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October 25, 2015

The stars aren’t “wysiwyg” kinds of objects — what you see isn’t always what you get. Consider Sheratan, the second-brightest star of Aries, the ram. It’s low in the east as night falls this evening, far to the left of the Moon.

To the eye alone, the star is a single pinpoint of light. It remains a single pinpoint even through the largest individual telescopes. Yet it’s actually a system of two stars.

Astronomers discovered that double nature by measuring Sheratan’s spectrum — spreading its light into its individual wavelengths. A star’s motion toward or away from us causes a shift in its spectrum. Astronomers see a double shift in Sheratan’s spectrum, revealing the presence of two stars in orbit around each other.

A second technique finally allowed astronomers to see the individual stars. Known as interferometry, it combines the light from several small telescopes to yield especially sharp images. Using an interferometer at Mount Wilson in California, astronomers precisely plotted the orbit of the two stars. That allowed them to determine the details of the individual stars.

One star is a little bigger, brighter, and heavier than the Sun. The other is more than twice as massive as the Sun, and more than 20 times brighter, so it accounts for almost all of the system’s light.

On average, the stars are closer together than Earth and the Sun — a closeness that makes Sheratan look like a single star, marking the horns of the ram.

Script by Damond Benningfield, Copyright 2015

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