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Space Facts

Star System Components

How can you have more than one star in a system?

Most of the star systems in the known universe are multiples. Most of these are double stars. Space is so large compared to the stars and planets within it, that the probability of a collision is vanishingly small.

Pairs of stars orbit about a common center of gravity — their mutual tugging on each other locks them into a dance about that invisible center. Both the Earth and moon, for instance, orbit about a common center of gravity that is embedded hundreds of miles, deep underground. If the moon had the same mass as Earth, the center of gravity of the Earth-moon system would be halfway between the two bodies.

Star systems with more than one stellar body typically label the brightest star with the letter "A." Where one of the stars in the system is later discovered to be a double, the secondary is typically given the lower-case letter "a." Such "spectral binaries" are sometimes difficult to detect, especially if the inclination of the orbits is shallow. The effect of the secondary star's mass on the spectrum of the primary is directed away from us when the inclination is close to zero.

Alpha Centauri — Three Stars

Discussing the orbits of stars in multiple systems can be confusing unless there is some way to know which orbit is being discussed. Combining the component letters into an "orbit" label helps clarify the discussion. For instance, in the Alpha Centauri system, with three stars, the brighter two orbit a common center of gravity roughly once every eighty years. This could be designated as, "the orbit of A-B has a period of eighty years." The "C" component, Proxima Centauri, orbits the central two in a period that is likely on the order of half a million years. This orbit would be labelled, "AB-C," meaning "C orbiting the pair AB."

Castor — Six Stars

The Castor system (Alpha Geminorum) contains six stars — three spectral binaries. The Castor A pair has an orbit of 9.2128 days (A-a). The B component orbit once every 2.9283 days (B-a), while Castor C has an orbital period of 0.8143 days, or about 19.5 hours (C-a). There are a number of estimates for the orbit of the AB pair of pairs, ranging from 380 years to 511 years. Because each spectral binary effectively acts as a unit, we typically don't find the secondaries listed in the orbit label; instead of "Aa-Ba," the simpler "A-B" would suffice. Castor C orbits the central pair of pairs in a period that probably exceeds 10,000 years (AB-C).

Much more complex systems are possible, but they would require much more space for stars to orbit safely about their various centers of gravity. The greater space required, though, makes such a complex system more unstable when the system as a whole orbits the galaxy within the galactic disk. Sharing the galactic traffic lanes with billions of other stars, any multiple system is going to be tugged on as it passes by other systems. The gravitational tugging would tend to disrupt a weakly held system.

Star Clusters — Hundreds of Stars

In fact, this is the reason there are few old star clusters within the galactic disk. They tend to "evaporate" long before they reach the age of our own star system. There are a few ancient "open" clusters that have maintained their stellar families, but they orbit the center of the galaxy at steep angles, passing through the galactic disk only once every several million years. This, of course, reduces the disturbance suffered by their stellar orbits about the center of each cluster. Two of the more ancient open clusters are NGC 188 (6 – 8 giga-years) and NGC 6791 (8 – 12.5 giga-years). Currently, the younger cluster is about 600 parsecs from the galactic plane. The older rides about 650 parsecs higher, but closer to the towering galactic hub.

Astronomy Data Book, by J.H. Robinson & J. Muirden — John Wiley & Sons, New York
A Field Guide to the Stars and Planets, by D.H. Menzel — 1964, Houghton Mifflin Company, Boston
Yale Bright Star Catalogue, 5th Revised Ed., D. Hoffleit & W.H. Warren, Jr. — 1991, Yale University, New Haven