Eclipses Elsewhere

It will be more than a century until we have another opportunity to witness Pluto—Charon eclipses. That pair comprises a binary planet with a mass ratio of about 8:1. Looking among the major planets, the next highest primary-to-secondary ratio is represented by the Earth and the Moon, weighing in at 81:1, so we may be justified in thinking of the Earth—Moon system as another binary planet.

Other binaries are known in the Solar System. Most asteroids are very irregular in shape (recall Figure 12-1). If they spin fast enough, asteroids may separate into component blocks that would then loop around each other, in a temporary gravitational embrace. ("Temporary" has an astronomical meaning here: it might be a million years before some close passage by a planet causes a separated asteroid to lose its grip on the fragments.) In 1993 the Galileo spacecraft, while on its voyage to Jupiter, visited the large asteroid Ida. It was a surprise to many that Ida was found to have a small moonlet, which has been named Dactyl. The large mass ratio, however, means that we cannot really claim Ida and Dactyl to represent a binary asteroid.

From various lines of evidence there has long been a suspicion that there are binaries among the asteroids that cross the Earth's orbit. For instance, several of the impact craters on our planet seem to be arranged in pairs formed at the same juncture. Direct evidence for such a binary object came in 1997, when observers following the brightness variation of the asteroid named Dionysus detected dips in its intensity curve characteristic of repeated mutual eclipses and occultations. Dionysus is a binary, with one lump larger than the other, and eclipses tell us so.

The astronomical context in which the term "binary" appears most often is far beyond the Solar System, in the description of binary stars. Such pairs are a well-known phenomenon. A large fraction of the apparent pinpoints of light one can see in the sky using only the naked eye actually display a dual nature if they are viewed instead through a telescope with sufficient resolution. For instance, the national flags of Australia and New Zealand both show the stars of the constellation known as the Southern Cross (or Crux), but the depictions are inaccurate on two counts. First, the colors are radically wrong. More important here, the binary properties are not shown on the flags. The brightest star in the Southern Cross is a multi-colored triplet, and the next brightest is a doublet. Similarly Sirius, the most luminous star in all the heavens, actually has a faint companion.

It was not until the early nineteenth century that the binary nature of many stars was widely accepted, despite earlier evidence for their existence. The largest available telescope at the time was that of William Herschel, which he said could not separate stars into discrete components. Observers reporting that some bright stars had luminous companions tended to be ridiculed. It was not until 1802 that Herschel agreed that binaries existed and could be distinguished telescopically. His son John spent much of the period between 1820 and 1840 drawing up catalogues of binary stars, initially with James South in London and Paris (recall our discussion of South in Chapter 12). John Herschel later took his family with him to Cape Town in South Africa, from where he scanned the southern sky for binary stars for four years.

These were visual binaries—stars that could be resolved by eye using a good instrument. Nowadays astronomers study more distant binary systems, too far to be separated directly, by analyzing their composite spectra. The spectrum emitted by each of the two stars, often of quite disparate types, will display varying red- and blue-shifts as first one star and then the other approaches and recedes from us in their locked orbits about the mutual center of gravity. (The speed-dependent shift in the spectrum of an object, due to the Doppler effect in light, was discussed in Chapter 12.) Such stars are called spectroscopic binaries. As with Pluto and Charon, those orbits allow the stars' masses to be investigated.

The first binary star to have its physical properties probed through such orbital data was not a spectroscopic binary, though. The star in question displayed not spectral changes, but rhythmic variations in its intensity.

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