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If our sun turned into a black hole, you might think our solar system would be doomed, but in reality that's just not how black holes work.

Hosted by: Stefan Chin

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Sources:

http://curious.astro.cornell.edu/86-the-universe/black-holes-and-quasars/general-questions/423-what-is-the-density-of-a-black-hole-advanced
https://science.howstuffworks.com/dictionary/astronomy-terms/black-hole2.htm
https://www.omnicalculator.com/physics/schwarzschild-radius
https://www.physicsclassroom.com/class/circles/Lesson-4/Mathematics-of-Satellite-Motion
https://www.nature.com/articles/s41550-018-0453-9.epdf
https://www.sciencealert.com/what-will-happen-after-the-sun-dies-planetary-nebula-solar-system
https://spaceplace.nasa.gov/review/dr-marc-sun/black-hole-sun.html
http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/blkhol.html

Images:

https://svs.gsfc.nasa.gov/10297
https://svs.gsfc.nasa.gov/12033
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[♪ INTRO].

Good news, everyone! The Sun is never going to turn into a black hole.

That only happens to really massive stars, and the Sun doesn’t make the cut. Instead, it will eventually expand into a red giant, and then will likely form a beautiful planetary nebula. I mean, you and I definitely won’t be around to see all that, but at least we don’t need to worry about getting sucked into a cosmic bathtub drain.

Except, even if someone did replace the Sun with an equally-massive black hole, we still wouldn’t need to worry about falling in. And no, the black hole sun would not come wash away the rain, either. Because, in both cases, that’s just not how black holes work.

The hypothetical black hole would be the same mass as the Sun, but not the same size. “Size” here is a little vague, though, since we don’t really consider a black hole a thing so much as a mathematically-determined region in spacetime around a singularity. In other words, they’re points of infinite density surrounded by an area where stuff gets pretty weird. In our “what if” scenario, the part we’d care about most is the “point of no return,” or the point where, if you got any closer to the black hole, you’re a goner.

That point is officially called the event horizon, and its distance from the center of the black hole is the Schwarzschild radius. So really, all we’d need to do to survive, ignoring the whole “no sunlight” thing, is not be within that radius, or already be accelerating towards it. According to the math, this radius increases as a black hole gets more massive.

And for a black hole the mass of the Sun, the Schwarzschild radius would be a whopping 2.954 kilometers. Basically, we’d be safe if we didn’t get within about 3 kilometers of the black hole. It would take an object almost 51 million times as massive as our star for the event horizon to reach Earth.

But just because we wouldn’t immediately fall in, you’d still think that our orbit would change at least a little. Or maybe we’d start gradually moving towards the event horizon. After all, our Sun just got replaced with a point of infinite density.

Except, we’d still be okay. Orbits in space don’t generally depend on the size of the object they’re going around; they depend on its mass. So if the mass doesn’t change, neither does the orbit.

You could create a scenario where there was a Sun-mass pomegranate at the center of the solar system, or a very round hamster, and not much would change. I mean, that would raise other weird questions, but neither would dramatically affect any orbits. Which is why the ridiculously massive black hole at the center of our galaxy hasn’t swallowed us and all the nearby stars: Once you’re in a stable orbit around anything, you’ll keep it up forever unless an outside force intervenes.

Nevertheless, can we all just agree not to replace the Sun with anything too weird anytime soon. If you liked this episode, you’ll probably really enjoy the Keplerian Orbits course from Brilliant.org. Orbital mechanics is a big field, and one thing I really like about this course is that there’s something in it for everyone.

You could start with the basic equations that determine how two objects orbit each other, or you could jump in and figure out how to get a space bus to Mars. Also, Brilliant is great at making concepts like this understandable, which is pretty impressive when you’re explaining things like Hohmann transfers. You can check it and other courses out at Brilliant.org/SciShow.

And right now, the first 200 people to sign up at that link will get 20% off of an annual premium subscription to Brilliant. Besides having a great time and learning about the universe, you can also know that you’re supporting SciShow. So, thanks for doing that! [♪ OUTRO].