Wish upon a star

Cepheus, according to legend, was the king of Ethiopia. His lovely wife, Cassiopeia, was not exactly reticent about her beauty. She boasted that she was more beautiful than any goddess. Naturally, this infuriated the gods and goddesses. Neptune, god of the sea, threatened to destroy Ethiopia unless Cepheus sacrificed his daughter Andromeda to the sea monster Cetus.

So Cepheus, poor man, chains Andromeda to the rocks. But before Cetus can devour her, Perseus flies down on Mercury's magic sandals, turns Cetus to stone by showing him Medusa's head, writhing with snakes, and rescues Andromeda.

The story -- and I've shortened it considerably -- leaves you breathless. But you can think about it as you look up at the heavenly memorials to all these characters, twinkling placidly in the night sky. Beautiful Cassiopeia is there, a gigantic "W", the constellation we call Sharmishtha in India. Hapless Andromeda is just beside her. Dashing Perseus, sword held high, is poised to carry her off. Cepheus, concerned father, looks on happily, for Andromeda will now marry Perseus.

But Cepheus has other reasons to be happy too. The scientific secrets in this constellation make a story that's at least as dramatic as the legend. In Cepheus' blinking stars -- and that's a rather apt description -- lie some of the keys to our understanding of the universe. Cepheus didn't give up his secrets easily. But when he did, the floodgates opened.

It all began when astronomers noticed that a star called Delta Cephei (meaning the fourth-brightest star in Cepheus) varied in brightness, and in a regular cycle. The star went from bright to dim and back again over about five days. Intrigued, astronomers looked around. Not only did some other stars in Cepheus behave similarly, so did many more, all over the sky.

Astronomers call such stars Cepheids. The periods over which their brightnesses varies range from less than a day to over a month. Delta Cephei, at five days, is about average for Cepheids.

In 1912, Henrietta Leavitt at Harvard studied about 25 Cepheids in the Small Magellanic Cloud, a small galaxy some 200,000 light years away. Because they were all about the same distance from us, Leavitt knew that if one Cepheid seemed brighter than another, it was because it was intrinsically brighter, not because it was closer to us.

With that in mind, Leavitt made an intriguing discovery. The brighter the Cepheid, the longer the period of variation in its brightness. And this led Leavitt and her Harvard colleagues to the "period-luminosity law", a major milestone of modern astronomy. Armed with it, we only have to observe the period of variation of a Cepheid's brightness to know how intrinsically luminous it is.

Why is this useful? Because the period-luminosity law lets us compare stars as if they were side-by-side. For example, it tells us that Delta Cephei -- or any Cepheid with a period of five days -- is about 700 times brighter than the Sun. If it was where our Sun is, we would be baking in heat 700 times hotter than we suffer today. (Though we probably would not be here to feel it).

Luckily, Delta Cephei is very far away, and so very faint indeed. This intuitively obvious fact -- that the further a star is, the dimmer it seems -- is confirmed by another law, the inverse square law. We know Delta Cephei's true brightness -- like 700 suns -- and how bright it appears to us. So this law tells us how far away it is.

So here's where Henrietta Leavitt's hard work with Cepheids leads us. By measuring Cepheids' periods and their apparent brightness, we know how far away they are. And since Cepheids are spread all over the sky, they are milestones as we explore the universe. For every time we observe a Cepheid, we know how far it is. So we also know how far other objects in its neighbourhood are.

Think of a captain on a storm-tossed ship, desperate to find land. He sees a flashing light through the rain. He knows that if he can estimate how far that lighthouse is, that's about how far away land is too. In much the same way, Cepheids are the lighthouses of the universe, blinking across the void.

Observing Cepheids gave astronomers some of their earliest reliable estimates of the distances to far away celestial objects. These stars allowed Edwin Hubble to make the observations that resulted in the famous law that bears his name: the farther away a galaxy is, the faster it moves away from us. In turn, that led to the birth of a whole new science, cosmology, that gave us most of our modern ideas of the creation of the universe itself.

All that, from a single nondescript star in Cepheus! Which, after all, is more incredible: the ancient Greek legend about Cepheus? Or the modern scientific edifice built on one of his stars?

Whatever your answer, there's a fascinating sidelight to this tale of Cepheids, and it might have been scripted from the legend itself. In 1925, Hubble found some Cepheids in what looked like a huge cloud -- the Great Nebula, it was called. Using those Cepheids, Hubble proved beyond doubt that the cloud was far outside our own Milky Way galaxy. It was, in fact, a separate galaxy altogether: the nearest full size galaxy to our own, about 2 million light years away.

And in which constellation is that Great Nebula, the constellation which then gave its name to the galaxy?

Andromeda, beautiful daughter of Cepheus.