MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
Astronomers have tracked, for more than 18 years, the motions of several key stars at the very centre of the Milky Way galaxy, and have found that they follow very tight orbits around something very massive and yet invisible. The object is suspected to be a black hole weighing in at about 4 millions times the mass of the Sun. The closest star passes within just 90 astronomical units (AU) of the black hole, which is too small to be seen on the scale of this image. It is represented by the yellow dot. The inset shows a close-up of the black hole, surrounded by an accretion disc.
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