You don’t need a thermometer to take a star’s temperature. All you need is your eyes. That’s because a star’s color is a direct result of its surface temperature. The hottest stars glow blue-white, while the coolest look reddish orange.

One example is the star Spica. It rises below the Moon this evening, and the Moon moves closer to it during the night.

Spica is a system of two stars locked in a tight orbit around each other. Both stars are bigger, heavier, brighter, and hotter than the Sun.

The main star, Spica A, is by far the more impressive member of the system. Its surface is a broiling 45 thousand degrees Fahrenheit, compared to just 10 thousand degrees for the Sun. That makes the star look blue-white. Under dark skies, with no moonlight, that color is pretty easy to see with the eye alone.

We can make out the color because Spica is both close and especially bright – more than 2,000 times the Sun’s brightness at visible wavelengths.

But if our eyes could see other forms of energy, Spica A would appear even brighter. Because its surface is so hot, the star emits most of its energy as ultraviolet light – wavelengths that are too short for the eye to see. So when you add up all wavelengths, Spica A glows more than 20 thousand times brighter than the Sun – a blue-white spotlight that tonight snuggles close to the Moon.

Tomorrow: bellying up to a cosmic bar.

Script by Damond Benningfield