Hubble solves old, discovers new mysteries in our neighboring galaxy
With the Hubble Space Telescope, researchers have now been able to definitively prove a black hole in the center of the Andromeda Nebula for the first time. In addition, they spotted a surprisingly high number of previously unknown clusters of very young blue stars in astonishing proximity to the black hole.
The blue light of this star cluster in the Andromeda Nebula, a good two million light-years away, was discovered by the space telescope a decade ago. Only now has a team of astronomers led by Ralf Bender from the Munich University Observatory revealed the exact nature of the blue starlight through analyzes with the telescope's imaging spectrograph (STIS): It radiates from a cluster of a good 400 stars that were present in a short phase of violent star formation formed only 200 million years ago and are very densely packed in an area only a light-year across. The disk-shaped region is embedded by an elliptical ring of older, redder, and cooler stars that had been identified in previous images.
The stars are so young that the probability that similar clusters will form is not a unique event in the lifetime of the neighboring galaxy, which is 12 billion years old, the scientists speculate. At the same time, the researchers cannot explain the formation of the clusters in this region: The very young stars also move very quickly around a common center of gravity, which the astronomers have now definitely classified as a supermassive black hole.
Near the hole, gas should be circling so fast that star formation should actually be impossible. The stars "are there," says Bender. Apparently, young stars near a black hole are not even uncommon, since they have also been detected in the Milky Way near the central black hole.
The black hole at the center of the Andromeda Nebula, with a mass of 140 million suns, is four times larger than previously thought, as the investigations further show with unprecedented accuracy. It had already been characterized in 1988 as the first of about forty supermassive black hole candidates, but this was not accepted by all observers and had contributed to a theory dispute. Other, less probable possible explanations could never be completely ruled out. For example, observations of supposed black holes could theoretically also be explained by a dense collection of burned-out stars, so-called dark clusters. This is now no longer possible, at least in the case of the Andromeda galaxy.
