Revolutionary Astronomical Words

Hello everyone, and thanks to the Young Astronomers for allowing me aboard despite being a comparatively crumbly non professional astronomer! Now, assuming I can get WordPress to bend to my will, my first post is going to be about words in astronomy that end up not meaning quite what they should, if they don’t want to be misleading. Like all sciences, astronomy is done pretty much in the dark (sorry about that) – and sometimes names stick before we know what we’re actually talking about. Here are a few.

Revolution

When we hear of revolts and revolutions, we think of noisy coup d’etats in which the angry mob displaces the, er, other angry mob – and either things improve for the country in question or they don’t, but in any case, it’s a radical change. But the word “revolution” actually means “going round in a circle”. The Earth completes one revolution round the Sun every – you got it – year. Doesn’t seem a very revolutionary word, does it?

It came from Copernicus. His revolution was, really, the ultimate revolution in Science: the recognition that we are not at the centre of the Universe; that, rather, we revolve around the Sun. The book he wrote (which was only published just before he died, as he knew it wouldn’t be popular!) was called “De revolutionibus orbium coelestium”, or “On the revolutions of the heavenly spheres”. It was a revolution, because it called into question the dogma of the day that the entire Universe was created for us.

(Picture credit: this online art gallery!)

Which, incidentally, led on to . . .

Planets

The word “planet” comes from Greek, and means “wandering star”. Apart from the odd motions planets made in comparison to the rest of the stars in the sky – which they did because they, like the Earth, were orbiting the Sun – there was no way of knowing in those days that they were any different from stars. Both looked like points of light. But stars are millions of times more massive than planets, and they give off light for entirely different reasons. To be fair on the ancient world, they couldn’t planet to happen . . .

Planetary Nebula

This is the name for beautiful nebulae such as the Cat’s Eye Nebula. They are actually nothing to do with planets, but were named as such in the 18th century when telescopes were not poweful enough to tell the difference.

(Picture credit: NASA.)

A planetary nebula is a much more gentle and orderly shell of gas than a supernova remnant. It is created when a small or medium star, like our own Sun, puffs off its outer layers at the end of its life. It’s often very hot, ionised gas, and is therefore an emission nebula – shining with its own light. It also contains elements such as carbon and oxygen, which are essential for forming rocks, planets, and life.

The word “nebulae”, however, does at least mean clouds. Astronomers referred to “spiral nebulae” many years ago, believing these to be beautiful spiral-shaped clouds at the same sort of distances as the stars in our Galaxy. They had no idea that these were galaxies millions of light years away from our own!

Astrology/Astronomy

Once upon a time, these two words meant the same thing. In the days when it was essential to know when to expect floods or plant your crops, and indeed when there were no TVs or streetlights at night, people would have known the sky very well. It would make perfect sense to think, “When such-and-such a constellation rises above that hill, it’s time to plant this out”, or “Oh dear, that one. The weather will be bad soon.” Into the Middle Ages, royals employed professional astrologers. A British tabloid newspaper claimed that Dr Brian May, the Queen guitarist who is also an astronomer, has a PhD in astrology . . .

Any word ending in “-ology” (biology, geology etc) usually means science. However, as the science and the myths separated, they needed two different names. They now have pretty well nothing to do with each other – but a lot of people don’t believe me when I say that!

Nova

The word “nova” implies newness. However, a nova is a star so old that it’s no longer strictly a star. It’s a massive explosion caused by the accretion of gas onto a white dwarf. This white dwarf is pinching this gas from a nearby star, usually in a binary system; every so often, it acquires enough for fusion to start again. It has to reach about 20 million Kelvin to do this, as a white dwarf is made of extremely compressed material which contains no hydrogen fuel to fuse (otherwise it would still be a star!). In order to make this even simpler, novae are not to be confused with supernovae, although a Type I supernova can result from the same sort of process.

The Big Bang

Time and again I’ve been told almost angrily: “It doesn’t make sense. The Big Bang was an explosion, so how could it create such an ordered Universe?”

The term “Big Bang” was actually coined as a derogatory joke, by Fred Hoyle, who preferred the steady state theory (that the Universe remains the same size and had no beginning). He said in the 1960′s on a radio program something along the lines of that he didn’t believe the Universe could have begun in one big bang. The name stuck!

We will never know what sort of noise it made – of course, even if we’d been around to hear it, it would have been so incredibly hot and violent that we’d have been smashed to bits. Certainly everything would have been bumping into each other a lot. There were no atoms and molecules as we know them, let alone solid objects or stars – everything was a seething plasma of atomic nuclei, electrons, and most of all radiation. It’s particles bumping into each other that make noise. But when the Big Bang occurred, any noise that occurred would have been inside it.

That’s because any explosion we think of today is nothing like the Big Bang at all. An explosion happens in one place, and its shock waves – flying shrapnel, for instance – fly out and damage their surroundings. The Big Bang didn’t have any surroundings. It’s easy to think of it as an expanding globe, with a centre and an edge. We think of the edge as rippling through something – perhaps the Earth! – at some point in time.

It sounds like it took place – in, well, a place. Somewhere we could go and visit. From there we’d see the evidence of destruction, perhaps everything rushing away . . .

That is everywhere and nowhere. The Big Bang happened right where you’re sitting. It happened across the room for you, and it happened on the other side of the Universe. It’s quite a mind-blowing thought. But it really wasn’t much like a bomb!

(Picture credit: good old Wiki.)

It was really quite complex too, with inflation, and a period of darkness (because all the atomic nuclei and electrons were flying around in too disorderly a manner to let light through. This is what happens inside a cloud – there’s too much stuff in the way, so light bounces off everything in random directions and goes any old where. It also means it’s relatively dark).

And guess what else? It wasn’t big at all. It was small. It was absolutely tiny – smaller than the head of a needle – perhaps smaller than an atom! How did all this stuff in the Universe today come out of something so small? We don’t know. In fact, theoretically, such an object shouldn’t exist. It’s called a “singularity”, and it means, because it’s too small even to have a size, it must have infinite density. But we know there are black holes which are also singularities – and, really, when we look at the earlier Universe and see how much smaller and hotter it was, and when we do the mathematics, it’s the only conclusion we can come up with.

It’s not only how we began, but it’s an immense – and immensely complicated – puzzle. It’s odd to think that something so huge and important could have such a jokey, normal name. But Universes happen before words do!

Alice