It may be a rare experience for city-dwellers, but a clear, moonless night in the country offers an astonishing sight: an amazing dark-velvet blanket studded by glistening stars, across which meanders the glowing road of the Milky Way.

Modern astronomers know that when we look at the Milky Way we are in fact seeing the edge of a galaxy, a pinwheel-shaped collection of more than 100 billion stars, among them an average one that we call our own Sun about two-thirds of the way out one of the galaxy's arms. Although other galaxies were visible for centuries to even simple telescopes as fuzzy smudges of light, it wasn't until the 1920s that astronomers reached a consensus that they too were groupings of enormous numbers of stars. This was thanks in part to the American astronomer Edwin Hubble, who observed pulsating stars in a blurry patch in the constellation of Sagittarius and concluded that it was far too distant to be part of our galaxy, and must therefore be one of its own.

What we know about galaxies
The past few decades have also been marked by remarkable advances in our understanding of how the universe formed. Astronomers now generally agree on a model called the Big Bang, which holds that all of the matter and energy in the universe was once contained in a compressed state of unimaginably high density and temperature that abruptly expanded in an explosion-like event 13.7 billion years ago. Within a million years the young universe cooled enough for atoms to form, leaving clouds of simple atoms of hydrogen. Perhaps 1 billion to 2 billion years after the Big Bang, much of this gas condensed into swirling galaxies, and then into separate stars.

Astronomers today estimate that there are at least 10 billion galaxies in the visible universe. Most galaxies are arranged in clusters. Our own Milky Way belongs to a small cluster of about 40 galaxies, called the Local Group. The group is about 3 million lightyears across, with most of its contents in two large spiral galaxies, our Milky Way and the neighboring Andromeda galaxy. Our Local Group is part of the Local Supercluster, a thin sheet of galaxy clusters. In between the various superclusters in the universe are voids, regions with few galaxies. Most of the universe consists of these voids.

Not all galaxies gather into groups or clusters, however; some appear like hermits far from neighbors. The "urban-dwelling" galaxies seem to have different properties than the loners. Some galaxies appear to form huge quantities of stars in a rapid flash of activity early in their history, while other galaxies display a slower, but gradual star formation.

Scientists have classified galaxies according to their shape, although they don't know why they form different shapes in the first place. Spiral galaxies, like our Milky Way, form stars gradually. They contain young, middle-aged and old stars, and large amounts of gas and dust. Elliptical galaxies are football-shaped and have little visible gas and dust; they appear to contain mostly old stars that formed fast and early. Irregular galaxies, with no regular shape or motion, have bursts of star formation. Peculiar galaxies are odd in shape and form, and are usually in the process of forming stars.

Astronomers also wonder how galaxies manage to churn tenuous cosmic gases and dust to the point that the matter collapses into the dense, fiery material that makes up stars. They know that a large portion of a galaxy is made up of seemingly invisible "dark matter." They don't know what dark matter is, how it got there, and what role it plays in the evolution of galaxies.

While scientists have been trying to decipher the mysteries of galaxies, these cosmic entities have even seeped into popular culture. Movie fans may have munched on a Milky Way candy bar while watching Star Wars, a story that takes place a "long time ago in a galaxy far, far away."

Although astronomers have learned a great deal about galaxies since the 1920s, there are still many mysteries waiting to be solved. NASA's Galaxy Evolution Explorer is an important part of that quest.

In search of stellar nurseries
The mission searches for galaxies in which young stars are forming by taking advantage of a peculiarity in the kind of energy that stars put out. A mid-sized, middle-aged star like our Sun throws off energy across a large spectrum of wavelengths, from infrared to visible light to ultraviolet. However, ultraviolet makes up less than 5 percent of the energy given off by the Sun.

Very massive stars, on the other hand, throw off an enormous amount of ultraviolet energy. Fast-trackers that live on the edge and burn the candle at both ends, these stars shine brightly and die early. Since they never get to be middle-aged like our Sun, any galaxy with a lot of these ultraviolet-bright stars must be one in which new stars are vigorously forming.

The telescope on the Galaxy Evolution Explorer thus peers out into the universe at ultraviolet wavelengths to look for these stellar nurseries. As it conducts its sky surveys, the telescope will observe millions of galaxies. Why so many?

Let's say an alien from some other planet wanted to learn how people on Earth age, but didn't want to spend 75 years watching a group of babies grow up and grow old. Instead, the alien could study 100 one-year-old babies, 100 two-year-olds, 100 50- year-olds, 100 75-year-olds, etc. The alien would not be able to write a specific biography of any one person, but it would learn an awful lot about the life of an "average" human. It would learn at what ages children grow fastest, how much time people spend eating and sleeping, and other characteristics that would help the alien describe the life of an average human. In the same way, the Galaxy Evolution Explorer will study many galaxies to learn about the life of an average one.

The telescope will examine galaxies both near and far. The farther we look out into space, the farther back in time we are seeing. The most distant galaxies that the Galaxy Evolution Explorer will see are about 10 billion light-years from Earth. Since the universe is thought to be 13.7 billion old, the mission will catalog galaxies across 80 percent of the history of the universe.

The mission will enable scientists to reconstruct the history of our Milky Way galaxy by studying similar galaxies. It will help answer questions about how the Milky Way began and how our star, the Sun, formed within it, an event that paved the way for the eventual development of our solar system, Earth and life.

Another topic of study for the mission is the history of how heavy elements formed in the universe. The original atoms created relatively soon after the Big Bang were simple ones of hydrogen and helium, each containing just one or two pairs of protons and electrons. "Heavy" elements, on the other hand, are the other hundred or so elements on the periodic tables that hang on the walls of science classrooms across the country -- in other words, any element that's heavier than hydrogen and helium. All of these other elements formed as part of nuclear fusion deep inside stars that later died. Elements that make up our bodies, such as carbon and oxygen, were all created in the forge of long-dead stars. We are literally made of stardust.

By using the mission to study other galaxies, scientists will determine which elements formed at various points in cosmic time. They will then apply this information to our Milky Way galaxy to pinpoint which elements existed when our solar system formed.

Blazing new trails in astronomy
The mission will provide the first-ever wide-area ultraviolet surveys of the sky, and the first wide-area spectroscopic surveys. Not only will it help us understand the cosmic events that shaped the history of our universe, but through its discoveries, the Galaxy Evolution Explorer will help design the future of astronomy.

The Hubble Space Telescope was named after the man who first discovered galaxies. The telescope that bears his name has imaged numerous galaxies, both individually and in such elegant groupings as the Hubble deep-field images, which show a tiny portion of the sky peppered with galaxies. The Galaxy Evolution Explorer, by observing large pieces of the sky all at once, will find the most rare and interesting objects in the universe. These objects may become the target of future observations by the Hubble Space Telescope.

When massive stars emit ultraviolet light, much of it is absorbed by dust. The dust then emits heat in infrared wavelengths, which, like ultraviolet, are not visible to the naked eye. The ultraviolet observations of the Galaxy Evolution Explorer will work hand in hand with those of future infrared missions, such as NASA's Space Infrared Telescope Facility, scheduled for launch in 2003, and the James Webb Space Telescope, planned for later this decade. Both of those infrared missions will observe some of the galaxies studied in ultraviolet light by the Galaxy Evolution Explorer. This will give a more complete picture of each galaxy. The mission's legacy will also benefit ground-based observations.

In addition to studying galaxies, the mission will compile a substantial archive of other objects of interest to astronomers. These include active galactic nuclei, often associated with massive black holes at the centers of galaxies; white dwarfs, old stars that have blown off their outer shells, leaving very hot cores that are bright in the ultraviolet; and quasars, thought to be associated with black holes and galactic nuclei.