Moon and Spica

To the eye alone, Spica is one of the 15 brightest stars in the night sky. It’s about 2,000 times brighter than the Sun, and 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 20,000 times brighter than the Sun.

Most of that energy is in the ultraviolet — wavelengths that are much 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.

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 of the temperature scale shine 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 at just the right wavelengths for the human eye.

And Spica is in good view this evening, below the Moon. They’re flanked by two bright planets. Jupiter, the giant of the solar system, is to the upper left, with Venus, the “evening star,” to the lower right.

Script by Damond Benningfield