Black holes sound very ominous, and they are – even though no human will ever see one. A black hole is a place in space where gravity pulls so much that even light can’t escape from it. The gravity, according to the US National Aeronautics and Space Administration (NASA), is so strong because matter has been squeezed into a tiny space; this can happen when a star is dying.

Because no light can get out, they are invisible, but space telescopes with special tools can help find black holes. The special tools can see how stars that are very close to black holes act differently than other stars.

Black holes can be big or small. Scientists think the smallest black holes are as small as just one atom; these are very tiny but have the mass (an amount of matter) of a large mountain.

Another kind of black hole, called “stellar,” has a mass of up to 20 times more than that of the sun. There may be many stellar mass black holes in Earth’s galaxy, the Milky Way. Fortunately, our sun will never turn into a black hole because it is not a big enough star to make one. Black holes don’t have a habit of wandering around in space gobbling up stars, moons and planets; Earth will not fall into a black hole because no black hole is close enough to the solar system for Earth to do that.

Even if a black hole the same mass as the sun were to replace the sun, Earth still would not fall in. The black hole would have the same gravity as the sun. Earth and the other planets would orbit the black hole as they currently revolve around the sun.

The largest black holes, called “supermassive,” have masses of more than a million suns combined. Scientists have found proof that every large galaxy contains a supermassive black hole at its center. The supermassive black hole at the center of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about four million suns and would fit inside a very large ball that could hold a few million Earths.

Scientists believe that the smallest black holes formed when the universe was created. Stellar black holes are made when the center of a very big star falls in upon itself, or collapses. When this happens, it causes a supernova, an exploding star that blasts part of the star into space.

There is now something new about black holes. An international team of astronomers, which includes researchers from Tel Aviv University (TAU), have developed a unique method for mapping gas clouds near a black hole.

Measuring the movement of matter around the giant quasar 3C273 – 2.5 billion light-years away from Earth – allowed the scientists to determine with an unprecedented accuracy the mass of the black hole. The new method, the team said, will allow astronomers to measure the mass of additional black holes across and deep into the universe.

This astonishing and “revolutionary” discovery has just been published in the prestigious scientific journal Nature.

Quasars are active supermassive black holes – that is, huge black holes that absorb gas from their surroundings at a dizzying pace. Using a new device called GRAVITY to observe the heart of the quasar 3C273, the team were able to directly see directly the gas surrounding the black hole.

“More than 50 years ago, astronomer Marten Schmidt identified a very bright but very distant object, the first quasar 3C273,” explained Prof. Hagai Netzer of TAU’s School of Physics and Astronomy, who was a partner in the new observation. “The energy emitted by this object is exponentially greater than the energy emitted from the entire Milky Way galaxy, for all the 100 billion stars in it. In fact, the energy is so great that the only way to produce it is by turning gravitational energy into heat, that is, by the flow of large amounts of gas into a huge black hole.”

Quasars in particular, and supermassive black holes in general, play a central role in the history of the universe. Their rate of growth is closely related to the development of most galaxies, and it affects their shape and size. Until now, it was not possible to directly map the position and velocity of the gas clouds surrounding black holes, except for the black hole in the center of our own galaxy.

To observe the gas disc surrounding the quasar 3C273, the international research group, headed by Eckhard Sturm and Jayson Dexter of Germany’s Max Planck Institute, used the GRAVITY device. Such measurements have not been possible until now because of the tiny size of the area where the material moves, about the size of the solar system, and its greater distance – 2.5 billion light-years away.

GRAVITY, however, makes it possible to connect four giant telescopes, each eight meters in diameter, at the South European Observatory in Chile, to an array called an interferometer that has the same separation capability as a single telescope with a diameter of 130 meters. This separation capability is equivalent to the ability to measure from Earth the diameter of a two-shekel coin lying on the moon,” added Netzer.

It should be noted that measurements of a completely different type of gas clouds, based on rapid changes in the light intensity of quasars, have been taking place for years. According to Netzer, the first measurements of 3C273 in the previous method were conducted at TAU’s Wise Observatory and were published in 2000 in the doctoral dissertation of Shai Caspi (currently a TAU researcher).

“The new method, and more accurate, allows us to determine many properties, such as the exact size of the area, the direction of the movement of the gas clouds around it and the precise mass of the black hole in the center,” said Netzer. “As we measure the mass of the sun by the speed of the Earth’s rotation around the sun and its distance from it, we measured the mass of the black hole by the movement of gas clouds around it and reached 300 million solar masses.”

Reinhard Genzel of the Max Planck Institute, who heads the research group that built the new device, noted the ability to apply GRAVITY methods developed to study the black hole in the center of the Milky Way can be applied to black holes in other galaxies. “The research group is working on five or six other bodies with similar characteristics,” concluded Netzer. “Later we will ask for more observation time, and I believe that after a number of years we will be able to generalize the result into smaller black holes, bigger and farther (that is, older).”