A Supermassive Black Hole is the largest type of black hole in a galaxy, in the order of hundreds of thousands to billions of solar masses. Most, and possibly all galaxies, including the Milky Way, are believed to contain supermassive black holes at their cores.
Supermassive black holes have properties which distinguish them from lower-mass classifications:
- The average density of a supermassive black hole (defined as the mass of the black hole divided by the volume within its Schwarzschild radius) can be much less than the density ofwater for very large mass black holes (the densities are similar for 108 solar mass black holes). This is because the Schwarzschild radius is directly proportional to mass, while density is inversely proportional to the volume. Since the volume of a spherical object (such as the event horizon of a non-rotating black hole) is directly proportional to the cube of the radius, average density decreases for larger black holes, being inversely proportional to the square of the mass.
- The tidal forces in the vicinity of the event horizon are significantly weaker. Since the centralsingularity is so far away from the horizon, a hypothetical astronaut traveling towards the black hole center would not experience significant tidal force until very deep into the black hole.
In June 2000, astronomers made an extraordinary discovery. One that promises to solve one of the biggest problems in cosmology – how and why galaxies are created. Incredibly, the answer involves the most weird, destructive and terrifying objects in the Universe – supermassive black holes. Scientists are beginning to believe that these forces of pure destruction actually help trigger the birth of galaxies and therefore are at the heart of the creation of stars, planets and all life.
Exactly how our galaxy was created has mystified astronomers and physicists for years. Although there have been many theories, there’s little evidence to explain how the gas in the early Universe condensed to form the galaxy we see today. Now scientists realise they’ve been missing a vital ingredient – a supermassive black hole. The immense gravity of a giant black hole might trigger the gas to collapse in the first place. By churning up the gas around it, a giant black hole would trigger the birth of stars, planets and life itself. Despite being the most destructive thing in the Universe, scientists now think our supermassive black hole could be crucial in creating the galaxy as we know it.
The supermassive black hole in our own galaxy may be the reason we exist, but recent work suggests it may also be our end. At present the Earth is too far away from this black hole for it to affect us, but physicist John Dubinski thinks all that could change. In January 2000 he graphically simulated the final fate of our galaxy. In 3 billion years we will collide with the next door galaxy, Andromeda. The resulting apocalypse will force the Earth and our Solar System out of orbit. Dubinski has calculated a worrying 50:50 chance that we’ll be sent hurtling in towards the black hole at the centre of this maelstrom. This would be fatal for the Earth.