What Is a Binary Star, and What Did Plato Say About the Stars?

A binary star is a system of two stars bound together by gravity, each orbiting a shared centre of mass. Plato knew nothing about them: the idea needed telescopes and Newtonian gravity, and was not established until William Herschel’s work around 1802-1803, more than two thousand years after Plato’s death. What Plato did say about the stars is nonetheless worth reading. In the Timaeus he called them living, divine beings; in the Republic he argued that astronomy should be pursued by thought rather than by gazing; and a late dialogue attached to his name, the Epinomis, makes astronomy the highest study of all.

What is a binary star?

In a binary, the two stars do not orbit one another so much as circle a point between them, the centre of mass, which lies closer to the heavier star. Many stars, particularly the more massive ones, have at least one companion, and some systems have three or more members. Astronomers sort binaries by how they are detected:

• Visual binaries are pairs a telescope can separate into two points of light. Their orbits are slow, often taking decades or centuries.
• Spectroscopic binaries look like one star, but the spectrum shows spectral lines that shift back and forth as the stars move toward and away from us.
• Eclipsing binaries are seen nearly edge-on, so each star periodically passes in front of the other and the combined brightness dips. Algol, in Perseus, is the best-known example.

Binaries matter because the orbit gives a direct way to weigh stars. Measure how fast two stars circle each other and how far apart they are, and Newton’s law of gravity yields their masses, which is hard to get any other way.

When were binary stars discovered?

Telescopes revealed that some stars come in close pairs soon after they were invented. The star Mizar, in the Plough, was seen as double in the early seventeenth century: Benedetto Castelli reported it to Galileo in 1617, and Giovanni Battista Riccioli noted it again around 1650, which is why older books often credit him. Pairs like this might have been chance lines of sight, though, two unrelated stars that merely look close.

The decisive step came from William Herschel. Comparing his own measurements of double stars made over some 25 years, he concluded in 1802-1803 that in a number of cases the two stars were changing position relative to each other in a way that looked like an orbit. That made them true physical binaries, bound pairs rather than accidental neighbours. In 1889 Edward Pickering, working with Antonia Maury at Harvard, announced that Mizar’s brighter star was itself a spectroscopic binary, the first found by that method.

Did Plato know about binary stars?

No, and it would be misleading to suggest otherwise. Plato lived from about 428/427 to 348/347 BC, long before the telescope. For him the stars were points of light visible to the naked eye. Greek observers could see Mizar and its neighbour Alcor as a naked-eye pair, but nothing in Plato’s writing treats any star as a double system, and he has no concept of stars orbiting one another. There is no “binary star” anywhere in Plato.

What the ancient sources do give us is Plato’s picture of the stars themselves, and that is very different from the modern one.

What did Plato say about the stars in the Timaeus?

In the Timaeus, his main cosmological dialogue, the stars are not balls of gas but visible gods. The Demiurge, the divine craftsman, makes them mostly out of fire so that they will be as bright and beautiful as possible, and gives each a pair of motions: a rotation in place, and a revolution along with the heavens as a whole (Timaeus 40a-d). The fixed stars, in his account, are divine, living and eternal creatures (Timaeus 40b). The planets, with the sun and moon, are made to define and keep the numbers of time (Timaeus 38c). For the craftsman himself, see what the Demiurge is.

The stars also play a role in Plato’s account of the human soul. The Demiurge, he says, sows each soul onto a star, one soul to each; and a person who lives well will return to the dwelling place of his companion star and enjoy a happy life (Timaeus 41d-42b). Here the stars are fixed points of reference, and a model of a stable, well-ordered life.

None of this is science in the modern sense, and Plato frames the Timaeus as a “likely account” (Timaeus 29d), not a proof. But it records the Greek habit of reading the heavens as intelligent and ordered. The Timaeus also says sight was given to us so that we could observe the revolutions of the heavens (Timaeus 47a-b). For the wider cosmology, read Plato’s Timaeus and cosmology.

What does the Republic say about studying the stars?

In Republic VII (527d-530c), astronomy is one of the studies that turns the soul toward the unseen. Socrates argues that the decorations of the night sky are beautiful but are only models, to be used like diagrams in geometry: the real subject is the true motions of speed and slowness that the eye cannot see, grasped by reason (Republic 529c-530c). On this view, a would-be astronomer who only stargazes learns little. The practical result is a very mathematical astronomy, and some historians treat it as an early statement of the idea that theory should lead observation.

What is the Epinomis, and is it by Plato?

The Epinomis is a short dialogue that follows the Laws and argues that the highest wisdom is astronomy joined to mathematics, with the stars and planets studied as visible gods. Whether Plato wrote it is disputed. Diogenes Laertius (III.37) says some report that Philip of Opus, a member of Plato’s Academy, copied out the Laws and was the author of the Epinomis, and many modern scholars regard it as not Plato’s own, partly on grounds of style and doctrine: it makes astronomy the crowning wisdom, whereas the Republic treats astronomy as a preparation for dialectic. Anyone quoting it as “Plato says” should add that caveat. Our separate essay on the Epinomis goes into its themes.

So how do Plato and binary stars connect?

Only by contrast, and the contrast is instructive. Binary stars show how astronomy changed once instruments let people measure what the eye could not. Plato’s own approach, set out in the Republic, favoured reason over eyesight, but he did not think observation was worthless: he thought it was only a starting point. Modern astronomy took the opposite route to the same end, using telescopes and spectra to uncover orbits Plato could not have imagined.

The one thing the two views share is a conviction that the heavens are ordered and can be understood. Plato took that order to be the work of a divine mind; Herschel found it in the mutual gravitation of pairs of suns. For the school where mathematics and astronomy were studied in his lifetime and afterwards, see what institution Plato founded and who Plato was. For another place where astronomy carries his name, see what the Plato crater is.