This Astronomer Was Right All Along
It often happens that an important observation or theory by a great scientist of decades or centuries past has to be extended or modified. But every once in a while, it turns out that the extension must be abandoned and we find that the original scientist was right after all.
Such a case of back-to-where-we-were has just taken place this year.
It began with the British astronomer Arthur S. Eddington. In the 1920s, he asked the question: Why doesn’t the enormous gravitational pull of a star like the sun simply force it to collapse into a tiny ball of crushed atoms?
The answer seemed to be that the internal heat of the sun kept it expanded against the pull of gravity. Eddington set about working out the balance between gravitational pull and internal heat and deduced that the sun’s core had to be at a temperature of millions of degrees. This was important in explaining the nature of nuclear reactions within the sun and how it and other stars obtained the energy necessary to keep them shining for billions of years.
Eddington found that the more massive a star, the more intense its gravitational pull inward, and the higher the temperature at the core had to be to balance that pull. By the time the star was somewhere between 60 and 100 times the mass of the sun, it would no longer be possible to maintain a balance. To keep the star from collapsing, the internal temperature would have to be so high that the star would actually explode.
Eddington therefore concluded that stars with a mass of considerably more than 60 times that of the sun could not exist. And for over half a century, indeed, there was no reason to think he was wrong. Stars with greater mass than that were not found.
But then, in the 1980s, stars were discovered that seemed to have a mass of several hundred times that of the sun, even more than 1,000 times the mass of the sun. How were such “superstars” possible? Eddington’s work had to be gone over and modified to account for these huge stars. (About three years ago, in fact, I wrote an essay about these superstars and how they were changing our views of stellar physics.)
But then the superstars fell apart--almost literally.
For instance, there is a star in the Large Magellanic Cloud called Sanduleak. Its distance was known to be about 160,000 light years, and it was so bright at that distance that it had to have a mass of at least 120 times that of the sun to produce all that light.
It was, however, observed and photographed with newer and better telescopes early in 1988. The image of the star was then analyzed with updated techniques to see how the brightness varied from point to point. It turned out that the star was not evenly bright and was therefore not a single star. It was, actually, a very closely spaced cluster of at least six stars. At the great distance of Sanduleak, this cluster seemed to melt together into a single star as seen under ordinary telescopic conditions.
Other such very bright and therefore very massive stars have also, by this technique, been resolved into tight groups of stars--and no one star in any of these groups seems to have a mass more than 60 times that of the sun. In other words, Eddington was right all along and the superstars have vanished from the sky.
Does this have any importance aside from the fact that it allows Eddington to rest peacefully in his grave?
As a matter of fact, it does. For one thing, it once again demonstrates that scientists must constantly be probing and testing their conclusions, and that their findings can be subject to change.
In this case, the confirmation of Eddington’s theories had relevance beyond the existence or non-existence of superstars. The discovery of star clusters, in turn, caused scientists to re-evaluate their estimates of the distances of galaxies from Earth.
It is important for astronomers to estimate the distance of faint galaxies in order to get a general notion of the overall size of the universe. To do so they try different techniques, working out the distance of the nearer galaxies, using that distance to work out the distance of farther ones and so on.
One technique has been to study those galaxies that were so near that individual stars could be made out within them, but so far away that the only stars that could be seen were the ones that were the very brightest in the galaxy. It was assumed that these “very brightest” stars in these distant galaxies delivered as much light as the very brightest star in our own galaxy. We knew just how far away and how bright very bright stars were in our own galaxy, so it was possible to calculate how far away distant galaxies were by working out how far they would have to be for their brightest stars to seem no brighter than they were.
But it may be that we have been deluding ourselves. It may be that while the brightest stars in our galaxy are seen clearly enough for us to be sure they are single stars, the brightest stars in distant galaxies are actually clusters that, taken together, shine much more brightly than single stars can.
If this is so, then some distant galaxies may be two or three times farther away than we at had thought, far enough so that a cluster of stars shines at about the same brightness as a single one would if it were closer. In that case, the universe is much larger than we thought and much older, and that would send astronomers back to their drawing boards.
