THE USM SOUTHWORTH PLANETARIUM
70 Falmouth Street Portland, Maine 04103
43.6667° N 70.2667° W
Altitude: 10 feet below sea level
Founded January 1970
Julian date: 2458591.35
"We'll leave the night on for you."
THE DAILY ASTRONOMER
Thursday, April 18, 2019
Black Holes IV or Sorry About That!
Consider this last Black Holes DA of the week to be something of a multiple apology. First, a subscriber asked a question that required such an involved answer that I will have to post the response on Monday. My apologies for the delay. Secondly, as we're about to discover, I will also be apologizing for our future, which is a bit brighter than some subscribers would prefer.
"Why won't the Sun become a black hole? I feel sort of cheated, because having the Sun become a black hole would be cool. What would happen if the Sun did become a black hole? Would Earth be sucked into it?"
-G.M.
Well, I have to admit that having the Sun turn into a black hole would be eye-catching. Suddenly, the golden orb fades to black and the sky would become immediately star-adorned. (If you live in Maine's vast flood/snow plains, you would notice the cloud banks darken slightly.) Why can't this happen?
First, the Sun will eventually perish in about 6.5 billion years. Its outer layers will form a planetary nebula that will expand away from a white dwarf remnant. This white dwarf will cool over an immensely long period of time. A white dwarf's structure is maintained by electron degeneracy, meaning that the electrons within it will repel each other and prevent any further collapse. The Sun's mass isn't high enough to create the compression forces necessary to overcome this electron degeneracy.
Stars that are at least ten times more massive that the Sun will be able to compress their remnant down to a much higher density: either a neutron star, which is sustained by another process called "neutron degeneracy," or, if it can overcome that, into a black hole.
Note that the size difference between white dwarfs and neutron stars is substantial: a white dwarf is planet sized, whereas a neutron star is about the size of a large city. A black hole contains a singularity: a mathematical point that is infinitely massive.
Also, as for what WOULD happen if the Sun became a black hole. If it did, we wouldn't last long, for one thing. We wouldn't die right away, for the same reason that we don't die when the sun sets each day. However, the lack of sunlight would dramatically disrupt the photosynthetic process on Earth, as well as the sudden loss of heat. We would all either freeze or suffocate within probably two weeks.
However, Earth would not be sucked into the black hole! The planet would continue revolving around it, just as we revolve around the Sun. Our year would be shorter because the Sun would lose some of its mass in the supernova explosion that created the black hole. When the parent body loses some of its matter, the velocities of the orbiting bodies are reduced.
We would certainly die if the Sun became a black hole, but not because Earth would be destroyed by it.