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Using parallax to measure stellar distance
Using parallax to measure stellar distance

Natural Selections: stellar distances

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Martha Foley and Dr. Curt Stager talk about stars and the very clever ways we can tell their distance from the earth.

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According to Dr. Curt Stager of Paul Smith’s College, scientists first examined stars that were very close to the earth and then used these basic techniques to tell the greater distances. “It turns out it’s really, really complicated, as you might expect,” said Stager. “It’s pretty neat. There was a lot of luck involved.”

There is a cluster of stars visible with the naked eye called the Pleiades. This group of stars is central to determining astronomical distances since it is one of the closest to earth at about 400 light years away. One light year is how far light travels in a year. “It’s a huge amount to us, but compared to the other stars it’s very, very small.”

Stager explained that scientists first determined the distance from the earth to the Pleiades using geometry. “What people did was look very carefully, looking at this cluster of this hundreds of stars, take the brightest ones, and just sort of map out how they’re arranged next to each other and do it very, very precisely,” said Stager. “You can do it with a telescope from the ground. People did it centuries ago even.”

Scientists waited six months to allow the earth to travel halfway around the sun. Given that they know the size of the earth’s orbit, they were able to know how far the earth had moved from the Pleiades. “It’s a 3D arrangement and if you look at it from a slightly different perspective, they’ll look as if they’ve sort of moved a little bit compared to each other,” said Stager. This information can be used to calculate the distance of the stars from the earth.

Once that distance was known, researchers looked for other stars of the same type that were farther out in space. These stars appeared fainter since the distance diluted the light. Another formula can be used to discover how much distance is required to dilute the light to that degree. After that distance is found, more can be discovered. “It’s like a cosmological ladder. You can go father and father out in space based on that principle,” said Stager. “So it’s pretty neat to be able to do that at such distances.”

Some of the stars and the Pleiades are spinning on their axes. Scientists determined this by studying the nature of the light on different parts of the stars, and can even tell how fast each star is spinning. Stager said, “If you think about it, if it’s spinning and you’re kind of looking at the equator, one side of the star is coming at you and the other is going away from you. And that effects how the light comes to you in the same way that a sound does. If an ambulance is coming at you, or a police car, and you hear the siren and it’s high-pitched when it’s coming at you and low-pitched when it’s going away.”

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