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BACKGROUND

Beyond the blue, beyond the violet, and into the ultraviolet part of the electromagnetic spectrum, the radiant universe appears strangely altered. At these shorter wavelengths and higher photon energies, the hottest stars shine brightly, while the cooler more common stars like the Sun virtually disappear from view. Clusters and associations of young stars are dominated by just a few massive ultraviolet-bright stars, while ancient globular clusters are reduced to their hottest, most rapidly evolving members. Spiral galaxies that look like graceful pinwheels in visible-light images are transformed into knotty webs --- each knot containing thousands of young hot stars. And the space between the stars, clusters, and galaxies appears profoundly black --- up to 100 times darker than the nighttime sky as seen at visible wavelengths.

Until the last decade, our view of the ultraviolet universe was impeded by the stratospheric ozone layer that protects life on Earth from the Sun's harmful UV radiation. Subsequent balloon, sounding rocket, and satellite flights have yielded fascinating glimpses of the ultraviolet universe. In 1990, the Space Shuttle Columbia flew the Spacelab/Astro-1 mission. Astro is the first and only true astronomical observatory to operate onboard the Space Shuttle. It consists of 3 ultraviolet telescopes which are attached to a single pointing system in the Shuttle bay. During the Astro-1 mission, these instruments obtained UV images and spectra of hundreds of astronomical objects. Learning resources relating to this mission include a new slide set and booklet, ``Beyond the Blue: Greatest Hits of the Ultraviolet Imaging Telescope,'' which is being distributed by the ASP through its mail-order catalogue. There is also a teacher's guide with activities, ``Astro-1: Seeing in a New Light'' which is available from the NASA Teacher Resource Laboratories. Sources of information on the upcoming flight of Spacelab/Astro-2 aboard the Endeavour (currently scheduled for February--March 1995) will be discussed.

All objects known to exist in our universe, from the nearest planet and most tenuous comet in our solar system to the farthest and most massive of the enigmatic quasars, which are probably the most distant and the oldest extragalactic objects we have observed to date, can be studied effectively in the ultraviolet region of the spectrum.

Ultraviolet astronomy also provides interesting information about the interstellar medium --the space between the stars. Although one tends to think of this space as a sort of vacuum, it contains the material from which all stars are formed.

Part of the EIT Telescope

Observations at ultraviolet wavelengths have shown that the very low-density material found in the interstellar medium is made in fact of rather similar composition throughout the galaxy, but that its distribution is far from homogenous. In fact, big gas bubbles have been blown into it by very strong solar winds emanating from the hot stars to be found in groups of recently formed stars.

In the same way as our sun is losing matter from its atmosphere through the solar wind, the atmosphere of younger and more massive stars is blown away by similar solar winds.

Knowledge of these winds is very important in developing our ideas of stellar evolution. Observations in the ultraviolet show that material from some stars are blown off in irregular bursts and not in a steady flow as was first imagined. Observations of the shift of spectral lines relative to the lines emitted by atoms at rest permit astronomers to determine the speed at which atoms are arriving or receding. This technique has been used to show that in some cases matter is first blown away from a star and later some of the matter returns to the star's surface. This outward and return flow is very similar to that seen in solar flares on our own sun.

It has been found that the ultraviolet wavelength region is the most appropriate for studying many phenomena occurring in distant galaxies. Galaxies are known to be made up of millions of stars. However, some galaxies harbor in their nucleus objects that emit more energy than all the stars in the rest of the galaxy together. Very little is known about those objects at present. Observations in the ultraviolet wavelength indicate that such objects are rather small and the fact that they generate such enormous quantities of energy puzzles astronomers. These objects are some of the strangest phenomena observed to date in the universe and probably are closely related to the quasars, which are considered to represent the light beacons for the very early stages of the formation of the universe.