The Cepheids: establishing the astronomical surveyor’s baseline

So what are the Cepheid variables, and what is their significance?

The Cepheids are very important in astronomy, especially for estimating the distances of remote objects such as the Andromeda Nebula and indeed the Magellanic Clouds. To get these distances right is the basis of the whole astronomical distance scale. This is, in a way, the surveyor’s baseline from which the rest of the universe is measured.

Cepheids are important for this because they are intrinsically bright stars – among the brightest stars around, many thousands of times brighter than the sun – and they are easily recognisable, because they pulsate and so they vary. They go in and out, in and out, and as they do, their light varies by a factor of two or three, which can be picked up a very long way away. Even in a faint star you can see if it is varying by that much. The periods of Cepheids range from about three days up to 30 days, and a 30-day Cepheid will be about 10 times as bright as a three-day Cepheid. This makes them very attractive to theorists, too, who like playing around with information like that. What also makes Cepheids so useful is that you can determine how bright the ones in the Galaxy are, because you know their distances from other arguments – which we haven’t got time to go into here but are good arguments.

The problem which I had been thinking of was measuring colours of the Cepheids in the Large Cloud. The Magellanic Clouds had been the province of Harvard Observatory for the whole of the century. They can’t be seen from the northern hemisphere, but Harvard had set up an observatory in Peru (later in South Africa). Harvard began work on Cepheids in about 1908, discovering lots of them in the Large Cloud, and that is when they discovered the famous period-luminosity law. But Harvard measured them only in blue light. They had never measured colours. Besides, the Harvard data that looked so good from 1950 were obtained by methods that were okay in 1910 but hadn’t changed their methods at all, and they just weren’t up to scratch. It wasn’t going to be hard to improve on the Harvard observations.

When you are comparing objects at different distances, the inverse square law holds. That is to say, if you shift a lamp to twice as far away as it was, it is then only a quarter as bright. If you shift it to three times as far away, it is only a ninth as bright, and so on. But if you know the candle-power of the lamp, its output in watts, you can work out how far away it is. Well, that is the way Cepheids work. We see all these Cepheids in the Magellanic Clouds, we know their period so we can correct for the period-luminosity law, and we know how bright they are, therefore we know how far away the Magellanic Clouds are.

Cepheid colours, galaxy dust and a triumphant insight

However, there is one major complication. There is dust within galaxies, though there is very much less between galaxies. Certainly within our flat Galaxy there are not only stars and a fair amount of gaseous hydrogen, but also a lot of dust. It is a very visible component of the Galaxy. The Southern Coalsack, well-known in the night sky as a big black hole in the Milky Way, does not represent an absence of stars in that direction, it is rather a cloud of dust obscuring the stars beyond it. The effect of this dust is to make your lamp look fainter, and before you can use it as an accurate distance indicator you must work out how much dust there is between you and it.

This is made possible because dust has another property: not only does it make your light look fainter, it also makes it look redder. If you can measure the colour of a star and it appears redder than you know it really is, you can use the amount by which it has been reddened to correct for the absorption by the dust. Measuring the colours of Cepheids is therefore very important. The question of their intrinsic colours – what were the colours of unreddened Cepheids – worried astronomers for a long time. Although they knew the distances of the ones in the Galaxy, they didn’t know how much dust there was.

Just as I was wondering how best to tackle this, two Americans from the famous Lick Observatory turned up – Gerry Kron and Olin Eggen (Eggen later became the director of the Mount Stromlo Observatory). Gerry was an electronic instrumentalist, a type then new in astronomy. He was astronomy’s leading expert with photoelectric cells, and in particular he had brought along some recently-developed multiplier cells, he knew just how good they were. His own program was to measure the colours of all the red dwarfs in the southern part of the sky. He had already measured all those in the north, but some of the most interesting ones are in the south and he wanted to complete the sample.

He thought he would do this on the 30-inch, and since it would have been a two-man job he asked if I would like to team up with him. He said, ‘We can measure my stars from May till August, say, and then we can measure yours. But don’t do yours with photography. One of my photoelectric cells should just about handle this.’ The stars I wanted to measure were up to that time the faintest which anybody had measured with a photocell, and the trick was going to be to pick them out, to recognise them, in the crowded fields of the Magellanic stars. If you have a map showing 100 stars, and you know one of them is a Cepheid, how do you find it in a telescope?

I really didn’t think I would be able to do this, so I used to go up and practise on part-cloudy nights when nobody wanted the telescope. I found it wasn’t as hard to do as I would have thought. You hop from star to star and then eventually, ‘There it is, for sure.’ And so we teamed up and went in to Woolley to ask for some time on the 30-inch. He said, ‘Well, you can have the next nine months all to yourselves’ – this is on the biggest telescope in the place! – ‘but then you, Gascoigne, get no more time for a year.’ It was the best bargain I ever struck in my life. Gerry and I got to work doing his red dwarfs in the winter, and the Magellanic Cloud Cepheids in the spring. We could measure them all right, and they turned out to be astonishingly blue, much bluer than the ones in the Galaxy. It was really hard to believe that the ones in the Galaxy were reddened as much as all that, but we pressed on regardless. I did most of the analysis, and after quite a while I thought, ‘Let’s assume that the ones in the Galaxy are the same colours, are as blue, as the ones in the Clouds’ – instead of being yellowish, as they seemed to us – ‘and see what happens.’ And that was the way to go.

Are the ones in our Galaxy more red-shifted than the ones in the Magellanic Cloud simply because we are looking through a relatively small distance of dust to get to the Magellanic Cloud?

Yes. The galactic Cepheids are in the galactic plane, while the Magellanic Cepheids are well above it. If you assumed the Galactic Cepheids did have the same colours as those in the Clouds, and if you assumed that we knew their distances correctly, it followed from our work that they were four times as bright as had previously been thought. This was startling, because it meant that the Magellanic Clouds were twice as far away as was previously thought, and if then the baseline is twice as long, the size of the universe is doubled. This was not altogether a new result. Walter Baade had proposed the same thing a year or so previously, but on quite different evidence and talking about the Andromeda Nebula, whereas we were talking about the Magellanic Clouds. At least we were able to give him good solid confirmation, and also greatly to strengthen the position of the Magellanic Clouds as distance indicators. By now we knew enough about them to be pretty confident about the answers they gave.

When suddenly all this dropped into place, after I had been working away at it for quite a while, measuring more Cepheids in our own Galaxy and some in the Large Cloud, the feeling of triumph, the great feeling that I had really done something, was wonderful. I had joined the professional astronomers. Not only that, but I truly understood a problem, a proper problem.

Focus questions

• When astronomers look into space, what is significant about the presence of dust within galaxies?

• When the data from measuring the Cepheids was understood, Gascoigne talks about his jubilation in truly understanding a proper problem. What do you think he meant by this?