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Across 2. A pulsar with a period of approximately a millisecond, a thousandth of a second 4. A flat, rotating disk of gas and dust surrounding a condensed mass, such as a young stellar object, a forming planet, or a collapsed star in a binary pair 5. Emission of particles, mostly photons, near the event horizon of black holes due to the quantum creation of particles from the gravitational energy of the black hole. 6. Objects that radiates tremendous amounts of energy in the form of gamma rays, possibly due to the collision and merger of two neutron stars initially in orbit around one another. 7. Theory proposed by Einstein to incorporate gravity into the framework of special relativity. 8. The leftover core of a massive single star after a supernova; these have so much gravity that not even light can escape 10. A point in which matter is infinitely dense, as in the center of a black hole or the universe at the very beginning. 12. The location around a black hole where the escape velovity equals the speed of light; the boundary of a black hole 13. The minimum velocity required to escape to infinity from the gravitational field of an object. 14. X-Ray source that radiates thousands of times more energy than our Sun in short bursts lasting only a few seconds. 15. A source of short, precisely timed radio bursts; thought to be a spinning neutron star

Down 1. A mass in the region of a few million to a few billion solar masses, is an object that forms when a very large mass collapses, or a number of black holes merge into one, in the core of a galaxy. 3. The radius of the event horizon of a nonrotating black hole of mass M, equal to 2GM/c^2 9. A small, highly dense star composed almost entirely of tightly packed neutrons; radius about 10 km 11. A class of neutron stars that have exceedingly strong magnetic fields; thought to be responsible for soft gamma-ray repeaters