If the Sun becomes a black hole!

In my recent interactions with some students, I realized that black holes get portrayed in extremely bad light. They are made out to be the villains of the universe, angry and monstrous eating away anything they can find, may it be stars or sometimes even galaxies! I wondered where this notion comes from among the students. Then I realized its in the way black holes are described. These are objects that have the most intense gravity we know of. In fact their gravity is so strong that nothing, not even light can escape from them. Everything gets sucked into them and can never escape out.

As much as the first two statements describing these mysterious objects are correct, the third is quite misleading and it is this statement that creates confusion amongst the general public. If everything gets sucked into a black hole, how are we still here?

“Because we are not close to one of course!”

Ah! So how close does one have to be? What if for some odd reason the Sun shrinks in size and becomes a black hole right now? Will the Earth get sucked into it? Let us explore the situation that this will create.

Remember that this is a hypothetical situation. As far as we know the Sun will never reach this stage because it does not have sufficient mass. Only extremely massive stars become black holes at the end stages of their lives. This happens when all the hydrogen and other elements that they burn as fuel are exhausted. The outer layers of such stars blow up and the remaining mass is enough to accumulate together and form a black hole.

Scientists currently estimate that the Sun does not have this necessary mass. But let us ignore that fact and imagine that this actually happening. I am sure some of you will think that life as we know it will come to an end because our source of heat and energy is no longer in existence. Its true that life cannot survive without the Sun shining but let us ignore this aspect as well. Rather, let us see what will be the effect on the planet instead of the effect on living and non living things on the planet.

So reviewing what we have understood so far quickly, we note that the Sun has shrunk and become a black hole. This is a hypothetical case and we are ignoring the fact that the Sun does not have sufficient mass for this to happen. We are also ignoring the effect of the loss of sunlight on the lifeforms that exist on the Earth. Let us simply see what will happen to planet Earth. Will it get sucked into this black hole?

The first thought that may come to mind is that the Earth will spiral in and get destroyed. But what might be the cause of this? The Earth revolves around the Sun because of a balance between the gravitational force if feels from the Sun and its own angular momentum. For it to spiral into the black hole this balance has to be broken and gravity has to dominate. Since the angular momentum cannot change, gravity has to increase. Can this happen?

Long time ago, Newton told us that the gravity you feel is determined by two parameters. One is the mass of the object and the other the distance of that object from you. More the mass, more will be the gravity and more the distance less will be the gravity. Additionally Newton also showed that the mass of an object can be considered to be concentrated at a single strategically placed point. This point is called the center of mass. For spherical objects it is the center. So the gravity we feel on Earth would be the same gravity we would feel if the Earth shrunk to a point but we remained where we currently are – about 6400 km from the center (Refer to Fig 1). In much the same way, the Sun’s center is its center of mass. The gravity that the Earth feels due to the Sun can be thought of as arising from the Sun’s center of mass. This is very important point and will lead us to the answer we are searching for.

Figure 1

Even though the Sun has shrunk to a point, there has been no addition or reduction in its mass. Thus the gravity will not change because of this factor. What about the distance? That hasn’t changed either. The distance between the Earth and the Sun’s center of mass is still the same. Its just that instead of the mass being all around that point as in the case of the Sun, it is now concentrated only at the center of mass. So this wont change the gravity either.

If the gravitational force felt by the Earth does not change, there will be no change in its behavior. It will continue revolving as always around the newly formed black hole! None of the other planets would change the way they revolve (or rotate) either. The entire solar system will continue existing, with the sole difference that the Sun would no longer be shining.

There will be no change in the gravitational force till the region where the surface of the Sun once lay. Only beyond this point will the gravity be more than it originally was. (Refer to Fig 2) And here lies the notoriety of a black hole. Normally, an object would have hit the Sun’s surface and burnt away. Now, because there is no surface this object can get closer to the center without hindrances. And as it gets closer and closer, the gravitational force will start becoming greater and greater. Soon it will cross a limit where the gravity will become so strong that this object would now be unable to return. Even light faces this limiting boundary. It is called the event horizon. An asteroid, comet or spacecraft unfortunate enough to cross a certain limiting distance from the black hole will be lost forever. In any case, this limiting boundary would be fairly close to the center of our black hole, a long way inside from where the surface of the Sun had been. All objects far away can escape.

Figure 2

Thus only objects coming very close would be eaten up. Since the Earth is well away from the event horizon and its own limiting boundary, it will continue revolving around the black hole.

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Tagged: astronomy, black hole, Earth, sun

Posted in: Articles, General Physics