Why do stars shine? Stars radiate energy because conditions near their centers are hot and dense enough to allow nuclear fusion to take place. This is the process whereby lighter elements are fused together into heavier elements, releasing energy as a by-product. This energy generation provides the pressure necessary to support the core of the star against the inward pull of gravity. Most stars are in the process of fusing hydrogen (the most abundant element) into helium (the second most abundant element). However, eventually most stars will develop an energy crisis, where the hydrogen near their cores becomes depleted. For very massive stars this can occur in "only" 10 million years or so, while for stars like the sun this so-called "hydrogen burning" phase is more like 10 billion years.
What happens next depends on the mass of the star, but the general scenario goes something like this: with the loss of energy input, the core of the star contracts and heats up. In so doing, the outer layers of the star actually expand outward and cool, and the star becomes what is called a red giant. When the core becomes hot enough, helium atoms can fuse into heavier elements such as carbon and oxygen, which again supplies the star with a stable, albeit relatively short-lived, energy source. In massive stars, this scenario repeats itself several more times, with the "ashes" of one burning cycle becoming the "fuel" for the next, shorter- lived cycle, generating heavier and heavier elements. This process of building heavier and heavier elements comes to an end with the creation of iron, because iron is an extremely stable atom. To fuse iron into still more massive elements actually requires energy input, rather than generating energy. Hence, the iron ashes just build up in the core of the star.
In the mean time, the outer layers of the star become so bloated and extended that they can actually be driven off into an expanding shell called a planetary nebula, leaving behind the hot, very dense core as a compact star called a white dwarf. This is basically the end of the road for many stars, as the white dwarf slowly cools and radiates its energy away into space. It is thought that our sun will eventually meet its demise in this manner, some 5 billion years hence.
In a few cases, however, something else happens. If the star is a member of a binary star system, the orbits of the two stars readjust themselves into a very close configuration whereby the "companion" star can dump material over onto the white dwarf. Now white dwarf stars have some peculiar properties, one of which is that the more massive a white dwarf is the smaller its diameter. Hence, dumping more matter onto such a star makes it denser and denser until it simply cannot hold itself up against the pull of gravity. In the ensuing collapse, the temperature becomes exceedingly hot and runaway thermonuclear reactions tear the star apart in a cataclysm known as a Type Ia supernova.
One of the main predictions of the theory of these supernovas is that a tremendous amount of iron (about half the mass of our sun!) should be present in the expanding remnant of the exploded star. And yet when we look at the young remnants of Type Ia supernovas using X-ray and optical data, we see little or no evidence of iron in the ejecta (the rapidly expanding material that was part of the star before it exploded). Hence, to paraphrase Clara Peller, the question has always been, "Where's the iron?"
The one place where the iron could be hiding is...right in the center of the supernova remnant! If the iron has yet to interact with the blast wave from the supernova explosion, it could remain relatively cool, and and hence go undetected. How could one find evidence of this dark, cold iron?
Nature has apparently provided us with one and only one case where this search can be made. A supernova occurred in our Galaxy in 1006 A.D. that was carefully recorded by Chinese astronomers, so carefully that we can tell from the way it's light varied that it was a Type Ia supernova. In addition, in 1978 astronomers discovered a faint but very hot blue star seen in projection near the center of the remnant. Originally thought to be associated with the supernova remnant, we now know that this star is behind the remnant from our point of view. (That is, it's light must shine through the supernova remnant on it's way to us.)
Ultraviolet spectral observations of this star with the International Ultraviolet Explorer satellite and with the Hubble Space Telescope have shown a number of absorption lines in the spectrum that are due to the material in the supernova remnant. Some of these lines are due to cold iron (iron that has had just one electron removed)! However, calculations of the amount of iron involved indicate that only a small fraction of the missing iron is present in this ionization stage. Most of the iron is expected to be in a slightly higher ionization level where two electrons have been removed, but the strong absorptions from this ionization stage are NOT accessible to Hubble or IUE. However, the strongest resonance absorption line of twice-ionized iron is accessible to HUT, at 1123 angstroms in the far ultraviolet!
This single, crucial measurement, which can only be made with a sensitive far-ultraviolet spectrograph like the one on HUT, will go a long way toward confirming or refuting our ideas about the late stages of stellar evolution and supernova explosions. The observation could not be made during the Astro-1 mission in December 1990 because this supernova remnant was too close to the direction of the sun. For an Astro-2 launch in March, the remnant will now be accessible, although largely in the daylit portion of the orbit. Still, the iron absorption line should be so broad and deep that it should be detectable anyway, assuming the iron is really there!
William P. Blair