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Moon and Spica

July 25, 2012

One of the brightest stars in the night sky snuggles close to the first-quarter Moon tonight. Spica, the leading light of the constellation Virgo, is to the right of the Moon at nightfall. Equally brilliant Saturn is about the same distance to the upper right of Spica.

Although Spica looks like a single pinpoint of light, it’s actually the combined glow of two stars, each of which is quite impressive in its own right.

The system’s primary star is more than 10 times as massive as the Sun and about 12,000 times brighter. The second star is about six or seven times as massive as the Sun, and about 1500 times brighter. On its own, it would still be visible to the unaided eye, but just barely.

The two stars are only a few million miles apart, so from Spica’s distance of 250 light-years, their light blurs together.

But that close proximity has more profound effects on the stars themselves. Their gravitational pull on each other is so strong that the stars puff out toward each other, giving each of them a “bulge” on one side. When the stars align so that these bulges are visible from Earth, Spica’s overall brightness increases by a tiny bit.

The brightness of the primary star also varies a bit because the star pulses in and out like a beating heart. So Spica’s light flickers like a light bulb with a slightly bad connection — the flickering of two brilliant stars.

Tomorrow: A steaming cup of starlight.

Script by Damond Benningfield, Copyright 2012


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