What did people used to think stars were?

[DarrenS]

I know that way back in antiquity - would that be Roman, Greek, ... times? - people thought the Earth was at the center of the celestial sphere, with the stars "glued" on it.

Actually - even then, what did people think the stars were, physically?

And between then, and our smug modern-day understanding of stars as huge balls of gas undergoing constant nuclear fusion - what did people think they were composed of? Was there a key moment when someone realized, Aha! Alpha centauri/Riga/Sirius is the same kind of thing as our Sun!

[kniz]

One belief was that they were holes in the sky (which was like an up-side-down bowl) thru which the gods looked down on humans.

^{Well they weren't rocket scientists!}

[bonzer]

Classical notions of what the stars consisted of were initially all over the place. Holes in the sky. candles, etc.
Things begin to settle down with Aristotle. He held that all the universe was made up of five elements: air, earth, water, fire and aether. The last of these was meant to be unchanging and was what the heavenly bodies, including the fixed stars, were made of. From De Caelo, Book I, chapter III:

For in the whole range of time past, so far as our inherited records reach, no change appears to have taken place either in the whole scheme of the outermost heaven or in any of its proper parts. The common name, too, which has been handed down from our distant ancestors even to our own day, seems to show that they conceived of it in the fashion which we have been expressing. The same ideas, one must believe, recur in men's minds not once or twice but again and again. And so, implying that the primary body is something else beyond earth, fire, air, and water, they gave the highest place a name of its own, aither, derived from the fact that it 'runs always' for an eternity of time. Anaxagoras, however, scandalously misuses this name, taking aither as equivalent to fire.

Later Roman authorities didn't necessarily agree. For instance, Pliny the Elder was pretty clear. In Natural History, Book II, chapter IV:

As regards the elements also I observe that they are accepted as being four in number: topmost the element of fire, source of yonder eyes of all
those blazing stars.

So only four elements, with aether missing and the stars made of fire instead. Lucretius was meanwhile a bit borderline. He thought that all matter was made up of atoms and most of the emphasis in De Rerum Natura is on the idea that there's nothing mystical about any of the natural world. It's all just atoms interacting. However, when he actually touches on what stars are made of (in De Rerum Natura, Book V), he's careful to emphasise that the atoms involved aren't quite like those that make up the Earth:

First came together the earthy particles
(As being heavy and intertangled) there
In the mid-region, and all began to take
The lowest abodes; and ever the more they got
One with another intertangled, the more
They pressed from out their mass those particles
Which were to form the sea, the stars, the sun,
And moon, and ramparts of the mighty world-
For these consist of seeds more smooth and round
And of much smaller elements than earth.
And thus it was that ether, fraught with fire,
First broke away from out the earthen parts,
Athrough the innumerable pores of earth,
And raised itself aloft, and with itself
Bore lightly off the many starry fires;
And not far otherwise we often see

But this is a quantitative, rather than a qualitative difference.

However, Aristotle prevails. It helps that Ptolemy endorses the idea of aether: for instance, in the discussion of circular motions in the heavens in the Almagest. This Aristotle-Ptolemy party-line survives through medieval thinking on the issue.

Where the current tradition of regarding stars as made of the same stuff as we find on Earth arises is during the Scientific Revolution, particularly as part and parcel of Copernicanism. The idea that the Sun is just another star is very closely tied to the adoption heliocentrism. Otherwise, things are very complicatedly tied into a general rejection of Aristotle. For instance, the observations of the supernovae in 1572 and 1604 was taken by some as evidence that the notion that the heavens were unchanging was false. This struck at the whole idea of what aether was meant to do. Certainly by the late 1600s the idea that there was just stuff, which was pretty much the same everywhere, was very widely accepted.

[DarrenS]

Excellent answer - thanks bonzer

[buckminster]

From the UniverseToday.com archive:

Astronomers Find a Huge Diamond in Space

Summary - (Feb 13, 2004) Astronomers from the Harvard-Smithsonian Center for Astrophysics have found a diamond in space, and it's big… really big. The object, technically known as BPM 37093, is a crystallized white dwarf star approximately 4,000 km across. The astronomers call it a diamond, because it's made up of crystallized carbon surrounded by a thin layer of hydrogen and helium gasses. It's believed that this is the final outcome for many stars, including our own Sun. In five billion years our Sun will become a white dwarf and two billion years after that the carbon should crystallize to form a gigantic diamond.

http://www.universetoday.com/am/publ...ond_space.html

best regards,

buck

[Jinx]

I wanted to add that the Greeks believed the fine, cloud of stars we now call the MIlky Way was, literally, milk spilled across the sky. I believe the term was Via Lacta, or something close to that...can't recall off-hand. But, hence we use that term today. Galileo is credited with turning his telescope on this mysterious "cloud" to say "My God! It's full of stars!"...much like Dave in 2001 Space Odyssey, But seriously, that's the straight dope. - Jinx