A protostar is a very young star that is still gathering mass from its parent molecular cloud. The protostellar phase is the earliest one in the process of stellar evolution. For a one solar-mass star it lasts about 1,000,000 years.
Brown dwarfs are substellar objects not massive enough to sustain hydrogen-1 fusion reactions in their cores, unlike main-sequence stars. Brown dwarfs occupy the mass range between the heaviest gas giants and the lightest stars, with an upper limit around 75 to 80 Jupiter masses.
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.