Although black holes were theoretically postulated in the 1920s, they remained outside the domain of observational astronomy until the 1970s.

Their closest cousins are neutron stars, which are composed almost entirely of neutrons packed against each other, making them extremely dense. A neutron star has the same mass as the Sun but its diameter is only a few kilometers. Therefore, the escape velocity from their surface is close to the speed of light.

Not until the 1960s, when the first neutron star was discovered, did interest in the existence of black holes become renewed. Evidence for black holes is based upon several types of observations, such as radiation analysis of binary stars (called X-ray binaries), gravitational lensing of the light from distant galaxies, and the motion of visible objects around invisible partners.

The closest and perhaps the most dramatic evidence for a black hole is at the center of our galaxy, the Milky Way. Using data obtained from the W. M. Keck Observatory, astronomers have been able to determine the orbits of several stars near the center of our galaxy. The periods and sizes of these orbits suggest that the stars are orbiting a mass of approximately four million solar masses. Moreover, this mass must reside in the region obtained by the intersection of the stars' orbits. This region is so small that it would fit inside the orbit of Mercury. However, no light is emitted from this region in the visible spectrum.

The only logical conclusion of these observations is that a black hole exists in this region. Astronomical observations of regions outside the Milky Way suggest that black holes with masses millions of times more massive than the Sun are common at the center of galaxies.

This text is adapted from Openstax, University Physics Volume 1, Section 13.7: Einstein's Theory of Gravity.