A minor revolution in astronomy occurred on April 6, 1992. It did not take place at a mountaintop observatory but happened at anunlikely location the Callaway Gardens Inn on Georgia's Pine Mountain (elevation: 820 feet). Astronomers had gathered therefor an international meeting on the normally slow-paced researchtopic of double stars, a field where discoveries often require decades to allow for many of these systems to complete their orbits. While azaleas flowered outside in the spring rain, astronomers inside presented results pointing to the startling conclusion that even the youngest stars are frequently surroundedby stellar companions. This realization was the product of painstaking observations by many different people using a host of clever techniques and new devices. That morning in Georgia, the separate works of these numerous researchers appeared magically to dovetail.

The finding that binary systems are at least as common for youngstars as for older ones might seem reasonable enough, but for astronomers it came as a shock. Most notions of double star formation had predicted that stellar companions are produced orcaptured well after a star has formed; hence, the youngest starswould be expected to exist singly in space. Such theories no longer bear weight. There remains, however, at least one idea forthe formation of double stars that holds up to the recent observations. It may be the sole explanation for why binary starsystems are so abundant in the universe.

The sun, a mature star, has no known stellar companions, even though most stars of its age are found in groups of two or more.In 1984 Richard A. Muller of Lawrence Berkeley Laboratory and his colleagues hypothesized that the sun is not truly a single starbut that it has a distant companion orbiting it with a period of about 30 million years. He reasoned that gravitational forces from this unseen neighbor could disturb material circling in the outermost reaches of the solar system, sending a shower of comets toward the inner planets every time the star neared. Muller suggested that this effect might explain periodic massextinctions: comets generated by the sun's companion would hit the earth every 30 million years or so and as with the demise ofthe dinosaurs would have wiped out much of life on earth. Because its approach would have sparked such widespread destruction, Muller called the unseen star "Nemesis."

Most scientists have not accepted Muller's interesting idea. Forone, the closest known stars (the Alpha Centauri triple star system, at a distance of 4.2 light-years) are much too far away to be bound to the sun by gravity. In fact, there is no astronomical evidence that the sun is anything other than a single star whose largest companion (Jupiter) is 1,000 times less massive than the sun itself. But living on a planet in orbit around a solitary sun gives us a distorted view of the cosmos; wetend to think that single stars are the norm and that double stars must be somewhat odd. For stars like the sun, this turnsout to be far from true.

Doubles, Anyone?

In 1990 the late Antoine Duquennoy and Michel Mayor of the Geneva Observatory completed an exhaustive, decade-long survey of nearby binary stars. They considered every star in the sun's "G-dwarf" class within 72 light-years, a sample containing 164 primarystars that are thought to be representative of the disk of ourgalaxy. Duquennoy and Mayor found that only about one third of these systems could be considered true single stars; two thirdshad companions more massive than one hundredth the mass of the sun, or about 10 Jupiters.

Binary star systems have widely variable characteristics. Starsof some double G-dwarf systems may be nearly touching one another; others can be as far apart as a third of a light-year. Those in contact may circle each other in less than a day, whereas the most widely separated double stars may take tens ofmillions of years to complete a single orbit. Duquennoy and Mayorshowed that triple and quadruple G-dwarf stars are considerably rarer than double stars. They counted 62 distinct doubles, seven triples and two quadruple groupings. They further determined that each of the triple and quadruple sets had a hierarchical structure, composed of a relatively close double orbited by either a more distant single star (forming a triple system) or another close double star (forming a quadruple system). The separation between distant pairs needs to be at least five times the gap of the close doubles for the group to survive for long. Arrangements with smaller separations are named Trapezium systems, after a young quadruple system in the Orion nebula. These arrangements are orbitally unstable they will eventually flyapart. For instance, if the three stars of a triple system comeclose enough together, they will tend to eject the star of lowestmass, leaving behind a stable pair.

Comments