The Black Heart of the White Swan

Astronomically Speaking #2, published in the York University Gazette, November 1999

Peer into the sky towards the constellation Cygnus, the Swan, near the horizon this month. One of the most interesting objects up there is one that, no matter how hard you try, you will never see. Up amongst the stars, hidden from us by its own gravity, is a black hole, and it's the closest one to the Earth at just one thousand light years away.

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Black holes are perhaps the strangest objects in the universe. To understand them, we should first talk a little about Einstein.

Early this century, Albert Einstein developed his theory of gravity known as General Relativity. Before Einstein, space and time were always part of the background, the playing field on which the games of physics were held. Objects moved through space, changing their position against the regular heart-beats of time.

Relativity says, however, that space and time, tÎogether known as space-time, aren’t just labels for where an object is found. Space-time itself is a real dynamic entity. In the vicinity of a massive object like a star, space-time is curved.

Curved space-time isn't easy to imagine, even for physicists. Try this: imagine you've got a nice big, soft king-sized bed. If you roll a bowling ball across the bed, the ball sinks down a bit, deforming the bed's surface into a curved dip as it goes. Place a bowling ball in the middle of the bed, and roll a marble across the mattress — the marble will, of course, follow the dip of the mattress down towards the bowling ball, along a curved path.

The mattress is a bit like space-time, the bowling ball like a massive object, such as a star. The mass of a star warps the space-time around it, and any object coming near the star will follow a curved path through the curved space-time. The Earth does this as it orbits the Sun; it traces out an ellipse in the space curved by the enormous mass of our nearest star.

Einstein became a bit of a celebrity, however, for suggesting that even light itself gets bent as it passes a heavy object. Careful observations have verified this effect. They have shown that a distant star seems to shift position in the sky a little when the Sun passes in front of it - the light coming from the star is bent by the Sun's gravity, and the amount of the bending is exactly the amount predicted by Einstein’s General Relativity. Headlines shouted "Einstein Proven Correct!", and his face began to appear in newspapers and magazines, and on coffee mugs and t-shirts.

What's all this to do with black holes? Well, what if you had an object so massive that passing rays of light would bend so much they would spiral down towards the object, never to escape? Any light trying to leave the surface of this object would be pulled back. And if light can't get away, then nothing else could either, since nothing in the universe can travel faster than light. Such things are perfectly acceptable results of General Relativity. Theorists dubbed them 'black holes' since light can't escape to show us what they look like.

You really wouldn't want to stumble across one of these things. The incredible gravitational pull of the black hole wouldn’t just drag you towards it, it would do very odd things to your body in the process. For one thing, the gravitational pull on your feet would be stronger than on your head. This is true if you're standing on the surface of the Earth too, the pull towards the ground is ever so slightly greater at your feet than up at your head. We don't notice this because the difference is incredibly small. But near a black hole, your feet would drop away and your body would stretch out like so much spaghetti.

So how do we know there really is a black hole in Cygnus? That's the thing  about black holes: they're black, we can't see them directly. We can, however, tell they're around by looking for the effects of their gravity on nearby stars.

Astronomers observed that a large star in the constellation Cygnus appeared to be orbiting around some other, very heavy object. When they looked to find the other object, though, there was nothing there. Just as odd, the star was emitting huge blasts of x-ray energy, as if it's surface was being ripped apart and dragged away. Astrophysicists calculated that these effects must be due to a black hole very near to that star, and dubbed the system "Cygnus X-1". The "X" is for the X-rays bursting from it, and the “1" means it was the first black hole they found.

So as you gaze up into the sky this month, just think what a wonderful, poetic twist it is, that deep within the constellation of Cygnus, in the very heart of that graceful white swan, lies one of the darkest and strangest denizens of the sky.