Stars that form iron cores quickly collapse as iron breaks down endothermically and some electrons are lost. Collapse first yields a neutron star with the mass of the Sun, but only the size of Austin. As protons are converted to neutrons, a huge flood of neutrinos is produced. If the collapse produces an explosion, the neutron star can be left behind. If the collapse is complete, a black hole results.
Smaller orbits have shorter period.
The total mass of the two stars can be determined by measuring the period of the orbit and the distance between the stars. Roche Lobes - Region of gravitational dominance of stars in a double system. More massive star reaches out further, has the largest lobe. Inner Lagrangian Point Connection point between Roche Lobes, point through which mass can be transferred between stars. Outer Lagrangian Points - Region beyond which matter orbits both stars or flows out of the system.