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  1. Black holes emit x-rays lights from within their event horizons (I believe this one is unknown because you cannot see what's going on inside event horizon)

  2. A 9 km black hole may be hidden between Saturn and Jupiter (I think it's true because the distance between these planets are larger than 9km).

  3. A black hole in a binary system will suck in its companion (Not sure about this one because I don't think the effect is large in a non-close binary system)

  4. Singularity of black hole has infinite density (I guess it is true but can't find evidence)

Could someone give any suggestion, please?

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    $\begingroup$ Appears to be homework question. $\endgroup$ – ProfRob Nov 20 '16 at 11:06
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    $\begingroup$ I'm voting to close this question as off-topic because this looks like someone's homework. $\endgroup$ – ProfRob Nov 20 '16 at 20:14
  • $\begingroup$ Though it’s interesting to read his thoughts annotated to each question. This shows that what he really needs is a deeper understanding of the particulars. $\endgroup$ – JDługosz Nov 20 '16 at 20:16
  • $\begingroup$ I agree with @JDługosz that the annotations are interesting, those cause this to be a higher quality question than the usual homework questions. It is still not good, but a small thing like that helps me to keep my faith in humanity. $\endgroup$ – SE - stop firing the good guys Nov 20 '16 at 23:30
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  1. Black Holes do not emit X-Rays from inside the event horizon. The X-rays are emitted from the black hole's accretion disk. This is a swirling disk of gas and dust which is spiraling into the black hole. As it falls inwards, it heats up, giving off energy in the form of x-rays and gamma rays.

  2. A 9km black hole could not lie within the solar system without disrupting the orbits of nearby planets. A black hole 9km across would have to be about 3 times more massive than our sun. You can calculate this by using a simplification of the formula for the Schwarzschild Radius: $r\approx 2.95 \frac{M}{M_{\odot}}$. If this were the case, then the orbital motions of the planets would be dominated more by this black hole than by the sun.

  3. A black hole doesn't really "suck" things in. It only has a gravitational pull relative to its mass. So if it was a part of a binary system and its neighbor is far away, its partner star will not notice any kind of change when the other star turns into a black hole. As an example, if the sun were to turn into a black hole right now, the Earth would continue orbiting around it without any kind of change. However, if the black hole and the other star are close enough together, matter from the star can fall off of it into the black hole. The black hole's proximity actually disrupts the internal structure of the star, causing matter to be ejected from the star. A similar situation happens when you have a white dwarf orbiting close to another star.

  4. Yes, you can consider the singularity as having infinite density.

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