The Hubble space telescope has found a black hole in one of the brightest star clusters in the sky. The discovery has forced scientists to reconsider the nature of Omega Centauri which is easy to spot from countries in the southern hemisphere.
Astronomers had thought the object was an unusual example of a globular cluster – tightly packed balls of hundreds of thousands of stars that are found orbiting galaxies like the Milky Way.
Now they believe that Omega is probably a dwarf galaxy itself but one that has been stripped of its outer stars.
The new discovery was made with the Advanced Camera for Surveys combined with data from a spectrograph on the Gemini South telescope in Chile. They show that Omega Centauri appears to harbour a medium-sized black hole in its centre – that is one of moderate mass.
Eva Noyola, of the Max-Planck Institute for Extraterrestrial Physics in Garching, Germany, led the team that made the discovery. She said: “This result shows that there is a continuous range of masses for black holes, from supermassive, to intermediate-mass, to small stellar mass types.”
Omega Centauri is visible with the naked eye as a blur almost the size of the full moon and is a favourite target for stargazers with backyard telescopes.
It lies 17,000 light-years away from Earth and was first listed in a catalogue drawn up by Ptolemy nearly two thousand years ago. The English astronomer John Herschel was the first to identify it as a globular cluster in the 1830s.
Astronomers are still trying to observe more closely a black hole at the centre of our own Milky Way. Circumstantial evidence has also already been found of a black hole in a satellite galaxy of our own.
Picture: A photo of the mysterious Omega Centauri. (NASA/ESA)