BackBlinking on the rocks
Blinking on the rocks
PremiumCepheids gave astronomers their earliest reliable estimates of how far celestial objects are from us. Eventually, that led to profound discoveries about the creation of the universe itself. Photo: Ian
The next time you’re out on a starry night, stop for a few minutes and look up. See that gigantic “W", or depending on the time of night maybe it’s a “M"? That’s the constellation Cassiopeia, called Sharmishtha here in India.
If you happen to be carrying a star guide, use it as you look around.
Ranged around Cassiopeia you’ll find Andromeda, Cepheus and Perseus. All names from an ancient Greek legend. Now Cassiopeia can help you find the North Star, but she also points forlornly…but I’m getting ahead of myself.
Cepheids, they called these stars.
Exactly a hundred years ago, Henrietta Leavitt announced the results of observing 25 Cepheids in the Small Magellanic Cloud (SMC), a small galaxy known to be about 200,000 light years away. Since all were in the SMC, all were about the same distance from us. This meant that if one Cepheid seemed brighter than another, it wasn’t because it was closer to us, but that it was intrinsically brighter.
With that understood, Leavitt made a dramatic discovery: The brighter a Cepheid, the longer the period of variation in its brightness. This was the “period-luminosity law", a major milestone—I use that word deliberately—of modern astronomy. Given a Cepheid’s period, the law tells us how intrinsically luminous it is.
This was a powerful tool, because it allowed astronomers to compare stars as if they were side-by-side, equidistant from us. For example, we know that Delta Cephei, with its period of five days, is about 700 times brighter than the Sun. If it sat where our Sun now sits, you and I would be toast.
Luckily, Delta Cephei is a long way off, and so very faint indeed (the fourth-brightest star in a nondescript constellation, remember). But how far away is it, actually?
Well, think of asking a friend to walk away, shining a torch in your direction as she goes. It gets steadily dimmer, right? In the same way, the dimmer a star seems, the further away it is. But we also know how bright Delta Cephei actually is—700 suns bright. Put this information together and we can estimate how far away Delta Cephei really is.
So consider where Henrietta Leavitt brought us. Measure Cepheid periods, and we know their intrinsic brightness. Compare with their apparent brightness, and we know how far away they are. And since we find Cepheids all over the sky, they are like milestones (now you know why I used the word) sprinkled across the universe. Every time we see a Cepheid, we know how far it is. If there are objects near it, we know how far those are as well.
It’s as if you were captain of a ship caught in a storm. Through sheets of rain, you see the blinking light of a lighthouse. If you can tell how far it is, that’s how far you are from land. Think of Cepheids like that: lighthouses blinking through the vast void.
Cepheids gave astronomers their earliest reliable estimates of how far celestial objects are from us. Eventually, that led to profound discoveries about the creation of the universe itself.
What the fourth-brightest star in a fairly nondescript constellation can do for us.
And the legend? Cepheus was the king of Ethiopia. His ravishing wife, Cassiopeia, boasted that she was more beautiful than any goddess.
Naturally, this got on the nerves of the gods. One, Neptune, threatened to destroy Ethiopia unless Cepheus sacrificed their daughter, Princess Andromeda, to the sea monster Cetus.
So there was poor Andromeda, chained to Ethiopian rocks, waiting to be eaten. But along came Perseus, flying on magic sandals. Holding aloft a head that writhed with snakes, he turned Cetus into stone and rescued Andromeda.
Lovely stuff, and that’s just the skeleton of it.
And then there was this. In 1925, Edwin Hubble found some Cepheids in a fuzzy celestial object then called the Great Nebula. They told him that the Nebula was actually far outside our own galaxy, the Milky Way. It was, in fact, a different galaxy altogether, the nearest full galaxy to our own, about two million light years away.
And in which constellation was this object now understood to be a galaxy?
Andromeda, young daughter of Cepheus.
Your star guide will show you how Cassiopeia points to the Andromeda galaxy. About what she might have been doing, there on the rocks where her daughter lay.
Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. A Matter of Numbers will explore the joy of mathematics, with occasional forays into other sciences. Comments are welcome at dilip@livemint.com
Also Read | Dilip D’Souza’s previous columns
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