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What are Black Holes?

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Two Types

Black holes are infinitely dense entities. There are two types…

  • Stellar Black Holes come from the collapse (death) of massive stars and have masses from 5 to 30+ times more massive than the Sun
  • Supermassive black holes are in the centers of galaxies and have masses from hundreds of thousands to billions time more massive than the Sun

No Light

They emit no light as their gravitational forces are so strong not even light can escape. We can ‘see’ them indirectly…

  • Since black holes are massive enough to bend light with gravitational forces, they can be observed by viewing the distorted light traveling to Earth from behind them
  • Objects can orbit black holes according to the laws of gravitation. The supermassive black hole at the center of the Milky Way galaxy is ‘observed’ via tracking the orbits of stars around it.
  • Orbiting and coalescing black holes emit gravitational waves (see below) that can be directly measured on Earth.

Undefined Size

While the mass of (the amount of matter in) a black hole can be determined with the above techniques, the physical size (space occupied) is not defined…

  • The size of a black hole can be defined to be a singularity, meaning it becomes so small that the density of the black hole becomes infinitely large
  • The size can also be defined by the radius to the event horizon (Schwarzschild Radius), which is the distance from the black hole at which objects (light, for example) crossing this threshold will not be able to escape the gravitational force exerted by the black hole.
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