Astronomers have observed a star speeding close to the massive
black hole at the centre of the Milky Way
for the first time.
The observations, made using the Very Large Telescope in Chile, tracked a star called S2 as it passed through the extreme gravitational field at the heart of our galaxy.
As the star approached its nearest point to the black hole on 19 May, it was accelerated to mind-boggling speeds, causing it to be subject to effects predicted by Einstein’s theory of general relativity.
Astronomers had been tracking the star and preparing to make the observations for the past 16 years ? the time taken for the star to complete a single elliptical orbit of the black hole.
“We have been preparing intensely for this event over several years, as we wanted to make the most of this unique opportunity to observe general relativistic effects,” said Reinhard Genzel of the
Max Planck Institute for Extraterrestrial Physics
(MPE) in Garching, Germany, who led the international team.
A major challenge was picking out the faint star 26,000 light years from Earth as it travelled in front of the black hole, which itself is surrounded by a halo of glowing dust and debris.
To achieve this required a telescope powerful enough to see a tennis ball on the moon from Earth and sophisticated systems to correct for vibrations of the telescope and interference from Earth’s atmosphere.
“We have put enormous effort into getting the instruments into good shape before the star approached the black hole,” said Thibaut Paumard, a researcher at the French National Research Institute, the CNRS, in Paris.
As the star made its closest approach to the black hole, at a distance about 120 times the distance between Earth and the sun, it reached a speed of 8,000km/s, or 2.7% of the speed of light.
Quick Guide
What are black holes?
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Black holes were first predicted by Einstein’s theory of general relativity, which reimagined gravity as the warping of space and time by matter and energy.
The equations predicted that, beyond a certain threshold, when too much matter or energy is concentrated in one place, space and time collapse, leaving behind a sinkhole through which light and matter can enter but not escape.
At first these were thought to be mathematical oddities, rather than real astronomical objects, but in the past century overwhelming evidence has confirmed that black holes are out there.
The edge of the black hole is defined by its so-called event horizon. This is the point at which escaping would require something to travel at faster than the speed of light ? which as far as we know nothing does ? so it is the point of no return.
Black holes are surrounded by an accretion disk of dust and gas, orbiting at close to the speed of light. A lot of this material is destined for oblivion, although some of it is ejected as powerful jets of radiation.
As it got close to the black hole the star appeared redder as the light leaving it was stretched out by the gravitational pull of the massive object ? as described by Einstein’s general relativity theory. The black hole at the centre of the galaxy is about 10 times the size of the sun in diameter, but has a mass of about 4m solar masses.
“It’s really the first time a [gravitational red shift] has been seen in such a clear fashion on an object that is moving,” said Paumard.
Closer analysis of the observations could reveal more about the distribution of stars and other objects close to the black hole.
The latest findings are published in the journal
Astronomy & Astrophysics
.