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To the eye alone, the stars that sparkle in the night sky are mere pinpricks of light in the dark cosmic background. And many of them are so small and so far away that they remain pinpricks even when you view them through a good-sized telescope.
One exception is Hamal, the brightest star of Aries, the ram. The constellation is in the east this evening. Aries isn’t much to look at, although Hamal is fairly bright, and in a fairly barren region of the sky. And tonight it’s due left of the Moon.
Hamal is close enough that astronomers can get an accurate measurement of its distance — about 66 light-years. That makes it a close neighbor. And the star is big enough to measure its angular size. It’s an incredibly tiny angle, though — the equivalent of a dime seen from a distance of about 30 miles.
Combining the star’s distance and its angular size in the sky allows astronomers to determine its true size — about 15 times wider than the Sun.
Hamal is so big because it’s nearing the end of its life. Changes in the nuclear reactions taking place in its core have caused its outer layers to puff up like a big balloon. That makes the star’s surface much cooler, so Hamal shines yellow-orange.
Millions of years from now, another set of changes in its core will cause Hamal to puff up even more — to perhaps dozens of times the diameter of the Sun. That’ll make it even easier for future astronomers to take the measure of this puffy star.
Script by Damond Benningfield, Copyright 2012
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