More Than Meets the Eye, Part II

1/21/07 – 1/27/07
by C. Zaitz

It’s hard to watch the birth of a star. Apparently stars like to form in private, shrouded by opaque clouds of gas and dust. When giant molecular clouds in space collapse from their own gravity, one outcome is a star with planets. Often, rather than planets, two or more stars form. Scientists wonder what causes one and not the other, but stars pull a self-made curtain around themselves during their birthing. With better telescopes, however, we’re beginning to see more. Over half the stars in the sky are multiple star systems. Here’s what we can see.

Sometimes we can detect a system of two stars, called binary stars, through spectroscopy. This is the study of the light we collect from them. Normally we can spread the light we collect from stars out into a spectrum so we can learn all manner of things about the star; its composition, perhaps its distance, speed and direction, and even how old it is. It’s amazing what a little starlight can do. But sometimes we notice that the spectrum from a star is odd. Part of it is shifted toward the blue end of the spectrum, and part toward the red. Shifting of spectra is a curious thing; it tells us that the star is both coming toward us and away from us. Impossible! Unless, perhaps, it’s not just one star, but two stars in orbit, with one coming toward us and one going away. In fact, this is how we discover many binary stars.

Sometimes we see stars dimming and brightening, apparently for no reason. However, as adults, we recognize that things usually have reasons, as bizarre as they may be. Sometimes when stars orbit each other, they happen to be in our line of sight such that they will eclipse each other. They are known as eclipsing binaries. One star, in the constellation Perseus, is a very famous eclipsing binary star. Algol is normally about 2.3 magnitude, easily seen with the naked eye, but every 10 hours or so it will dim about 68%. This is when its companion moves in front of our line of sight, Since the companion star is dimmer, the total amount of light we get from Algol lessens. Perhaps that’s why it is named in Arabic, “the ghoul.” Algol is often referred to as the winking eye of the demon.

If a star has several companions, it becomes a star system. A famous system of stars is the middle star in the handle of the Big Dipper. Just using your eyes, you can usually detect two stars instead of one. Mizar and Alcor are close, but not close enough to be a double star system. We call that a visual binary- they appear close in the sky, but are not that close physically. However, the brighter of the two, Mizar, is actually composed of two stars, Mizar A and Mizar B. Furthermore, Mizar A is composed of two stars, as is Mizar B, so we have a system of two sets of twins orbiting each other. Amazing. But you can’t see this with your eyes. It’s fun to look at Mizar and Alcor anyway and imagine the complicated gavotte those stars must be executing.

Look to the north to see the Big Dipper, and if your eye is caught by a shiny light in the west at sunset, don’t be alarmed. It’s only the eye-catching, attention hogging Venus, showing off in the fading twilight.

Until next week, my friends, enjoy the view.