These notes will help you understand Ch 4 in Wheeler, and
will help you fill in the gaps in your notes from class.

Ch 5 in Wheeler is implicitly discussed in our work here, and in
our discussion of novae and supernovae, so if you read
it, you'll find most of it to be familiar.

When we discussed binary star evolution, we implied that mass
will be directly transferred from one star onto the other.
This is difficult to achieve. the reason is that, since the stars are
moving (orbiting each other), they have angular momentum.
In order for material to fall directly onto the other star, it
has to have zero angular momentum. That's hard to do.

Let's give the example of trying to dump nuclear waste into the Sun.
You know of the controversy over storing nuclear waste in the
ground for thousands of years before it's safe to be around?
Look up "Yucca Mountain" in GOOGLE if you don't know the story (not needed
for this course, needed to be an informed citizen).
Imagine that we had rockets that were big enough to get rid of all this junk.
Well, the sun wouldn't care, human waste is pretty puny.
So why not?

We the Earth orbits the sun at 30 kilometers per second. That's a lot
of orbital energy. If you want to hit the Sun you have to get rid
of this entire 30 km/s. You could do it by launching a rocket directly opposite
the orbital direction of the earth, at 30 km/s in that opposite direction, 
and then the rocket would have zero net angular momentum and will
fall straight in. We don't have big enough rockets to do this routinely.
There are ways to do it, but some "activists" would freak out,
perhaps legitimately.

So the point of the previous paragraph was to give you a concrete example of why
material coming off of one star doesn't directly hit the other.
So it goes into orbit about the other. Since there's one preferred
plane, it'll form a disk. Actually, it'll form a ring, like the rings
of Saturn. Turning a ring into a disk apparently is done through
friction or large scale motions in the rings.

By the laws of gravity, orbits of different sizes move at different speeds.
So each piece of the ring moves at a different speed, causing the
gas particles to "scrape against" each other. This scraping is
friction, which steals energy from the orbits and turns it into heat
and light. Ultimately, you end up with a disk.

Well, it turns out that pure friction isn't efficient enough to make
what we see. MAKE SURE YOU STUDY THE FIGURES IN CHAPTER 4.

A possible additional source of friction comes from magnetism.
Imagine (see fig 4.4) a blob of material help together by magnetism.
The part of that blob closer to the compact object moves
more quickly because of the laws of gravity. This stretches the
magnetic field. Eventually the lines of magnetic force snap (we stretched
them too hard) and deposit energy into the disk. This drives turbulence,
which is an added source of friction. 

Astronomers often think of the disk as a "flat third star" in
these systems. The disk gives off light, sometimes a substantial
amount of light even compared to the stars themselves.
The disk can change in brightness. how exactly it changes in brightness
is understood, but not at the level of detail we'd like. THE PHYSICS
IS HARD, so give us 20 or 40 years.

How a disk might change brightness depends on the atomic properties of matter.
Matter is more or less transparent to light (we call this the OPACITY
(opaque-ness)) dpeending on what it's made of and on its temperature
and density. 

When the temperature is low, hydrogen is relatively transparent.  But
the low density, outer parts of the disk are constantly receiving new
material from the other star. The matter becomes more opaque to light, trapping
that energy, changing the temperature. It gets hotter. Well, hotter
material is more opaque to light (there are more possible orbital transitions).
So the temperature goes up. 

The layer just inside the outermost part of the disk gets hotter, too,
since it can't dump out all of its energy. So, a wave of heating
travels from outside to inside. Friction increases, matter moves from
outside to inside. 

Now the outer part of the disk is transparent again. The end result,
in the same spirit as described above, is that a cooling wave
travels inward.

So the entire disk can change its brightness on some timescale.
What exactly that timescale is depends on the mass accretion rate
and the structure of the disk.

Observationally, what we know is that certain sorts of disks change
their brightnesses on timescale sof weeks or months, some on time
scales of years or decades, perhaps some on timescales of centuries or
thousands of years. As Jane discussed recently (another anachronism in
these notes), there is some real fine tuning needed to reproduce in
our computers some certain types of these stars.

Objects which change brightness on different timescales, not surprisingly,
have received different names. Yet, at least in a hand-wavey way,
all of those variables associated with accretion disks seems to
be similar in spirit, if not in detail.

AGAIN, LOOK AT AND STUDY THE FIGURES, BOTH IN THE TEXT AND ON THE WEBSITE.

What happens when matter finally makes it to the surface of a white dwarf
(to use a specific concrete example)? Well, it can sit there and pile up,
essentially doing nothing except adding to the weight of the white dwarf.
It can eventually fuse on the surface of the white dwarf, creating a nova
(as Jane discussed, we see carbon and oxygen in the spectra of novae,
so more than just this new hydrogen layer is expelled in the surface
explosion), or it can pile up till the central temperature
and density of the white dwarf are extreme enough to star uncontrolled
central nuclear fusion, which results in the explosion and
destruction of the star.

We'll talk about matter falling onto neutron stars or black holes later.