In 1687, the English scientist Isaac Newton published his monumental work, Philosophiae Naturalis principia Mathematica (Mathematical Principles of Natural Philosophy), containing his theory of gravitation and the mathematics to support it. In essence, Newton’s law of gravitation stated that the gravitational force between two objects, for example, two astronomical bodies, is directly proportional to their masses. Astronomers found that it accurately predicted all the observable data that science at that time was able to collect, with one exception – a very slight variation in the orbit of the planet Mercury around the sun.

It was 228 years before anyone was able to offer a refinement of Newton’s law that accounted for the shape of Mercury’s orbit. In 1915, Albert Einstein’s general theory of relativity was published. Using the equations of general relativity, he calculated the shape of Mercury‘s orbit. The results predicted astronomical observations exactly and provided the first proof of this theory. Expressing it very simplistically, the general theory of relativity presumes that both matter and energy can distort space-time and cause it to curve. What we commonly call gravity is in fact the effect of that curvature.

Among other phenomena, Einstein’s theory predicted the existence of blackholes, although initially he had doubts about their existence. Black holes are areas in space where the gravitational field is so strong that nothing can escape them. Because of the immense gravitational pull, they consume all the light that comes near them. And thus they are “black.” In fact, neither emitting nor reflecting light, they are invisible. Due to this, they can be studied only by inference based on observations of their effect on the matter – both stars andgases¹ – around them and by computer simulation. In particular, when gases are being pulled into a black hole, they can reach temperatures up to 1,000 times the heat of the sun and become an intensely glowing source of X rays.

Surrounding each black hole is an “event horizon,” which defines the area over which the gravitational force of the black hole operates. Anything passing over the lip of the event horizon is pulled into the black hole. Because observations of event horizons are difficult due to their relatively small size, even less is known about them than about black holes themselves.

Black holes exist in three sizes. Compact ones, called star-mass black holes and which have been known to exist for some time, are believed to be the result of the death of a single star. When a star has consumed itself to the point that it no longer has the energy to support its mass, the core collapses and forms a black hole. Shock waves then bounce out, causing the shell of the star to explode. In multiple explosions within the first few minutes of its existence. So-called super-massive black holes, also well documented, contain the mass of millions or even billions of stars. And just recently one intermediate black hole, with about 500 times the mass of the sun, has been discovered. Scientists have postulated that the intermediate black hole may provide a “missing link” in understanding the evolution of black holes.

Current scientific data suggest that black holes are fairly common and lie at the center of most galaxies. Based on indirect evidence gained using X-ray telescopes, thousands of black holes have been located in our galaxy and beyond. The black hole at the center of the Milky Way, known as Sagittarius A* (pronounced “A-star), is a supermassive one, containing roughly four million times the mass of our sun. Astronomers suggest that orbiting around Sagittarius A*, 26,000 light years from Earth, may be as many as tens of thousands of smaller black holes. One possible theory to explain this is that a process called “ dynamical friction” is causing stellar black holes to sink toward the center of the galaxy.

It is thought that the first black holes came into existence not long after the big bang. Newly created clouds of gases slowly coalesced into the first stars. As these early stars collapsed, they gave rise to the first black holes. A number of theories proposed that the first black holes were essentially “seeds” which then gravitationally attracted and consumed enormous quantities of matter found in adjacent gas clouds and dust. This allowed them to grow into the super-massive black holes that now sit in the centers of galaxies. However, a new computer simulation propose that such growth was minimal. When the simulated star collapsed and formed a black hole, there was very little matter anywhere near the black hole’s event horizon. Being in essence “starved,” it grew by less than 1 percent over the course of its first hundred million years. The new simulations do not definitively invalidate the seed theory, but they make it far less likely. On the other hand, it is known that black holes a billion times more massive than our sun did exist in the early universe. Researchers have yet to discover how these super-massive black holes were formed in such a short time, and the origin of these giants poses one of the most fundamental questions in astrophysics.

It has become practically a hallmark of the research on black holes that with each new study, more is known, more theories are generated, and yet more questions are raised than answered.