WHAT A CLEVER STAR - The Cepheid Variable

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This month's title doesn't seem at all connected with last month's offering about how to measure a stars distance, but bear with me, it is. I talked last time, as part of the 'how do we know that' series about using parallax to find some stars distances from us, but that only works for stars that are relatively close. So, there are two important things we need to consider, firstly there are billions of stars, mostly a very long way off, and secondly most stars are variable. A variable star is one that changes in brightness as seen from Earth, and yes, the Sun is a variable star, but only by a miniscule amount.

Stars can change brightness for several reasons, and there is one type that is important to us, the Cepheid variable, first discovered over 100 years ago by a lady called Henrietta Swan Leavitt in the constellation Cepheus, hence the name. The thing that's special about these stars is that they are totally regular in the time that it takes to go from bright to dimmer to bright again. Any of them that have a period of for example, 34 days will be the same real brightness. If an astronomer looks at two of them and one has a maximum brightness that's dimmer than the other it must be further away. If you are clever, and have been taught how, you can then get a Cepheids distance from its brightness. Modern instruments for measuring all this have become so accurate that it's possible to get incredibly accurate results, in fact to about 0.001 percent. All this is also a perfect example of the hard slog in taking measurements and doing calculation the professional astronomers have to do.
That leaves us with the problem of measuring all the other stars. Study of the Cepheid variables over the decades led to a lot of work on the make up of stars, for example the mix of hydrogen, helium and a lot of other stuff. Eventually it became possible to work out that stars of a particular mix were very likely to have a certain absolute brightness and from that to estimate its distance.
If that last bit sounds like a bit of a fudge, that's because it is, and would make a professional shudder. The truth is that the actual process is very complicated and full of math, but it's the result of the work of hundreds, probably thousands of people over a long time, and it works.
That's quite enough of the heavy thinking, so a bit of news. As I write this the Europa Clipper spacecraft had lifted off to go to the smallest of Jupiter's Galilean moons. This might just be the cleverest probe ever launched and in six years when it gets there will probably change so much of what we believe we know, just maybe even finding traces of microbial life from the oceans under Europa's frozen surface. We live in astonishing times, things have advanced so much since I saw the first space launch. I can't help wishing that world leaders had progressed as much.

Charles Oates, Vega Baja Astronomy Group.

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