By David Siegel Bernstein, PhD
If a black hole got married, would it still be considered a singularity?
(joke from astropixie’s blog)
This S4F is dedicated to black holes; a worthy topic for science fiction fans and writers. Black holes come in different sizes—they can be as small as an atom (with the mass of a mountain) or enormous with the mass of over 1 million suns, labeled as supermassive. It is believed every large galaxy contains one of these supermassive black holes in its center. The one in the center of the Milky Way is called Sagittarius A.
Let’s begin with how a black hole is created. The executive summary goes something like this: a star runs out of fuel to puff itself out and, thanks to gravity, it collapses. If it collapses to its Schwarzchild radius it will continue to collapse to a singularity and a black hole is created.
Whoa! What’s the Schwarzchild radius? Or for that matter what’s a singularity? Well since you happened to ask: the Schwarzchild radius (rs) is the radius of a given mass where if the mass could be compressed to fit within this radius, no force could stop it from continuing its collapse to a singularity. A singularity is an infinitely small and dense point—something not clearly defined in physics (darn). When an object is compressed smaller than its Schwarzschild radius it is called a black hole.
Fun facts: the estimated rs of Earth is 9mm and it is 3km for the Sun.
Okay now for the longer “how” of a black hole. The power of a star comes from the nuclear reaction taking place in its core which causes outward pressure. This pressure is balanced by the weight of gravity from the star’s mass. In other words, the nuclear power pushes out and gravity pushes in. As the star gets older it goes from combining hydrogen into helium (where our sun is at) to fusing helium into carbon, and then carbon into oxygen, and then oxygen into silicon and then finally silicon into iron (a cosmic dead-end, more one this in a future post). At this point fusion stops and the outer layers of the created elements (hydrogen, helium, carbon and silicon) burn around the iron core. The core continues to build up until it explodes. This is called going supernova. Now for the interesting bit, after the supernova explosion the core compresses further, ramming atoms against each other, changing protons into neutrons until—voila, a neutron star. But if the neutrons can’t prevent the star from collapsing further, from gravitational forces, it will shrink to beneath its Schwarzchild radius and you get yourself a stellar black hole
Black holes devour anything that dares to pass its outer boundary, called the event horizon. This includes light and electromagnetism. Its core is a singularity.
Now that we have the basics, in my next post I’ll provide some more black hole facts and perhaps a few tips on how to incorporate these stellar objects into science fiction stories.
See you in time and space… and S4F
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