Are there many more astronomical discoveries out there?

NEWS from the astronomical frontier tends to run to extremes. Recent reports include finding what may be the greatest single mass yet observed in the universe -- a concentration that's a millon, billion times the mass of the sun. This prompted one reader to ask if the English language would run out of superlatives before astronomers run out of discoveries. According to Martin Harwit of Cornell University and Roger Hildebrand of the University of Chicago, however, the lexicographers are safe. They estimate that ``the number of [astronomical] phenomena already discovered represents an appreciable fraction of those that can be found.'' By ``appreciable fraction'' they mean something like a tenth to a third of all discoverable astronomical phenomena.

That may sound like the hubris of late 19th-century physicists who considered they had their subject all wrapped up just before they discovered the inner world of the atom. But Harwit and Hildebrand point out that fields of exploration do sometimes mature. Reporting their assessment recently in the journal Nature, they explain:

``It is generally assumed that the potential scope of any scientific discipline is quite incalculable. Yet the discovery of the Earth's major rivers, mountains and seas, an enterprise that may have seemed endless to Ptolemy's contemporaries, is now essentially complete.''

They add that, ``barring the possibility of interstellar or intergalactic space travel, there is a limited range of events which astronomers can observe.''

These limits are inherent in physical law and the structure of the cosmos. Whether astronomers view the cosmos through infrared (heat) radiation, visible light, or high energy gamma rays, the distance of the farthest object they can observe is limited by the age of the universe. Radiation from anything more distant wouldn't have had time to reach us yet. Also, interstellar and intergalactic dust can obscure objects. Moreover, the range of electromagnetic radiation, with which astronomers can study the universe, is strictly limited. Whenever a new radiation range -- such as radio waves or X-rays -- has become available to astronomers, a new observation window has opened to reveal unsuspected phenomena. But virtually all of those windows now are at least partially open.

Taking these and other limitations into account, Harwit has compiled a list of 43 major phenomena. They range over such things as stars and planets, galaxies, and unidentified radio sources. There are statistical techniques that can be applied to such a list to infer how many major types of phenomena remain to be discovered. To use a statistician's terms, these techniques are used to estimate a total population from suitable samples. In this case, it is the population of discoverable types of astronomical phenomena that would be involved.

Harwit and Hildebrand have done this with Harwit's list. First, they tested the list itself to see if it is a reasonable sample of discoverable astronomical phenomena. It passed that test. Then they estimated how far astronomers have already come in their explorational quest. As astronomers refine their techniques, as they move their instruments out into space, there are still many wonders for them to find. But that list is not endless. There will very likely come a time when, as in the exploration of Earth, discovery of a truly new type of phenomenon will be rare.

But again, as with Earth, that will be only the beginning of a larger scientific challenge. The search for new rivers, mountains, or seas has given way to a larger search to understand Earth as a whole. Biologists and geophysicists are beginning to join in integrated studies to learn how Earth and its living systems operate and interact as an interdependent unit. They are just beginning to search for the answer to how that unit fits into the larger context of the solar system. This search is expected to be one of the major scientific enterprises of the coming century.

Astronomers will face an even more daunting quest. The more they know of the phenomena of the universe, the more they will be challenged to understand the universe as a whole -- how it functions, whence it came, how it evolves, and what our role is in it. No one can foresee the end of that exploration.

A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.