Black holes were originally no more than the mental inventions of theoretical physicists. Now, however, astronomers can actually identify and observe them. They offer a wealth of information on the nature and evolution of galaxies. The latest discoveries in this field have been among the great success stories of European astronomy.
The thrilling story of black holes began back in 1963, with the first identification of quasars – very distant objects emitting a phenomenal amount of energy. The brightest ever recorded, 3C273, located in the constellation of Virgo, is a billion billion times brighter than the sun. Only gamma-ray bursts produce more energy than quasars (see p. 42).
An all-consuming attraction
How can an energy emission of such enormous power be explained? Scientists put forward the hypothesis that such a phenomenon might be related in some way to black holes, those objects postulated in cosmological theories derived from modern physics. One might describe them as strange ‘zones’, fields of attraction situated at various points in the universe which ‘capture’ or ‘suck in’ stars passing within reach. These objects are so dense that not even light can escape from them.
We now know that black holes do indeed exist, and that the energy they sporadically emit – corresponding to the phenomenon of the quasar which is associated with them – actually comes from the matter or stars they have absorbed. When a star falls into a black hole’s field of attraction, its gravitational energy is released in the form of intense heat and radiation – first in the form of radio waves, then infrared, then light, UV and X-rays.
The black hole which has exercised the most powerful fascination for astronomers is located at the centre of our own galaxy, the Milky Way. For a long time astronomers had wondered about an enigmatic and very powerful source of radio emissions called Sagittarius A and regarded as a candidate for the title of the "supermassive centre of our galaxy", but had not been certain whether it was in fact a black hole.
Precise, irrefutable and definitive proof was eventually found by an international team at the controls of one of ESO's four VLT telescopes in Chile, using its adaptive optics system (see box). After ten years of observation (1992-2002), the researchers finally obtained striking images of the star S2. This star, with a mass fifteen times that of our own sun, orbits the famous Sagittarius A at a speed of 5 000 km/s (that is to say 200 times faster than the Earth’s movement around the sun).
Reinhard Genzel, Director of the Max-Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, an active member of the team, explains: "The mass of the star S2, and above all its perfectly elliptical orbit, which has been observed with great precision, imply that the centre of attraction located at one of the focal points of its orbit has an enormous mass. The mass of this object must be 2.7 million times that of our own sun within a tiny volume no more than 10 light-minutes in diameter – a little less than the orbit of Venus, the second closest planet to the sun." These features being compatible with no other phenomenon except a black hole, the final proof had thus been found that Sagittarius A is indeed our galaxy's central supermassive black hole.
Proof – in a falling star
Other recent discoveries demonstrate that the era of direct observation of the physics of black holes is well and truly with us. On 8 May 2003 one of the astronomers in Genzel’s team, gazing at the control screen of his giant telescope, was suddenly startled by the presence of an unexpected star. He received a second surprise, just a few minutes later, when the star suddenly disappeared. The team had just observed – for the first time ever – a very intense flash in near infrared, at precisely the location of the supermassive black hole at the centre of our galaxy.
This wavelength of radiation matched the signature of an accretion of heated matter which, during its ‘fall’ into the black hole, begins to heat up and thus give off near infrared rays. After years of research, therefore, the elusive evidence of absorption of matter by a black hole had finally been found. Hitherto no one had actually witnessed this last signal from matter caught by a black hole and crossing the point of no return towards an unknown destiny.
The most striking result of this observation was that this emission varied very rapidly in intensity in just a few minutes, proving that these infrared signals must come from a miniscule region. This zone is situated right at the frontier of the black hole, beyond which no more radiation can escape. The phenomenon also reveals the existence of periodicity in the radiation, due to the spiralling movement of the matter orbiting the black hole before vanishing within it. It must revolve extremely rapidly. "This is a major discovery," adds Reinhard Genzel, "which allows us to confirm current theories about black holes. Until recently such a possibility was still unimaginable."
When astronomy meets theory
21st century astronomy concerns itself not only with questions of astrophysics – such as cosmic inflation, dark matter, dark energy and so on – but also with the great questions of physics at its most theoretical. Thus one can now imagine testing, by observation, the theory of quantum gravitation, the evolution of the cosmos, the Big Bang itself, or even the stability of fundamental constants throughout the history of the universe. In just fifteen years staggering progress has been made. One of the next steps will be to understand when and how these supermassive black holes were formed, and why almost every large galaxy seems to have one.
The Yepun telescope, one of the four making up the VLT at Mount Paranal in Chile, is equipped with NaCo and SINFONI adaptive optics systems. This technology compensates for defects in the image due to atmospheric disturbances by distorting the mirror and producing an exact opposite of the defect; combining the two effects...
The Yepun telescope, one of the four making up the VLT at Mount Paranal in Chile, is equipped with NaCo and SINFONI adaptive optics systems. This technology compensates for defects in the image due to atmospheric disturbances by distorting the mirror and producing an exact opposite of the defect; combining the two effects cancels both out. This is done with the assistance of a star occupying a predictable position in the observation field, which serves as a fixed point of reference. As soon as the tiny image of that reference point, the star, becomes distorted or unclear, ultra-rapid servomechanisms (performing 1000 operations per second on a distorting mirror within the instrument) restore its ‘clarity’, and thereby that of the rest of the field of observation.
When astronomers are observing an area of space where no reference star is available, they make use of an artificial star. This new feat of technology was introduced in 2006. A powerful laser beam (with a precise wavelength) stimulates the sodium atoms present in large numbers in a layer of the atmosphere about 90km above the Earth’s surface, generating a characteristic bright light.