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ABC of Astronomy D Is for Double Star
We're used to having just one Sun, so the planet Tatooine in George Lucas's Star Wars seems exotic with its double sun. Yet at least half the stars we can see in the sky are multiple star systems, usually double. Confusingly, when astronomers talk about double stars they could mean either one of two quite different types of star pairs true binaries or merely optical doubles.
Optical doubles are artifacts of observation. Two stars that have no connection happen to appear close together, a coincidental alignment when seen from Earth. An example is α Capricorni. You can see both α1 Capricorni and α2 Capricorni in good viewing conditions without binoculars. Even though they seem rather cozy together, in fact α1 Cap is 690 light years from us and α2 Cap is 109 light years away. Because of its great distance from us, α1 Capricorni seems dimmer than its companion, but is actually over twenty times more luminous.
The two stars in a binary system are held together by their mutual gravitational attraction. They're often described simply as orbiting each other. If, using a telescope or binoculars, you can see each of the two stars, it's a visual binary. Otherwise it's a non-visual binary. This is a working definition, because further observation or improved instruments can move a binary from the non-visual to the visual class.
It can still be difficult to tell if you have a single star or a double star, and if double, whether it's binary or not. In the past it was impossible most of the time, so optical doubles and binaries got lumped together.
William Herschel and double stars
William Herschel (1738-1822) began a systematic hunt for double stars in 1779. In fact, he was looking for double stars when he discovered the planet Uranus in 1781. But he wasn't the first person to take an interest in the doubles. That was Czech astronomer Christian Mayer (1719-1783).
The prevailing assumption was that double stars were chance visual pairings. Over 150 years before Mayer began to study double stars, Galileo was the first to see a double star through his telescope. He thought that doubles were unrelated stars along our line of sight, and that the angle between them would seem to change as we orbit the Sun, because of parallax. (Here is a diagram that shows parallax, though using just one star.)
Mayer however theorized that double stars were close together and orbiting each other. The dimmer star was the one that was farther away at a given time. But Herschel subscribed to the Galilean view, and his interest in studying double stars was measuring stellar distances by parallax. Although he cataloged well over eight hundred multiple stars, he was in for a surprise.
Over twenty years after his original observations Herschel remeasured a number of them. Far from seeing a parallax, in 1803 he announced that double stars were "not merely double in appearance, but must be allowed to be real binary combinations of two stars, intimately held together by the bond of mutual attraction." Herschel was credited with the first use of the term binary to describe such pairings. More importantly, it confirmed that Newtonian gravitation was at work beyond the Solar System.
There are ways of detecting a binary system other than visually. The classes of binary systems are based more on the means of detection than the characteristics of the system. The two most common types are spectroscopic and eclipsing binaries.
No one knew that the star Mizar had an invisible companion until 1889. E.C. Pickering (1846-1919) discovered that at Harvard College Observatory, making Mizar the first spectroscopic binary.
If there are two stars, there are two sets of spectral lines. Sometimes this is enough to establish that there's a binary. But there may also be a Doppler shift as the stars orbit. While an object is moving away from us, it has a red-shifted spectrum, i.e., its spectral lines are closer to the red end of the light spectrum. Moving towards us shifts the lines towards the blue. (For more about this, click on the link "Searching for Extrasolar Planets" below this article.)
If two stars orbit in our line of sight, so that one moves in front of the other as they circle, we have an eclipsing binary. It appears to be one star with varying light output as the two stars in turn pass in front of (transit) each other. This diagram shows how the light varies in the eclipsing variable star Kepler-16.
Importance of studying binary systems
Since at least half of the stars we see are binary, understanding them is an essential part of our knowledge of stellar evolution. Astronomers have discovered binary protostars, which suggests that multiple-star systems are produced as the molecular cloud breaks up during star formation. Much of what we know about stellar masses comes from binary systems, because it's not possible to make a direct measurement of the mass of a single star. The methods for detecting these systems, such as Doppler spectroscopy and transits, are also useful in discovering extrasolar planets.
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