In the late 18th century, two scientists (John Michell and Pierre Simon Laplace) separately theorized that a star could be so massive that not even light could escape its surface. But nothing more was made of these “dark stars” until the 20th century, when Albert Einstein published his general theory of relativity.

His idea turned the notion of gravity on its head: It isn’t really a push/pull force, but instead describes how mass affects the fabric of space-time. And general relativity made possible the reality of those dark stars. In 1916, just one year after Einstein published his theory, physicist Karl Schwarzschild calculated general relativity’s gravitational equations in an extreme case (when an object has a mass near infinity) and found that the fabric of space-time would fold in on itself, creating a “singularity” — a region with zero volume and infinite density. Such a point would not allow matter or even light to escape.

Today’s term for this object — a black hole — came 50 years later, around the same time researchers began discovering hints of them. Now, after 40 years of research, scientists are positive these extremely dense bodies exist, and that there are two different types. Astronomers categorize black holes as either stellar mass (which range from three to tens of times our Sun’s mass and mark the end state of a massive star) or supermassive (which are millions to billions of times our star’s mass and sit at the centers of galaxies).

The smaller variety

Over the years, different methods have convinced scientists that both types of black holes exist. One of the most

successful techniques astronomers have used to find both stellar and super-massive black holes is monitoring the movements of stars near suspected black holes. They use the visible companion in a binary system as a tracer.

This method arose in the late 1960s – when X-ray detectors aboard satellites spotted sources of X-rays in our galaxy. Many of these changed brightness over fractions of a second, so astronomers called them “X-ray transients.” Scien-tists soon realized that some of these objects were binary systems.

In X-ray binaries, if the two bodies are close enough, the compact object pulls hot gas from the star. The stellar material heats up to millions of degrees due to friction and emits X-rays. The unseen compact companion could be a neutron star or a black hole; both are small enough to appear “invisible” until gas falls onto them, which then heats up and produces a bright X-ray source.

To distinguish between the possibilities, scientists need the invisible object’s mass, which they can determine using basic physics laws. Astronomers collect data through the visible star’s spectral emission, which tells them about the star’s composition and movement.

As the star moves away from Earth in its orbit around its invisible companion, its – emission shifts slightly toward the red end of the electromagnetic spectrum; as it speeds toward our planet, its emission shifts to the blue end. From the amount the spectrum shifts, astronomers can determine how fast the visible star is moving and how long it takes to complete one orbit. Then, using the same law of physics that dictates how the planets orbit the Sun, they can calculate the mass of the unseen companion.