The name 'black hole' was invented by John Wheeler, and seems to have stuck because it was much catchier than previous names. Before Wheeler came along, these objects were often referred to as 'frozen stars.'

So what is a black hole?

It's a region of space that has so much mass concentrated in it that there is no way for a nearby object to escape it's gravity. It is suspected that most of the black holes out there were produced in the deaths of massive stars that collapsed and couldn't resist it's own gravitational pull. There is no limit in principle to how much or little mass a black hole can have, in principle any mass can be turned into a black hole if compressed to a high enough density.

A black hole has a horizon from where it's gravity becomes stronger, outside of this horizon you could avoid being pulled into the whole since the gravitational pull would be as large as any other objects. Once you have entered a black holes 'horizon' there is no turning back however. The pull is too strong and anything entering the black hole will be ripped apart.

You can't see a black hole so you have to determine its existence by using indirect evidence. Black holes shrink, they do so because they radiate and use energy for doing so. The more energy it uses the more it shrinks until it evaporates totally.

Let's take a look at the star constellation Hydra now, we'll start with the origin of its name. Hydra, also known as the water serpent, concerns one of the stories of Heracles' labours, the second one.

Heracles was off to slay Hydra, which was a gigantic dog-like beast with somewhere in-between seven and fifty heads. One of these heads was immortal, meaning so was Hydra. Its breath was deadly and anyone who would even smell its tracks would die.

When Heracles finds Hydra, he cuts one head off...only to see two new ones grow back out. He then realises that the only way to slay the Hydra will be to chop off the immortal head. When he manages to do that, he buries the head under a huge rock and cuts the Hydra in to pieces, which he scattered around.

As a constellation, Hydra winds down from the northern hemisphere bordering the cancer to as far off as Centaurus in the southern hemisphere, at one point its body is actually cut off by another constellation (the south-western tip of Crater).

There are quite a few nice double stars in the Hydra but all in all, you could say that the stars are of average brilliance, with the exception of Alphard. Alphard is a bright giant, about 25 times larger than our sun; it shines 95 times as bright.

In the constellation there are a few star clusters and nebulae as well as a spiral galaxy.