Will the supermassive black hole at the centre of our galaxy consume us all?

The headline of an online article1 posed this question: “Will Our Black Hole Eat the Milky Way?” It is a good question to ask. Should we, here on Earth, be afraid of the supermassive black hole at the centre of the Galaxy? With it acting like some sort of a super cosmic vacuum cleaner will it eventually suck up our home planet and the rest of the galaxy? The short answer is no. But let’s review why that is so, and you’ll see it is not quite the same answer that a secular astronomer would give.

Our galaxy, called the Milky Way, has a supermassive black hole at its centre. The black hole has a mass of about 4 million times the mass of the sun.2,3 The Galaxy as a whole has a mass of about 20 billion suns (assuming no dark matter4,5), which is about 5,000 times the mass of the super-massive black hole. This makes the mass of the black hole 0.02% of the mass of the whole galaxy. It’s very small but also the stars around the black hole, at the centre of the galaxy, remain in very stable orbits. Few are consumed by the black hole, and those which are, represent a very small consumption of the mass of the whole galaxy as a function of time.

So don’t worry. You have absolutely nothing to worry about. The amount of time it hypothetically would take the black hole to consume the Galaxy is practically longer than the age of the Galaxy, assuming only natural processes of decay, and collision with any nearby galaxies.

Essentially that supermassive black hole, located near Sagittarius A* (see Fig. 1), presents no problem just sitting there at the centre of the Galaxy. The orbits of the stars around it are stable.

sgrastar

Figure 1: Sagittarius A*. Credit: Chandra

Back in the 1970s, the astronomers Bruce Balick and Robert Brown realized that there was an intense source of radio emissions coming from the very center of the Milky Way, in the constellation Sagittarius. They designated it Sgr A*. The asterisk stands for exciting.1

In 2002, astronomers observed that there were stars zipping past this object, like comets on elliptical paths going around the Sun. Using Newtonian physics the mass of the central object can be calculated from the speeds of the stars orbiting, though Einstein’s relativistic physics is more accurate. So even though the central object could not be seen directly its mass could be calculated. And because of the permissible size that such a central object could be its density can be estimated. The only possible object with such density and gravity to affect the orbital speeds of the observed stars means it must be a black hole. In this case, it worked out that the black hole must have a mass several millions times the mass of our own sun. See Fig. 2. Continue reading