For this discussion, however, the black holes we're interested in are much, much larger, with masses of millions to billions
times that of the Sun, and they are found in the centres of galaxies. The best-studied example of such a supermassive
black hole lies at the heart of our own Milky Way. This object, known as Sgr A*, is identified as a weak and variable radio
source right at the dynamic centre of our Galactic bulge. In the last twenty years infrared observations, which can peer
through the veil of dust in the disc of our Galaxy, have revealed a population of stars orbiting the location of the radio
source. By measuring the orbits of the stars, we can deduce that the central object has a mass of roughly four million
times that of the Sun, despite being remarkably faint (Figure 4). Decades of study of this object have led to the
overwhelming consensus that it is indeed a supermassive black hole.
For this discussion, however, the black holes we're interested in are much, much larger, with masses of millions to billionstimes that of the Sun, and they are found in the centres of galaxies. The best-studied example of such a supermassiveblack hole lies at the heart of our own Milky Way. This object, known as Sgr A*, is identified as a weak and variable radiosource right at the dynamic centre of our Galactic bulge. In the last twenty years infrared observations, which can peerthrough the veil of dust in the disc of our Galaxy, have revealed a population of stars orbiting the location of the radiosource. By measuring the orbits of the stars, we can deduce that the central object has a mass of roughly four milliontimes that of the Sun, despite being remarkably faint (Figure 4). Decades of study of this object have led to theoverwhelming consensus that it is indeed a supermassive black hole.
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