If the Sun was suddenly replaced with a black hole of the same mass, what would happen to Earth?
Let’s take a few steps back into the black hole’s and study the star that it once was. Stars form when matter accretes within a cloud of gas and dust, heats up as it contracts under its own gravity and, at some point, begins nuclear fusion. At first, hydrogen nuclei are fused into helium. The mass of four hydrogen nuclei, i.e. four protons, is a little more than the mass of the resulting helium nucleus, and the surplus is converted into energy that the star radiates.
Once the star runs out of hydrogen, it begins to fuse heavier elements; the heavier the star itself is, the heavier the nuclei it can fuse. For example, once a Sun-like star exhausts its hydrogen, its core contracts to enable helium fusion while its outer shells expand to form a gigantic, redder star called a red giant. The helium fusion in its core produces carbon; once it runs out of helium, the core can’t shrink any further to fuse carbon. So the outer layers continue to expand and form a planetary nebula while the inner carbon core becomes a white dwarf. This process is extremely slow, panning out over about 50,000 years.
Heavier stars are able to fuse elements like neon, oxygen, silicon and iron. However, it’s impossible to fuse elements heavier than iron because the iron nucleus is extremely stable. A star full of iron is bound to have become a supergiant – very large and very bright – with a core that is rapidly contracting and thus heating. Since the star can’t fuse iron, the core contracts until it has become hot enough to disintegrate iron, and any lighter elements therein. Eventually, the core is left with just protons, electrons and neutrons, even as it continues to contract.
At some point, it becomes so hot that the protons and electrons fuse to form neutrons, and the core is transformed into a super-dense object called a neutron star. Neutron stars that are more than thrice as heavy as the Sun collapse under their own weight to create black holes.
As it happens, our Sun can’t become a black hole because it is not massive enough. But let us assume that by some conspiracy, it does become a black hole. What would happen to the Solar System?
A rocket on Earth’s surface possesses some gravitational potential energy that binds it to Earth. When it lifts off, it keeps accelerating and increasing its kinetic energy until it exceeds the gravitational potential energy. At this point, the rocket will be able to escape Earth’s gravitational influence. This minimum velocity threshold is called the escape velocity.
With a black hole, the escape velocity can be staggeringly high. For example, the distance from the centre of the black hole where the escape velocity is equal to the speed of light is called the Schwarzschild radius (named for Karl Schwarzschild). If the Sun were to become a black hole, it would have a Schwarzschild radius of about 2.96 km, which is 0.0004% of its current radius.
What happens to objects outside this radius?
When the core of a neutron star collapses to form a black hole, the neutrons resist the contraction and produce a shock wave. This wave of energy throws the former star’s outer layers outwards in a spectacular event called a supernova. Supernovae can be very powerful; each one can outshine the light of all the stars in the rest of the galaxy put together. The energy of a supernova triggers explosive nucleosynthesis, a process in which elements heavier than iron are formed within the gas and dust.
If the Sun went supernova, it is possible the Solar System would not survive.
At this juncture, let us assume – either by the same conspiracy or a different one – that the Solar System’s planets somehow survived the cataclysmic radiation and heat from this explosion, and remain in their orbits as usual. Would they get sucked inside the resulting black hole? They would if they got within 2.96 km of the black hole’s centre, but if they stay farther away, they will simply continue on in their orbits. Let’s recall that Mercury is 46 million km – i.e. 15.5-million-times the Schwarzschild radius – away from the Sun even at its closest approach.
In fact, the orbits of Earth and the other planets will be shifted by a minuscule amount when some of the stellar matter is thrown outwards in the supernova, nothing more. They certainly wouldn’t get pulled into the black hole any more than they are forced towards the Sun right now.
The gravitational force between two objects decreases as they grow further apart. Thus, for an extremely dense object – as with a black hole with a mass equal to that of our Sun (i.e. a density of 200 billion billion billion kg/m3) – the change in gravitational forces over very small distances can have drastic effects. For an object near the black hole, the difference in gravitational strength between two points, one closer to the black hole than the other, can be high enough for it to become stretched out. This is a phenomenon evocatively called spaghettification.
However, supermassive black holes like the one at the centre of the Milky Way galaxy aren’t so dense, and so don’t spaghettify in-falling objects. The object would simply continue to fall inwards, enveloped by more and more darkness, until it reaches a point at the centre called the singularity. We don’t yet know what happens there because that’s where the laws of spacetime break down completely, becoming utterly meaningless and unable to explain the nature of reality. While this is somewhat frustrating, it is also equally exciting.
To quote Jules Verne, “Reality provides us with facts so romantic that imagination itself could add nothing to them.”
Sakhee Bhure graduated with a BS in astronomy and astrophysics from the Florida Institute of Technology. She is interested in writing about science.