What would happen to a galaxy should the black hole at its centre 鈥渓eave鈥� for whatever reason?
Herman D鈥橦ondt
Sydney, Australia
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In the short term, astronomically speaking, nothing much would happen. Without the black hole, there would only be some missing mass at the centre. Depending on the galaxy, this could be from 100,000 to a few billion solar masses. Galaxies have a total mass of between a billion and 30 trillion solar masses, so the missing amount would be a fraction of 1 per cent.
If the black hole vanished from the centre of our galaxy, the effects of its missing gravitation would travel outwards at the speed of light. Hence, the sun wouldn鈥檛 feel any difference for about 26,000 years. As the sun鈥檚 galactic orbit is determined by the bulk mass of our galaxy, and only to a small extent by its central black hole, at that time, it would experience a minute change in its orbit. Stars that were close to the black hole would, of course, alter their orbit more quickly and more violently. They might even be flung out of the galaxy altogether, as their orbital speed would be too high to be kept in check by the distributed mass of the galaxy.
Over the (very) long term, the fact that the galactic material would no longer be influenced by pressure waves when the black hole absorbed stars and nebulae might mean the formation of new stars was affected. So there might be fewer new stars in the galaxy. The Milky Way also couldn鈥檛 become a quasar, a galaxy emitting powerful radiation, as this depends on the existence of a central black hole.
Peter Bubenzer
Parksville, British Columbia, Canada
If the black hole suddenly 鈥渓eft鈥� a spiral galaxy like ours 鈥� that is, it just vanished 鈥� then, while it is a very large mass, it would still be outweighed by the mass of the rest of the galaxy. The galaxy would continue to rotate, but the centre of rotation would be shifted to the centre of mass of the entire galaxy as determined after the removal of the black hole.
I assume the orbits of many suns would be affected, so there would be some upset (collisions, orbits shifted, stars and planets ejected from the galaxy etc.) until the new configuration reached some form of stability.
Of course, if the black hole 鈥渓eft鈥� by taking a path out of the galaxy, rather than vanishing, that would also disrupt those parts of the galaxy it passed through, and take some parts with it.
Mark Thompson
Tewkesbury, Gloucestershire, UK
The Milky Way isn鈥檛 dominated by the supermassive, destructive force of the Sagittarius A* black hole 鈥� it may have a mass equivalent to over 4 million suns, but the Milky Way contains billions of suns.
If Sagittarius A* donned its hat and went off to play, the surrounding stars might suddenly find themselves setting off in all directions due to a massive change in gravitational pull, but the rest of the galaxy wouldn鈥檛 change.
In fact, for us earthlings approximately 30,000 light years away, we may see some of the aforementioned stars in new orbits, but we would be too far away for any gravitational changes (or radiation) to affect us.
For all we know, our black hole may have already gone, as the Vogons needed to build that hyperspace express route after all.
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