Electromagnetic waves propagate in all directions from a source, like ripples from a pebble dropped in the middle of a serene pond. After traveling for millions of years the waves reach the earth, but the atmosphere and ionosphere absorb all of the energy in the electromagnetic spectrum, except for light and radio waves. Before 1950 all that was known about the cosmos was limited to light detected by optical telescopes. With the development of radio receivers after World War 11 and the launching of scientific instruments into earth-orbit, astronomers began to study the non-visible waves from the sky. They discovered many new kinds of objects in the universe that were previously unknown.

These first arrays showed that most of the radio emission from radio galaxies and quasars was emitted many thousands of light years outside of their parent galaxies, from regions which contained no stars and emitted no visible. What was producing the enormous quantities of radio waves in these dark regions outside the galaxies? To answer these questions, a large array had to be constructed.

The 28th, and last telescope, was constructed in 1979, and the fully completed VLA was dedicated on October 10, 1980. Even during the ceremony the VLA was produc-ing radio images of unparalleled quality and in record time. When the VLA was approved in 1972 its estimated cost was 76 million dollars. Even though it was built during an era of double-digit inflation and its performance has exceeded specifications in every way, the VLA was completed essentially within its original budget at a total cost of 78.6 million dollars.

From afar our galaxy looks like a flat pancake with prominent spiral arms. It contains about 100 billion stars, glowing hydrogen gas, and small, dense cold clouds which con-tain simple molecules and grains of dust. Most of the stars are similar to the sun but are too distant to be detected by existing radio telescopes. Strong radio signals in our galaxy do come from interacting binary star systems, huge clouds of gas, stars in their infancy or stars that explosively complete their evolution. Near the center of our galaxy there is something which emits a large amount of radio energy. Its nature is still unknown; it may be a black hole or a compact cluster of a million stars.

   

On a clear winter's night, the Orion Nebula can be seen just below Orion's belt. The red light is produced by a hydrogen cloud which is heated by four young stars within it. The cloud is shrinking under its own gravity, and it will continue to form stars over the next million years. Filaments and loops at the edge of the Nebula are caused by shock waves moving through the gas. These shocks are produced by strong winds from embryonic stars.

The radio photograph of the Nebula has a similar form to that of the optical photograph, because hot hydrogen gives off both light and radio waves. The colors represent the intensity of radio emission from red (brightest) to yellow, green, blue and black. From a comparison of the two images, astronomers can estimate the temperature in the cloud, its thickness, density, and dust content.

Astronomers believe that supernovae are associated with the violent death of very massive stars. When thermonuclear reactions in the center of these stars exhaust their fuel (mostly hydrogen and helium), stellar material can no longer support itself, and within a matter of minutes, the star collapses under its own gravity. This infall heats up the interior to 100 million degrees and heavy elements are quickly produced in nuclear reactions. As a result, shock waves are created which move outwards from the interior expelling most of the material into space, contaminating the cosmic environment. This material is rich in heavy elements and metals (e.g. oxygen, sulfur, silicon, and iron) which will be eventually recycled into new stars. All of the material in the universe, except hydrogen and helium, was created billions of years ago in supernovae.

   

The brightest radio source in the sky, Cassiopeia A, was produced by ejected material of a supernova which occurred in 1680 and which has now expanded to 12 light years in diameter. New clues to understanding the cataclysmic death of massive stars are captured in the supercomputer-processed radio image. A powerful explosion blew off the star's outer layers and as the material moves outward in an expanding shell at 4000 km/s, it emits radio energy when it collides with clumpy interstellar material. The regions where the most energetic collisions are occurring also emit light as well as radio waves. The optical photograph contains many faint stars, which are in the direction of the supernova; none of them are a relic of the catastrophe.

The NRAO is in process of building a dedicated VLBA of ten telescopes, exclusively for the high resolution imaging of radio sources. The center for operations of the VLBA will be in Socorro, NM and its operation will be combined with that of the VLA in the Array Operations Center, recently built on the campus of the New Mexico Institute of Mining and Technology. Although it would be possible to use satellite or other continental networking systems to transport the data from each telescope to a central location for processing, the data rate will be so large that 20,000 continuous inter-continental telephone lines would be required. Instead, the radio signals will be recorded on videotape, similar to that used to record TV programs, then shipped to the NRAO in Socorro, where the tapes will be played back to produce, ultimately, a radio image of unprecedented resolution and detail.