To the eye alone, Spica is one of the 15 brightest stars in the night sky. And it really is brilliant. At visible wavelengths, it’s about 2,000 times brighter than the Sun. It looks white with a hint of blue.

When you look at Spica with special instruments, though, it’s even more impressive. It consists of two stars, not one. Both are much bigger and heavier than the Sun. And when you add up all wavelengths of light, they shine about 20 thousand times brighter than the Sun.

Most of that energy is in the ultraviolet – wavelengths that are too short for the human eye. Spica’s two stars produce so much of it because their surfaces are tens of thousands of degrees hotter than the Sun’s.

In fact, the type of energy a star emits depends almost entirely on its surface temperature. And so does the star’s color. To the eye alone, the hottest stars look blue. But they emit huge amounts of ultraviolet. The coolest stars look orange or red. They emit huge amounts of infrared light – wavelengths that are too long for the human eye.

Stars in the middle are white or yellow. They emit most of their light at visible wavelengths. So with a star like the Sun, we see most of the energy it produces – light that’s just right for the human eye.

Spica is quite close to the Moon as darkness falls this evening. The Moon will slide away from the star during the night, but they’ll still be close as they set, around dawn.

Script by Damond Benningfield