What Are Binary Stars?

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  • 0:01 What Is a Binary Star?
  • 0:34 Binary Star Types
  • 3:14 Binary Star Masses and Orbits
  • 5:05 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson defines binary stars and describes the different kinds, including visual binary, astrometric binary, spectroscopic binary, and eclipsing binary stars. We'll also describe how orbits and mass relate to one another in a binary star system.

What Is a Binary Star?

Have you ever had double vision? Maybe it was because you had a massive headache. Perhaps you had one drink too many. Or, maybe your vision had to be checked. But in the world of astronomy, double vision may be completely normal. If you look up at the night sky with your unaided eyes, what looks like one speck of light, one star, may actually be two at once! A binary star is a pair of stars that revolve around their common center of gravity.

Binary Star Types

Binary stars are classified in terms of how they are observed. A visual binary is a form of binary star that can be resolved into two separate stars with a telescope. Basically, for a visual binary, all you need is your vision and a little help from the telescope. Tens of thousands of visual binaries are known to us. If you'd like to see one for yourself, check out the star called Mizar in the big dipper, which is in the constellation Ursa Major.

But, as you can only imagine, not all binary stars are so easily visible. A case in point is an astrometric binary, a kind of binary star that has a visible star with an unseen companion star. You're surely wondering how we even know there is another star if we can't see it. Well, astronomers infer this invisible star's presence from the variation in the motion of the visible star due to its silent partner's gravitational influence.

Another kind of binary star that cannot be truly appreciated with only a telescope is a spectroscopic binary, a type of binary star whose nature is revealed by its spectrum. A spectrum is like the rainbow of colors that comes out of a prism when you shine a white light into it. Well, if the colorful details of a spectrum are changed due to the motion of the source of light, something called the Doppler effect, it clues us into the fact that there are two stars there.

Further still, there's something known as an eclipsing binary, a binary star where one star passes in front of the other, thereby cutting off the star's light at regular intervals. What this means is that an eclipsing binary has regularly scheduled, if you will, changes in brightness, just like a solar eclipse here on Earth momentarily changes the apparent brightness of our sun.

As one star passes in front of the other, the brightness dips
Eclipsing Binary Star

Just look at the animation shown on screen to see what I mean when it comes to an eclipsing binary. Watch how the stars move and the brightness changes as they do so. A real world example of this is the eclipsing binary Algol, in Perseus. Note how Algol 'winks' at you as a result of its eclipsing partner.

Finally, we have the optical double, which actually isn't a binary star, but I thought it needed mentioning in this lesson. It's when a pair of stars appear to be close to each other when viewed from Earth, but are actually very far apart and have no relationship with one another.

Binary Star Masses and Orbits

Every binary star system varies in the sizes of the orbits involved and the orbital period, which is the time it takes the star to make one orbit. If the orbits are small and the orbital period is short, then the stars' gravity must be very strong in order for each star to hold one another in orbit. The only stars that can do something like this have to be massive!

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