COBE Searches For Fossil Starlight

FOR astronomers trying to learn about the origin of our universe, the hunt is on for fossil starlight. That is radiation left by a race of stars that may have shone and faded before even the most ancient of presently observable galaxies appeared.

The Cosmic Background Radiation Explorer (COBE) satellite that the National Aeronautics and Space Administration orbited in November will be looking for this fossil. No one has yet reported an unambiguous trace of it. But David Meyer and colleagues at Northwestern University and the University of California, Berkeley, have found a strong hint that this ancient radiation may, indeed, be out there awaiting discovery.

If found, it would help illumine what astronomers call the ``dark ages'' of cosmic history. That is the billion-year period between the formation of the first atoms and the appearance of stars and galaxies as we know them.

The best record we have of the early universe is the primordial microwave radiation that now permeates the cosmos. This is the cosmic background radiation from which the COBE satellite takes its name.

Astronomers believe this radiation is left over from the the so-called Big Bang explosion of primordial energy that created our universe 10 billion to 20 billion years ago.

This expanding mass was so hot at first that particles of matter and photons (particles of radiation) freely exchanged energy. Within 300,000 to 500,000 years, the expanding fireball cooled to 3,000 Kelvin (4,940 degrees F). Photons and matter particles no longer interacted. Atoms could form and the radiation could evolve on its own. This is the background radiation astronomers now detect as pervasive microwave noise. It has cooled to a frigid 2.75 or so degrees above absolute zero (minus 455 degrees F).

The radiation is highly uniform all over the sky, reflecting its condition when it parted company with matter particles. However, there may be small variations within it that would reflect the influence of early clumpings of matter as galaxies began to form. Also, it may be slightly enhanced at wavelengths in the far infrared - that is, wavelengths a little shorter than microwave.

Such infrared enhancement could be the fossil starlight astronomers seek. If it does exist, it may well have been produced by stars that arose during the dark ages. They would have left behind dust heated by their energy - dust that now reradiates that energy in the far infrared.

Toshio Matsumoto, working with colleagues at Nagoya University in Japan and the University of California at Berkeley, thought they saw hints of such fossil starlight in rocket measurements they made of the background radiation several years ago. They reported a slight enhancement of that radiation at a wavelength of 1.16 millimeters.

Now Mr. Meyer and his associates have confirmed that finding. They measured radiation from a form of cyanide consisting of one carbon atom and one nitrogen atom in interstellar dust. This emission is excited by the background radiation. Hence it is an indirect measure of the temperature of that radiation. The study confirms that there is a blip in the background radiation just where theorists expect it to be if a pre-galactic group of stars had existed.

Now it is up to COBE to hunt for the fossil starlight. The $150 million satellite went into a near polar orbit some 559 miles high on Nov. 18. It is equipped to survey the background radiation over the entire sky for a year. Besides sensors to pick up the microwave radiation, it carries infrared detectors that can seek out any signal a dark-age star population may have left behind.