Cosmic ladder (Part 2)

To measure distances in deep space, you need to look for candles in the darkness. [2 of 2]

Cepheid variable star
NASA, Public domain, via Wikimedia Commons.

In Part 1 we explored the parallax method of measuring distances in outer space. This works really well… for stars close to our own. As you go further out, you need other tricks. One such trick is the standard candle.

In 1912 an astronomer at Harvard named Henrietta Swan Leavitt noticed something special. She was studying a class of stars called Cepheid variables. These stars pulse and flicker, getting brighter and then dimmer and then brighter again in a predictable cycle. What she noticed was this: the stars that pulsed brighter also pulsed faster. The stars that pulsed slower were also dimmer. There was a clear and measurable relationship between their brightness and the rate at which they flickered.

Why was this important? Well, it all hinges on the difference between luminosity and brightness. Luminosity is the absolute brightness of a star – how it would look if you were right up close to it. “Brightness,” in contrast, is how bright a star appears in our night sky. Luminosity is determined by the type of star. Brightness is determined by the type of star but also by its distance from us.

The further a star is from us, the dimmer it appears. Say you have two stars of equal luminosity. If one is twice as far away, it appears four times dimmer to us than the other. If it’s three times as far away, it’s nine times dimmer. So, if you have just two pieces of information…

  • The luminosity of the star (the brightness if you’re right up close to the source)
  • The brightness of the star as it appears to us

…well, then you can calculate the distance to that star.

Some Cepheid variables are in our galactic neighbourhood – we can measure their distance with the parallax method. And the luminosity of those local Cepheids is easily observed. Because of the relationship that Leavitt discovered, we know that a distant Cepheid with the same pulse will have the same luminosity. Suddenly, we have all the variables necessary to work out the distance to Cepheids a long way away. Suddenly, our measuring stick becomes much much longer.

The Cepheid variables became markers in the cosmos. That’s why they’re called “standard candles” – we know their brightness and their distance from us, so they can be a measuring stick for the stars and galaxies around them.

We used the first method – parallax – to work out the distance to nearby stars. That method then enabled us to create a second method – standard candles – to work out the distance to other stars. And the standard candle method calibrated another step, and so on and so on. To measure distances in space today, we rely on more than a dozen different methods – each one a separate rung on a “cosmic distance ladder.” Together, they can chart the cosmos.

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