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Moon and Aldebaran

February 7, 2014

To the eye alone, the stars that twinkle across the firmament are no more than pinpoints — they’re so far away that it’s impossible to see them as disks. Some may seem bigger than others, but that’s just because they’re brighter. Most stars remain pinpoints even when viewed through a telescope. But a few are big enough and close enough that astronomers can actually measure their size.

A prime example is Aldebaran, the bright orange eye of Taurus, the bull, which is close to the lower left of the Moon at nightfall.

The Moon is one of the main tools that astronomers use to measure Aldebaran’s size. The Moon periodically passes directly between Earth and Aldebaran, blocking the star from view. Measuring how long it takes it to disappear behind the Moon reveals Aldebaran’s angular size.

The other tool for measuring a star’s size is interferometry. Astronomers combine the light from two or more telescopes that are far apart. That produces especially sharp images of the sky, allowing us to see stars as disks.

These techniques tell us the angle that Aldebaran covers in the sky — how big it looks from Earth. To convert that to a true size, astronomers must also know the star’s distance — a difficult measurement in its own right. The current value says that Aldebaran is about 67 light-years away. Combined with the angular measurements, that tells us that Aldebaran is 44 times the Sun’s diameter — a giant among stars.


Script by Damond Benningfield, Copyright 2013

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