Back in the 18th century, scientists like Laplace has already pointed out that highly compact objects might prevent their nearby light from escaping. Soon after Einstein published his General Theory of Relativity in the beginning of 20th century, Karl Schwarzschild found a mathematical solution of the theory to describe the spacetime structure of such object with spherical symmetry. That was the prelude to the study of black holes. Later on, Oppenheimer and others through calculation proved that supermassive stars under gravitational force could really collapse to form black holes. By the 70’s, astronomers started to carry out systematic observations to look for evidence of black holes in binary system. The flush of observational evidence from the Hubble Space Telescope launched at the end of the 20th century further convinces us that black holes really exist. To the surprise of astronomers, black holes come in various sizes and origins, and are far more complicated than we can think of. For example, the sizes of black holes can vary immensely from a few to a few billions of solar masses! What is more important, the existence of these different kinds of black holes and the respective astronomical phenomena associated with them always brings far-reaching revelation to our understanding of the evolution of stars, galaxies and at last the whole Universe.
In the early days, man turned their eyes to the binary systems to look for black holes. From the spectral analysis of the orbit, if the invisible companion of a star in a binary system is 3.5 times heavier than that of the Sun, this dark celestial object is most probably a black hole. We take 3.5 solar masses as the benchmark for judging whether something is a black hole because we know that theoretically the mass of other compact objects (like neutron star) cannot exceed that maximum threshold. Otherwise those bodies will collapse under their own gravitational force into black holes. However, it is never an easy task to determine the mass of companion stars in binary systems just by way of spectral analysis. Miscalculations did always happen, as it is difficult to accurately measure factors like the luminosity of the visible stars in the pairs and the tilting of the orbits. Accretion disks formed when compact objects suck in matters of companion stars may also betray the existence of black holes. For neutron stars and black holes, accretion disk will emit high-energy X-rays when matters are spiraling in the compact objects, since immense gravitational force can cause substantial heat up of the matters. Searching for X-ray sources in the sky becomes the most important ways to locate neutron stars and black holes among binary system.