GRASP THE VAST - IT'S A BLOOMING LONG WAY TO THE STARS

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In last months 'how do we know that' I talked a bit about how we know what stars are made from, this month it's how to measure how far away they are.

It saddens me that even today many people have no idea of just how far it is to anywhere in space, but I am rather biased about astronomy. We don't use kilometres to talk about a star’s distance away, the numbers just get too big to be meaningful, we use light years. A light year is a measure of distance, not time, it is simply how far light can go in one year, so, how far is that? Everyone knows Jupiter is a very long way away, over 700 million kilometres but that's only about 39 light minutes. One light year is nearly 6 trillion kilometres, so how the heck can we measure anything so far away? Actually, there's several ways, but the first is really simple and you have probably noticed the basis of it in everyday life, it's called parallax.
Close one eye and stretch out your arm with a finger pointing upwards, now line up your finger with something, maybe a door edge. Now close the open eye and open the other one, you'll notice that the door edge seems to have moved, keep swapping eyes and the edge keeps moving. If you did that whilst sitting in a bar, don't blame me for the strange looks you're getting.
What's happening is probably obvious, each eye is seeing the door edge from a different angle because your eyes are a little way apart. If you did this scientifically, with lenses and something to measure the angles, you can work out the distance using the elementary maths you did at school, thank you trigonometry.
Of course, even a nearby star is a huge distance away, but there is a simple way of using this same method to measure the distance of quite a lot of stars, we just need to look much further apart than our eyes are. The Earth orbits (goes around) the Sun in one year, so every six months the Earth is on opposite sides. If we measure the angle of a nearby star at each of these positions, we have 2 angles measured about 300 million kilometres apart and that is big enough to get quite an accurate measurement. Needless to say, there is a lot more to this, for example stars all have their own movements, called proper motion, and maybe a star is at a difficult angle to observe. Then there's the problem of getting an accurate measurement through our atmosphere, the twinkling of a star makes measurement tricky. Fortunately professional astronomers are a clever lot and there are ways of allowing for these problems..... up to a point. In the 180 or so years since the first star's distance was measured a lot of work has been done, especially since special satellites were made to get more precise measurements. Where a big problem arises is with very distant stars, the angular measurements are too small to work, this is where one of the wonders of the universe comes to our rescue and we can measure the distance of really distant stars, but that will have to wait until next month.

Charles Oates, Vega Baja Astronomy Group.

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