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Starlight Echoes Make Cosmic Measuring Stick

Larry O'Hanlon, Discovery News

Photos

It's All in the Echo

Feb. 13, 2008 -- Astronomers have borrowed a technique from bats and dolphins to ultra-fine tune the distance to another star.

By using not only the direct light from the star, but also the starlight "echoes" created by nearby gas and dust, astronomers calculated the distance to the star in the constellation Puppis to an accuracy of 99 percent.

The technique is particularly useful for studying the distance from Earth to stars in the Milky Way and the nearby Magellanic Clouds, which are companion dwarf galaxies. It's also critical for calibrating the light of similar stars in other galaxies, and measuring distances between galaxies.

The reason the technique has not been used before on other stars near gas clouds is that the very bright star in question, called RS Pup, is a special sort called a Cepheid.

That's a variable star, which means its light pulses brighter and dimmer at a rate, or period, that is directly related to how much light it puts out. RS Pup pulses brighter by a factor of five every 41.4 days. This period-luminosity relationship has allowed astronomers to use Cepheids as cosmic measuring sticks for almost a century.

What makes RS Pup special is that its pulsing can be also seen in the light reflecting off the gas clouds. The reflection is somewhat delayed, however, because it has to travel from the star to the cloud, and then to Earth.

It's this delay, when compared to the direct, straight-line light from RS Pup, that allowed the astronomers to use basic geometry to refine their measurement of the star's distance.

"This is the only (Cepheid) with a nebula around it," explained astronomer Pierre Kervella of the Observatoire de Paris-Meudon, France. So it's the only one for which this sort of measurement can be done.