X-ray Binaries

I've previously mentioned binary stellar systems. They're pretty neat. But like good physicists, let's make them even neater by taking one of the stars to its logical extreme. Say it's massive - more than a few solar masses. It goes through the normal stellar evolution process and eventually becomes either a neutron star or a black hole. If we look in the rotating frame of the binary, the potential energy (from a combination of gravitational potential and centrifugal potential as a result of doing physics in a noninertial frame) behaves roughly as we would expect. There are deep wells where the stars live, and the potential energy drops off as you move further away. The interesting bit is the saddle point in between the two stars. It's a sort of tipping point - a little bit of mass near the saddle falls into the closer well.
Interesting things start to happen when a star extends past this saddle point. It's called exceeding its Roche lobe, and it means that the material past the saddle point starts to spiral in towards the other star (in our case, a very compact object). Because the mass already has some angular momentum, it is forced to spiral inwards instead of just falling. This creates an accretion disk, with lots of mass swirling around the compact object. As it does so, it falls in slowly, just based on viscous effects as particles run into each other.
The radiative efficiency of accretion disks is stunning. For comparison, the efficiency of hydrogen fusion is 0.7% (that's 0.007). That means that 0.7% of the mass energy of the hydrogen atoms is converted to heat/radiated energy. But for accretion disks, the efficiency can be as high as 40%, depending on rotational speed, inner radius of the disk, and the nature of the compact object (neutron stars can have way higher-efficiency accretion disks because of surface effects). That's almost two orders of magnitude more efficient than fusion. Wow.