Billions of miles away, deep within a cool, dense cloud of seething dust, gas and molecules, a new babe is being born.

Completely hidden from human eyes, this infant will grow in size, power and brilliance until, some untold millions of years from now, it may burst forth full-blown from its cosmic cocoon as a new star.Incredibly, the conception, birth and early evolution of stars the basic components of our observable universe remain as mysterious today as they were nearly 400 years ago when the first telescopes were trained on the sky. The details of a star's formative years are unknown simply because the vast, dusty shrouds surrounding their birthplaces are impenetrable to optical telescopes.

Now, however, a new breed of telescopes sensitive to submillimeter radio waves promises to help solve the puzzle of stellar birth, as well as a host of other astronomical mysteries ranging from the energy source of quasars to the temperatures on other planets. (A millimeter is one-thousandth of a meter, or about four-hundredths of an inch. A meter is 3.3 feet.)

"The submillimeter portion of the spectrum provides the best means for probing the physical and chemical conditions in molecular clouds," says Dr. Thomas G. Phillips, director of the California Institute of Technology's new submillimeter telescope on Mauna Kea in Hawaii.

"Star formation is a key to processes occurring throughout the universe," Phillips notes. "And submillimeter instruments may finally permit us to observe how an interstellar cloud actually collapses to form a protostar."

Stars, both new and old, emit light in a virtual rainbow of colors, or wave-lengths, ranging from radio waves a few thousand feet long to X-ray and gamma-ray waves no longer than the diameter of an atom. Most of this radiation is invisible to the human eye. While modern astronomers using instruments both on the ground and in space now routinely observe the heavens across almost this entire electromagnetic spectrum, the submillimeter band a portion between radio and infrared waves remains largely unexplored.

"In a real sense," says Dr. Irwin Shapiro, director of the Smithsonian Astrophysical Observatory in Cambridge, Mass., "this is the last frontier of ground-based astronomy."

And it is a rich frontier. Those astronomical sources emitting most of their radiation in the submillimeter band are considered "cool objects" with average temperatures no more than a few score degrees above absolute zero (-459.7 degrees Fahrenheit). Such temperatures are found in the churning clouds of gas and dust where new stars are born, as well as in the disks of primitive material that may be forming planetary systems and other young stars.

In our own solar system, submillimeter observations will allow scientists to sample the cool atmospheres of the planets and the icy halos of comets without the need for space probes.

One reason the submillimeter band (wavelengths from about 0.1 millimeter to 1 millimeter) has been so little explored is that observations must be done from very dry sites, usually on high mountaintops, where water vapor in the Earth's atmosphere will not block out so much of this radiation from space.

More important, because the submillimeter wavelengths are so close in frequency to infrared (and visible) light, the large dishlike antennas used for observations need curved surfaces nearly as precise as the primary mirrors of optical telescopes.

Fortunately, recent technical advances offer both better reflectors for gathering submillimeter waves and more sensitive receivers for detecting, amplifying and recording the focused radiation.

The Caltech 10.4-diameter telescope in Hawaii, for example, is one of just a few to apply this new technology; the other include a 15-meter-diameter telescope built and operated by a British-Dutch-Canadian collaboration on Mauna Kea and a 10-meter telescope planned by the University of Arizona and Germany's Max Planck Institute for placement on Mount Graham in Arizona.

The most ambitious, and potentially most powerful, system is an array of six, movable, 6-meter-diameter telescopes proposed by the Smithsonian Astrophysical Observatory. Astronomers at the University of Massachusetts, Amherst, may also collaborate with SAO on this telescope array project.

Mounted on tracks each several hundred meters long, the telescopes would form an instrument now as an "interferometer" in which the separate instruments would work together to create the equivalent of a single huge telescope with a resolution some 30 times better than any of the individual telescopes. Indeed, its resolution, or ability to see fine details, could approach that of the best ground-based optical instruments.

"This kind of resolution is comparable to being able to read the headlines of a newspaper in New York City when you are standing in Boston," Shapiro says. "It would allow us to see galaxies with a new clarity and to trace their spiral arms as outlined by molecular clouds."

Another area where submillimeter astronomy could make major contributions is in the study of quasars, the brightest, most distant and perhaps most puzzling objects known. Twenty-five years after their discovery, no one can yet explain convincingly how an object with a size about that of our solar system can emit more energy than all the stars in the Milky Way combined.

"Since many quasars are spewing out tremendous amounts of submillimeter radiation," Shapiro notes, "the array may help us understand what kind of energy source could drive these powerhouses." This telescope array is still on the drawing board; however, the design of detectors and the search for the best mountain location are being pursued. If sufficient funds were to become available, construction could begin by 1990.

Appropriately, that year is also the 100th anniversary of the observatory's founding by Samuel Pierpont Langley. Langley was the third Secretary of the Smithsonian and the inventor of the bolometer, a forerunner of the modern detectors that will be used in the array.

Perhaps the most exciting prospects for submillimeter astronomy may be serendipitous surprises that cannot even be imagined now. Indeed, ever since Galileo's first crude optical tube revealed the moons of Jupiter, astronomical discovery has followed the introduction of new observing tools.

"Repeatedly, discoveries of novel cosmic phenomena have been made by men and women who looked at the universe with new instruments and came to see it from a fresh perspective," Dr. Martin Harwit, a former Cornell University astronomer, wrote in his history of astronomical research, "Cosmic Discovery."

"If we are to take seriously the lessons learned from the discoveries of the past three decades, urges Harwit, now director of the Smithsonian's National Air and Space Museum, "We must make a deliberate effort to . . . introduce radically new observational techniques, permitting a view of the universe through brand-new channels."

The newest channel submillimeter astronomy promises to open exciting, and maybe even unexpected, windows on the cosmos.