If Our Sun was a Black Hole

Back in 1994, Soundgarden released a song off their album Superunknown called "Black Hole Sun". Since I want to talk about what would happen if our Sun became a black hole, it gives me an excuse to post this video.

from Soundgarden's Official YouTube Channel

 

Remember, the Sun will never become a black hole as it is too small, but for fun, let's find out what would happen if it did.

First, we should determine what the Schwarzschild radius of the Sun would theoretically be. When a black hole evolves, not all the mass of the main sequence star is in the black hole, but for our purposes, we will assume that somehow the Sun we see shining right now will have all its mass converted into the black hole. We will need to know two things: the Sun's mass in kilograms and the Sun's radius in meters (we do everything in metric).

• Sun's Mass (M₈) = 1.99x10³⁰ kg (we will say 2x10³⁰ kg for simplicity)
• Sun’s Radius (R₈) = 696,342 km = 6.96x10⁸m

Note: the symbol 8 refers to the Sun. When that subscript follows M or R, it just means the unit is in solar masses or solar radii.

Recall that the Schwarzschild radius is this:

Plugging in the values for the Sun's mass, G (6.67x10^-11 m³kg^-1s^-2), and c (3x10⁸ m/s), we find that if all the mass of the Sun became a black hole, the Schwarzschild radius would be 2,960 meters or about 3 km.  Compare that to the actual radius of the Sun and you can see how much smaller it would be.

So what would it mean for the Earth if the Sun became a black hole? In a couple of words, not much. The Earth would still be orbiting the same mass, so our orbital radius and speed would not change. However, life on Earth would die out because we would not be receiving the same radiation from the Sun to which we have been accustomed. No visible light from the Sun warming us, heck, no radiation of any kind. Black holes do not give off radiation in that manner. Due to Hawking radiation, particles near the event horizon of the black hole (the theoretical distance where light could escape from the black hole) would evaporate off. Some might hit Earth, but there wouldn't be anything to keep our planet warm.