from a small telescope, these images
that I've shown were obtained through quite
powerful telescopes. If you use a small telescope
like the ones that people used in the early
days, when you look at them they appear to
have bluish-green color, the same color as
the color of planets Uranus and Neptune, and
has the name planetary nebula.
The name has nothing to do with planets or
planet formation. Historically, when they
were detected for the first time, these fuzzy
objects, their color looked just like the
color of those distant planets -- Uranus and
Neptune. Therefore, they were called planetary
nebula.
