Spike to Spica

May 18, 2015

mars is red / spica is blue
Credit nasa

  This is “Looking UP! in southern Colorado,” from the Colorado Springs Astronomical Society. I’m Hal Bidlack, and there are lots of reasons to look up!

A few episodes ago we talked about the star Arcturus, and how you could find it by following the curve of the handle of the Big Dipper across the sky. Today let’s talk about what happens if you keep following that same curve past Arcturus. You can speed on, to Spica

Spica is not hard to find if you keep following that same arc, in part because there are few other bright stars near it. In fact, one Arabic name for this star was Azemech, which means solitary or defenseless star.

Spica is strange. It is at least two stars, a binary system. While double stars orbiting each other are common, they are usually farther away from each other than Spica. Now, the Earth is about 93 million miles from the sun. But the two main stars that combine to be Spica in our sky are only 11 million miles apart. That means that those two stars are three times closer to each other than the scorched planet Mercury is from our sun.

One of the Spica stars is seven times bigger than our sun and the other is at least four times bigger, and both are much hotter. And, while Mercury takes 88 Earth days to orbit the sun once, these two huge stars spin around each other once every four days. They revolving around each other so fast that there is evidence to suggest that they have become egg shaped, due to the tremendous forces involved in so rapid a rotation.

The combined light from these two stars is more than 2200 times brighter than our sun, so it’s a good thing Spica is about 262 light years away.

Because of where it happens to be in space, the moon can pass in front of, or eclipse, Spica. Analysis of the last time the moon eclipsed Spica suggests there may be as many as three more stars there, which means when you look at Spica, you could be looking at as many as five individual stars, all combining to make one bright point in the sky. Pretty cool for a solitary star.

If you’d like to take a closer look at Spica or any of the other wonderful and amazing things in the sky, please visit KRCC.org or CSASTRO.org for a link to information on our monthly meetings and our free public star parties! 

This is Hal Bidlack for the Colorado Springs Astronomical Society, telling you to keep looking up, Southern Colorado!