If you were going to travel close to a black hole in order to study it, which type should you choose? Most people would probably pick a smaller black hole, because it seems easier to avoid. But that's a fatal mistake. Small black holes can be far more dangerous than big ones, due to a terrifying process called "spaghettification."
Spaghettification and the Tides
If you're moving towards a black hole, you're probably falling. The only important thing to establish is whether all of you is falling at the same pace. Black holes are matter concentrated into a infinitely small point. The exact amount of matter that's concentrated is what determines the size of the black hole.
In theory, a black hole can be any size. In practice, the smallest black hole yet found is 3.8 times the mass of our sun. The largest is about 40 billion times the size of our sun.
There's an old joke that falling doesn't kill anyone — only hitting the ground does. Not with black holes. As you approach the black hole feet first, your body starts to fall at different rates. Just as the moon tugs harder at the side of the Earth closest to it, causing the tide to rise up towards it, the black hole pulls harder at your feet than it does your head.
The difference in pull is like being tied to two different cars, both going the same direction, but one going sixty miles an hour and one going forty miles an hour. The process affectionately dubbed "spaghettification" is when your body gets elongated into a long strand of goo.
Big Black Holes and Small Ones
Spaghetiffication is an unavoidable consequence of approaching the singularity at the center of a black hole, but some black holes are more approachable than others. The event horizon of a black hole is the point at which nothing returns. Let's say you put your foot right on that horizon. What matters isn't how fast you're falling, but how fast your head is falling compared to your feet.
A small black hole with an event horizon roughly your own size will exert a massively larger force on your feet than it does on your head. A black hole ten thousand times your size will exert a great deal of force on both your head and your feet, but the difference between the force it exerts on your head and on your feet will be minuscule.
So while they will both rip you apart eventually, the point at which a small black hole will spaghettify you is well outside its event horizon, and the point at which a large black hole will do damage is well inside its event horizon.
And that's assuming the big black hole is as dense as the small one. As we saw last week, doubling the mass of a black hole doubles the radius of the black hole. This is unusual. Smash two equally massive balls of putty together and the radius will only increase a small amount. The large increase in radius means that, as you add mass, a black hole's volume goes up much more than its mass goes up. Average density is mass divided by volume, so a black hole's average density will drop and drop as its mass and size increase. Some black holes have the average density of water, or even air.
So if you want to get up close to a black hole, and you aren't keen on being shredded like tissue paper, pick a large black hole.
Circling the Drain
By going big, you can avoid another danger of smaller black holes. How can astronomers see a black hole of only 3.8 solar masses? The sun is not a large star, so 3.8 solar masses isn't an impressive size, astronomically speaking. And the defining characteristic of black holes is that light doesn't escape from them. It seems like they should be invisible.
Small black holes are surprisingly easy to see. As they pull in matter, the tidal forces stretch that matter out, spaghettifying it. This matter could be anything from stars to dust. If a star plunges straight towards the black hole, it will get stretched out like a rubber band. If it is circling the black hole, it gets pulled into a spiral, a bit like water circling the drain. The process of stretching the matter causes massive amounts of friction, heating the matter up until it shines. Small black holes are some of the brighter objects in the universe, because of the heated material flowing around them. Its only when they get big, and can swallow a star whole, that they go dark again.
If you're near a black hole, you are already dealing with a lot of dangers. You don't need the sizzling remnants of stars circling around you as you try to peer into the darkness. Far safer to watch a huge black hole take in a brown dwarf in one big gulp... from a relatively safe distance, of course.