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Moon and Spica
Spica, the leading light of Virgo, is one of the brighter stars in the night sky. It’s also one of the more thoroughly studied. The file compiled by astronomers shows that it consists of two stars that are only a few million miles apart. They’re so close, in fact, that their gravity distorts both stars, making them look a bit like eggs. And both stars are bigger, heavier, and brighter than the Sun.
Astronomers amassed much of that file with spectroscopy, a technique that breaks Spica’s light into its individual wavelengths, creating a rainbow of colors. Each element in the stellar atmospheres imprints patterns of dark lines on the rainbow, revealing the stars’ compositions.
Since there are two stars in the system, there are two sets of lines. In fact, that’s how astronomers discovered the system’s binary nature. As the stars orbit each other, the sets of lines shift back and forth. The size of the shift reveals the stars’ orbital speed, while the timing of the shift reveals their orbital period — about four days.
The spectrum also reveals the temperatures of the two stars, as well as how fast each of them is turning on its axis. And it shows that the heavier star is pulsing in and out like a beating heart — one more entry in the file on this bright system.
Spica stands close to the lower right of the Moon as night falls this evening. The brilliant planet Jupiter is to their upper right, completing a beautiful triangle in the night sky.