So far, we have only considered the evolution of single stars. However, the most amazing and bizarre objects in the Galaxy come from binaries. Consider two stars orbiting each other at a separation of a few A.U. For a while, the two stars will be perfectly happy. However, eventually one of the stars will become a red giant. Its size will increase greatly, until it approaches its companion star.

When this occurs, many different things can happen. If the un-evolved star is not very massive, and the separation between the two stars is large, maybe nothing will happen; the two stars may remain detached. However, it is possible that part of the surface of the giant star may find itself closer (gravitationally speaking) to the companion (secondary) star. If this occurs (i.e., if the red giant overflows its Roche Lobe) mass will be lost from the red giant and accreted onto the secondary. (This is sometimes called a semi-detached system.) Alternatively, the red giant may become so large that it engulfs its companion and forms a contact binary system. In a contact binary, the secondary star orbits within the atmosphere of the primary!

In contact binaries, things get crazy, and the results can be astounding. For instance, the friction caused by one star orbiting inside the atmosphere of the other can splash the atmosphere right off the red giant, leaving a naked core. (This is akin to stirring up the water in a bath-tub so much that it splashes out of the tub.) At the same time that this is happening, the secondary star will lose some of its orbital energy, due to friction, and spiral in towards the core of the primary. The result may be either a merged core, or two stars that are very close together. Note that, if the outside of the red giant is stripped away, what's left is nothing more than a white dwarf star.

Now things get interesting. Suppose the result of the contact binary system is a white dwarf and a companion star that are very close together. It is possible that the companion star will overflow its Roche Lobe and accrete matter onto the white dwarf. To some extent, this accretion is similar to the accretion of a semi-detached binary. However, there is a fundamental difference between the two events. To understand this, you have to consider what accretion really means.

In an accreting binary system, the secondary star is not standing still in space -- it is orbiting the white dwarf. As a result, when matter comes off the star, it, too, will orbit the white dwarf. So, if the matter is orbiting, how does it wind up accreting onto the other star? The answer is that is doesn't, unless something can take away its orbital motion. Is there something that can do that? Yes, there is --- its the accreted matter itself. If a lot of gas from the primary star is orbiting about the secondary, there will be friction, as some of the particles rub up against each other. This friction will heat up the gas, and also slow the gas down, so that it gradually spirals in towards the white dwarf. The gas will form an accretion disk.

Now note: the amount of friction occuring in the accretion disk depends on how fast the gas is orbiting around the star. But the orbital speed, of course, depends on the gravity (Kepler's and Newton's laws): the more massive the body, the faster the orbital speed; the closer to the body, the faster the orbital speed. If a secondary star is accreting onto a compact object, such as a white dwarf, the orbital speed and friction can be extremely high. The friction will heat the gas to extremely high temperatures -- in fact, from the blackbody law, the accretion disk will glow in the x-ray portion of the spectrum. When a ``star'' is observed to emit x-rays, it's very likely that it's not the star that's emitting the x-rays, but an accretion disk.

Finally, let's consider some of the strange objects that are formed by accreting systems. First, let's accrete onto a normal white dwarf. White dwarfs are white dwarfs because they have no hydrogen to burn. However, through accretion, the star can find itself with a substantial layer of hydrogen on its surface. After a while, as more matter is piled on it, bang -- the accreted matter (explosively) fuses. We call this a nova.

Now, let's fast forward a few years. White dwarfs are white dwarfs because electron degeneracy holds up the star against gravity. But electron degeneracy only works if the star is less than 1.4 solar masses. Normally, this is not a problem, but, over time, an accreting white dwarf may get enough material to put it over the Chandrasekhar limit. If this happens, crush, ka-boom, the white dwarf collapses in a supernova type event. (This is called a Type Ia supernova.)

Even a neutron star can be affected by accretion. Just like a person who strokes a basketball and makes it spin faster and faster, the accretion of matter onto a neutron star can make the star spin faster and faster. In the process, the neturon star/pulsar can be spun-up to rotate at thousands of times per second, Moreover, all this time, the accretion disk is radiating hard x-rays. This high energy light will blast the secondary star, and the added heat may literally evaporate it! The result would be a bare, millisecond pulsar.

And, of course, the binary systems with the most energetic x-rays involve accretion onto a black hole. Near the even horizon, the orbital speed of the gas is enormous, so extremely high temperatures are achieved. By looking for the brightest and most energetic x-ray sources in the sky, astronomers find black hole candidates.

THE CYCLE OF STAR FORMATION

The matter ejected into space by stars consists of both gas and dust. Some of this interstellar medium is far from any star, and is cold. When atoms encounter each other at low temperatures, they can form molecules, and this is what happens. Out in space, there are Giant Molecular Clouds that may contain over 100,000,000 solar masses of material. Dust, intermixed with the gas can be seen in silhouette, as it blocks out light from background stars. This dust also shields the inside of clouds from any heat from the outside. Within these regions, the temperature can drop to just a few degrees above absolute zero.

If there's alot of matter sitting it space, it won't just lie there. Whether it be a star, a black hole, or a large cloud of gas and dust, gravity is gravity, and slowly, gravity will begin to pull the interstellar matter together. Since the gas is so cold, there is virtually no gas pressure to resist gravity. The cloud will slowly collapse, and, begin some regions of the cloud are denser than others, the cloud will fragment into protostars.

As these clouds collapse, their central pressure will increase. This (through the equation of state) will cause their temperature to increase. The protostars will begin to glow, first in the microwave, and then in the far infrared. The dust surrounding the cloud will prevent any optical light from escaping, but very long wavelength light will make it through. We can see protostars with observations in the far infrared.

As the collapse progresses and the cloud gets smaller, the shape of the cloud will change. Because every atom of this cloud is gravitationally attracting every other atom, the matter in a protostellar cloud will gradually be drawn together. Meanwhile, if there is even the smallest amount of rotation associated with the cloud to begin with, this rotation rate will increase, as a consequence of the conservation of angular momentum. (This is the same physics we applied to pulsars.)
So now consider what will happen to this rotating cloud of gas. The gas that is near the poles of this cloud will not be rotating very rapidly (just like the north pole of the earth is standing still), so gravity will work to pull the gas towards the center. However, gas near the equator of the cloud will have a harder time falling towards the center, due to centripetal force. (For an example of centripetal force, imagine yourself in a car going around a racetrack at high speed. You will feel a force pushing you out away from the center of the track. This is actually due to Newton's first law [bodies in motion will continue in a straight line motion, etc.], but we will call it centripetal force.) So, the gas near the poles will collapse towards the center, while the gas in the equatorial plane will not. When the gas falling in from the top of the gas cloud collides with gas falling in from the bottom, the energy of the collsion dissipates, trapping the gas in the plane of rotation. In effect, the circular gas cloud collapses to a disk.

The physics of this disk is similar to that associated with mass-transferring binary stars. Friction in the disk will cause the gas to lose energy, spiral in, and finally accrete onto the protostar. This will increase the core's mass, which will increase the core's central pressure and temperature. The protostar will get hotter, and this heat will warm the surrounding dust. In the HR diagram, this evolution procedures from the red giant branch area (when the star is large and cool) to the main sequence. But we don't see these stars in the optical -- they're still surrounded by a large amount of dust.
Finally, the centers of these protostellar cores (which, up to now have been producing energy via their gravitational collapse) begins to fuse hydrogen to helium. The light emitted from these cores will then begin to disperse the surrounding interstellar medium, via radiation pressure from the photons. This may allow us to see these cores (although may times the stars will be heavily reddened, as the dust will cause the blue light to be scattered away.)

Eventually, the matter in a gas cloud is converted to the next generation of stars. The dust and gas that is blown away may compress nearby gas, and cause it to star forming stars. A chain reaction may result. Note that occasionally, you'll see a blue-ish haze surrounding these stars. That's due to left-over dust, and it appears blue for exactly the same reason the sky appears blue: the scattering properties of dust.

If any O and B stars are formed into a gas cloud, its high energy light will ionize the gas, lighting it up via a series of emission lines. This is called an H II region. Many of the pretty pictures you see of star forming regions are pink -- the pink is caused primary by a bright red (and some blue) emission lines of hydrogen.