A black hole is a region of spacetime exhibiting such strong gravitational effects that nothing – including particles and electromagnetic radiation such as light – can escape from inside it.
A pulsar (short for pulsating radio star) is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The beam is emitted along the magnetic axis of the pulsar, which spins along with the rotation of the neutron star.
A neutron star is a type of compact star. Neutron stars are the smallest and densest stars known to exist in the Universe. With a radius of only about 11-11.5 km (7 miles), they can, however, have a mass of about twice that of the Sun.
A white dwarf, also called a degenerate dwarf, is a stellar remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to that of the Sun, while its volume is comparable to that of Earth.
O-type stars are very hot and extremely luminous. These are the rarest of all main-sequence stars. About 1 in 3,000,000 (0.00003%) of the main-sequence stars in the solar neighborhood are O-type stars.