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H-R Diagram
 Introduction
 The Brightest Stars
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Hipparcos Satellite Data

Parallax in action
Notice how the thumb jumps relative to the galaxy picture depending on which side the camera was placed.

The Hipparcos satellite finds the distances to stars using a technique called parallax.

You have probably seen parallax before. Hold your thumb at arms length. Close one eye and look at your thumb. Now switch which eye is closed. You will notice your thumb appears to "jump" relative to the background. Your thumb appears to jump because you are looking at it from a slightly different angle. The distance between your eyes is called the "baseline" and the angular distance (in degrees or radians) that your thumb appears to jump is called the "parallax angle." The length of the baseline determines the smallest parallax angle you can resolve; longer baselines can resolve smaller angles.

Stars are extremely far away, so we need a very large baseline to determine parallax angles. In fact, the baseline needs to be substantially larger than the radius of the Earth. Astronomers use the Earth's entire orbit to get a large enough baseline. Astronomers observe a star on one night and then again about six months later, when Earth has moved halfway around the sun. Using this technique, astronomers can find parallax angles with a baseline of 186 million miles!

Even with such a large baseline, the parallax angles of stars are very small. Proxima Centauri, the closest star, has a parallax angle of 0.772 arc seconds (each degree is divided into 60 arc minutes and each arc minute is divided into 60 arc seconds. Therefore, 1 arc second is 1/3600 of a degree!) Due to atmospheric blurring, you can measure parallax angles down to about 0.01 arc seconds from the surface of the Earth.

The Hipparcos satellite measures the parallax angle of a distant star

The Hipparcos satellite, which makes its measurements from Earth orbit, measured the parallax distances to about 120,000 stars with an accuracy of 0.001 arc seconds, and about 2.5 million stars with a lesser degree of accuracy. This gives accurate distances to stars out to several hundred light-years.

Much like SDSS data, all Hipparcos data are available online. On the next page, you will use Hipparcos data to help make an H-R diagram. You will calculate the distances to several stars from their parallax angles. You will then use this distance, along with each star's visual magnitude, to calculate its absolute magnitude.