Dwarf Stars: Back to the Drawing Board

Fortunately for scientists, the universe is a delightfully complex place. Every once in awhile, an explanation arises that seems to explain rather neatly some facet of the universe. If the explanation were really adequate then that facet would no longer be of much interest to the scientists. However, in almost every case something arises that shows the explanation to be wrong, or, at best, incomplete. Joyfully, scientists then get back to work.

That sort of thing has recently happened in connection with white dwarf stars.

Occasionally, an ordinary star that is unusually massive can explode, causing part of the star to collapse into a tiny object. Such a collapse can produce any of three kinds of objects: a white dwarf star, a neutron star or a black hole.

White dwarf stars are the largest of these three types. They are therefore the most easily observed and have been known for three-quarters of a century. A white dwarf star has the mass of an ordinary star but has collapsed to the size of a planet such as Earth, or smaller.

Because white dwarf stars are so small, they tend to be dim and can only be seen if they are in our own neighborhood of the galaxy. Still, there are so many of them that they can be studied in considerable numbers, and certain details can be made out.

The light we get from white dwarf stars comes from the outermost surface. This light can be spread out into various wavelengths and from the nature of those wavelengths, conclusions may be reached as to the chemical nature of the surface of the white dwarf star.

It turns out that about 80% of the white dwarfs studied have surfaces made up of hydrogen--pure hydrogen. Apparently, in such stars, not more than 1 part in 100,000 is anything other than hydrogen.

This is perhaps not surprising. Ordinary stars like our sun are made up almost entirely of hydrogen and helium, the two simplest atoms (hydrogen is the simplest, helium the second simplest.) Other stars are undoubtedly the same. As a star ages, it uses up its hydrogen, converting it to helium. By the time a star explodes and becomes a white dwarf, almost all its hydrogen would probably be gone.

However, a white dwarf’s gravitational field contracts just as the star does and becomes much more intense. The gravitational field of a white dwarf star is about half a million times as intense as Earth’s. All the matter in a white dwarf star is held together very firmly, and the lightest matter is bound to float on top. The lightest matter is hydrogen, and it therefore makes up the surface of the white dwarf star. Everything else is below the surface.

In scientific notation, such hydrogen-surface white dwarfs are called “DA stars” (for dwarf star, type A). However, there are the remaining 20% of the white dwarfs that contain no hydrogen in their surface, only helium. They are called “DB stars.” These stars too might still be explainable. If all the hydrogen is used up by the time the white dwarf star is formed, then helium is the next lightest material, and that’s what floats on the surface.

However, astronomers are not entirely happy with the thought that some white dwarfs form with a little hydrogen at their surfaces, and some with none. They feel it would be neater if all white dwarfs formed in the same way and became different afterward.

But how could they become different? Well, the space between the stars contains thin wisps of gas, mostly hydrogen. Perhaps all white dwarfs start with helium, but some, located where interstellar matter is a bit denser, attract hydrogen from the wisps, gradually building up a layer of hydrogen over the original helium surface.

Sure enough, there are a few white dwarf stars that are “DBA stars.” They have a helium surface, but one that contains hydrogen as well, to the extent of about 1 part in 10,000. These stars must have begun to pick up hydrogen from interstellar space, but have not yet completed the task. In fact, one white dwarf star recently studied is a DBA star that contains not only hydrogen in addition to helium in its surface layer, but even a tiny quantity of calcium. Calcium also exists in interstellar matter in amounts just large enough to account for its presence in that star. So the matter seemed settled.

In a very few cases, however, there are twin white dwarf stars; two of them close enough together to circle each other. A case of this sort was studied earlier this year by astronomers at the Florida Institute of Technology and it turned out that one twin was DA, with a pure hydrogen surface, and the other DB, with a pure helium surface.

The question is, if one of them picked up enough hydrogen from interstellar space to become DA, why didn’t the other twin do the same? They are passing through the same regions of space, after all. It is possible that if the twins are of different mass, the larger one might be able to attract the hydrogen from the smaller one and hog it all--but the stars are a bit too far apart for that to be likely.

There must be an explanation, of course, and scientists are back at the drawing board, looking for one.