Are black holes and neutron star non luminous?
1 Answer
Yes for black holes (but the region surrounding a black hole can be highly luminous), no for the neutron stars.
Explanation:
Black holes are indeed nonluminous because nothing, not even light, can escape.
The region surrounding a black hole, however, can be highly luminous as infalling objects, drawing energy from the gravitational field, become hot enough to produce intense radiation, especially X rays (as in quasars, objects as small as stars but bight enough to outshine entire galaxies).
While such intense radiation is seen as evidence for black holes, it cannot come from black holes as such. The recent experimental discovery of gravitational waves, which black holes can emit directly, is thus a key development in identifying and verifying black holes.
Neutron stars, however, are not quite as concentrated; light (and other electromagnetic radiation) can escape and we even see some of them as rapidly pulsating stars.
