There is probably a giant black hole at the center of our galaxy as well
Avishai Gal-Yam
Studies conducted in recent years indicate that black holes - objects whose existence was predicted by Einstein's theory of general relativity, but were considered for years to be only theoretical entities - exist in the universe and are apparently quite common.
Most of the stars in the universe emit different types of light and radiation. Einstein's theory of general relativity predicted the existence of completely different objects, with a large mass and a very small volume, which do not radiate at all: their gravity is so great that even light rays cannot escape from them. That is why they are called "black holes". For many years their existence was considered only a theoretical possibility, but in recent years much evidence has been accumulating that these objects do exist.
The hypothesis that huge black holes exist in the centers of galaxies first arose with the discovery of galaxies known today as "active galaxies". In the center of these galaxies, extremely powerful sources of radiation were discovered, the properties of which indicated that its origin was not in ordinary stars. In some cases this radiation is much stronger than the light emitted by all the other stars in the galaxy combined.
The objects from which such radiation is emitted were called "quasars" by their discoverers. To explain the source of the enormous energy of the quasars, some researchers have proposed that at the center of these galaxies exist enormous black holes. It is likely that in the process of the creation of the galaxies, a lot of matter flowed into their center and accumulated in a relatively small volume. Under these conditions, a black hole can form whose mass is very large - many millions of times the mass of the Sun, for example. Additional matter flowing into the center of the galaxy will begin to swirl around the black hole, become very hot and emit the radiation we see. Over the years, much evidence has accumulated for the correctness of this explanation, and it is now accepted by most researchers.
One of the strongest evidences that in the center of active galaxies material indeed falls into a giant black hole was obtained from X-ray observations. In recent years, satellites sensitive to X-rays have observed a large number of active galaxies. An analysis of the intensity of the radiation at different frequencies indicates that its characteristics match the radiation expected to be emitted from matter spinning very close to a black hole.
However, it seems that not only the centers of active galaxies have black holes. A group of scientists led by John Magurian from the University of Toronto recently used the "Hubble" space telescope to observe a sample of galaxies relatively close to our galaxy, the Milky Way. The superior resolution of the space telescope allowed scientists to estimate the density and velocity of stars in areas very close to the centers of galaxies. From the analysis of the observations it becomes clear that in the center of most galaxies there is a huge concentration of mass. The accepted explanation is that the concentrations of mass in the centers of galaxies are black holes.
Observations towards the center of our galaxy indicate that we are probably not unusual, and also in the center of the "Milky Way" there is a huge black hole. First evidence of the existence of the black hole was observed when radio radiation was detected from the central region of the galaxy. Such radiation is sometimes emitted from the surroundings of massive black holes in the centers of galaxies.
A group of German astronomers led by Andreas Eckhart and Reinhard Ganzel observed for several years the infrared light coming from the center of the galaxy using state-of-the-art equipment, which allowed them to determine with great precision the position of the stars in this region. In this way, the two researchers were able to follow the orbits of the stars in the central region of the galaxy. The analysis of the orbits of the stars indicates that a huge and invisible mass concentration affects these orbits - evidence of the existence of the black hole at the center of the galaxy.
Observations from the new "Chandra" satellite, launched by the American space agency NASA, were recently published, which observed X-rays coming from the direction of the "Andromeda" galaxy - the largest galaxy closest to our galaxy. The observations show that there is probably also a giant black hole at the center of this galaxy.
The "black hole", that creation of Einstein's theory of general relativity which for many years was considered only a theoretical oddity, has become in recent years another, interesting but rather routine component, in the increasingly clear picture of the universe.
{Appeared in Haaretz newspaper, 22/5/2000}
