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More massive the body is more is the gravity.Still often smaller black holes are considered to be very dangerous than the bigger ones,why?

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All black holes are dangerous if you get close enough, but smaller ones are more dangerous because they cause stronger tidal effects, so they can cause spaghettification well before you reach the event horizon, whereas you can cross the event horizon of a supermassive black hole without the tidal effects becoming noticeable.

Very small black holes would also be dangerous due to the large amount of Hawking radiation they'd emit. E.g., a 1 million metric ton hole has a luminosity of over 356 terrawatts. However, there's no known mechanism to produce black holes that small, apart from waiting for a large hole to get small by evaporating via Hawking radiation, and that won't even start to happen for a very long time, since the cosmic microwave background is currently around a billion times hotter than even the smallest stellar black holes.

Actually, the main danger in the neighborhood of a black hole is if the hole is active, that is, if it is currently surrounded by matter that it's absorbing. That matter gets accelerated to very high speeds due to the gravitational acceleration and when it collides with other matter large amounts of dangerous radiation is released.

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  • $\begingroup$ Tidal effects is the main one. Tides are the difference in the force of gravity at 2 points. It depends on the radius to the third power. Stellar mass black holes are much smaller in size than super-massive black holes, so the difference in gravity between say a persons head and feet is smaller. Imagine doing chin ups, now with somebody else hanging onto you feet, then 10 people ... the population of London. That's equivalent to the tidal force felt by a person going feet first near a stellar mass black hole. Near a million mass $\endgroup$
    – TazAstroSpacial
    Jun 15, 2018 at 0:43
  • $\begingroup$ @TazAstroSpacial Certainly, if you're close enough. But if you were, say, 1 AU from an active stellar mass BH, the tidal effects would be negligible, but the radiation from the accretion disk would be dangerous. $\endgroup$
    – PM 2Ring
    Jun 15, 2018 at 1:13

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