The Sun Is a Star; Not All Stars Are Suns

PART III: Well more than a century ago, science again started to directly challenge the idea of a Plurality of Worlds by demonstrating that stars are not all other suns

LEFT: Anonymous, “Portrait of Agnes Mary Clerke.” RIGHT: Starry Sky.
LEFT: Anonymous, “Portrait of Agnes Mary Clerke.” RIGHT: Starry Sky. (photo: Wikimedia Commons / Public Domain - Pexels / Pixabay / CC0)

UFOs, ETs and the Strange History of Other Earths


Blockbuster movie franchises, media coverage of UFOs and Navy pilots, and claims about what the discovery of “extraterrestrials” would mean for humankind’s place in the universe all hinge on the idea that our universe abounds in other worlds like our Earth. This prominent idea is commonly associated with scientific progress, dating back to Copernicus in the 16th century. But as this four-part series shows, science in fact has never supported this idea of a “Plurality of Worlds.”

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Scientists adopted the idea that the universe is full of other suns, orbited by other earths, long before there was scientific evidence to support this idea of a Plurality of Worlds (see Part II of this series). In the early 19th century, there was great enthusiasm for the idea that all planets were inhabited. William Herschel, the first astronomer to discover a new planet (Uranus), even argued that the sun itself was inhabited:

The sun ... appears to be nothing else than a very eminent, large, and lucid planet, evidently the first, or, in strictness of speaking, the only primary one of our system; all others being truly secondary to it. Its similarity to the other globes of the solar system with regard to its solidity, its atmosphere, and its diversified surface; the rotation upon its axis, and the fall of heavy bodies, leads us on to suppose that it is most probably also inhabited, like the rest of the planets, by beings whose organs are adapted to the peculiar circumstances of that vast globe.... [N]ow I think myself authorized, upon astronomical principles, to propose the sun as an inhabitable world, and am persuaded that [my observations and conclusions] are sufficient to answer every objection that may be made against it.

Thomas Paine in his Age of Reason wrote of “the inhabitants of each of the worlds of which our [solar] system is composed” and of “millions of worlds equally dependent on his protection” (“him” being, for Paine, the “one Deity” of “the true Deist,” not Christianity’s trinitarian God). 

In the 1830s one writer, Thomas Dick, even attempted to estimate the population of the solar system. He reckoned that our neighboring planets fairly swarmed with intelligent life, putting the population of the solar system in the tens of trillions (with no solar population included). He proceeded to put the population of the universe at trillions of times larger still. 

This is the sort of universe that Mark Twain portrayed in his 1907 Captain Stormfield’s Visit to Heaven. Here the Captain comes to find out that the universe is so full of inhabited worlds that no one has even heard of Earth, to say nothing of the America, California or San Francisco that he so greatly esteems. When he identifies Earth as “the world” he is answered with “the world! H’m! there’s billions of them!” 


Agnes Mary Clerke on Diversity in Stars

But science never actually supported the sort of Plurality of Worlds universe seen in Captain Stormfield, even if science ceased to directly oppose such a universe once the star size problem was resolved (see Part II). This is because science did not provide any positive information concerning the stars and what they were. Toward the end of the 19th century, Agnes Mary Clerke, who was a sort of multi-lingual, Catholic, one-woman hub for astronomical communication and knowledge at the time, described what was known about the stars at that century’s start, and what had been learned about them by its end. 

At the start, she wrote, very little was known; astronomers largely viewed the stars as a background against which the motions of planets were measured. William Herschel, she noted, operated on the assumption that “the brightness of a star afforded an approximate measure of its distance” — meaning that one star was more or less the same as another, so those stars that appeared faint to us he reckoned to be more distant, and those that appeared bright to us, less distant. This assumption certainly meshes with the Plurality of Worlds idea that all stars are (more or less identical) suns. But it was an assumption. Nothing stood against it, since so little was known about the stars, but there was no science to particularly support it, either.

Throughout the 19th century, however, more and more was learned about the stars, largely thanks to advances in technology. Astronomers finally developed telescopes powerful enough to be able to determine stellar distances. By the end of the century, Clerke wrote, the distances to roughly 100 stars had been measured. Moreover:

[T]he list [of stars with measured distances] is an instructive one, in its omissions no less than in its contents. It includes stars of many degrees of brightness, from Sirius down to a nameless telescopic star in the Great Bear.

Many of the brightest stars had been found to be too far away for their distances to be measured, while most of the stars that were found to be nearest to Earth were quite faint. And so, she wrote,

The obvious conclusions follow that the range of variety in the sidereal system is enormously greater than had been supposed, and that estimates of distance based upon [brightness seen from Earth] must be wholly futile. Thus, the splendid Canopus, Betelgeux, and Rigel can be inferred, from their indefinite remoteness, to exceed our sun thousands of times in size and lustre; while many inconspicuous objects, which prove to be in our relative vicinity, must be notably his inferiors. The limits of real stellar [brightness] are then set very widely apart.

In other words, stars are not all suns. To be clear, today we understand that the sun is a star. That is, we understand that the sun is a globe of hot, dense gas, held together by its own gravity, heated to incandescence by nuclear reactions occurring deep within it; we understand that all stars are these nuclear-powered, incandescent, gas globes. 

But as Clerke observed, the range of what counts as a “star” is very wide. We have learned more about more stars in the century-plus since she wrote, but her observation still holds true. There actually are stars that are enormous, comparable in size to the sizes Johannes Kepler determined (see Part I of this series), but not at all dim like Kepler calculated; these are rare. More common are stars comparable to the sun. But most common of all are those stars “notably inferior” to the sun. Of the 100 stars currently known to be the sun’s nearest neighbors in space, roughly 80 have less than one one-hundredth of the sun’s power output. All these combined would not equal the sun, and most are not even visible to the naked eye.

Thus, following the downfall of Kepler’s and Jacques Cassini’s star-size arguments (see Part II), science was neutral on the question of whether stars were other suns. But in the 19th century science came back to show that Kepler and Cassini were right conceptually, if wrong on the particulars. There is a great diversity in the universe of stars, so stars are not, at least broadly speaking, just like our sun.

And if other suns are not so common, then what of other earths?


Further Reading

Clerke, A. M. (1887). A Popular History of Astronomy During the Nineteenth Century (2nd ed.). New York: Macmillan & Co.