Journey through the universe: a gallery of observations.

Like many places since Sept. 11, the Smithsonian Institution's National Air and Space Museum in Washington, D.C., feels like a different world. Security guards wearing latex gloves use batons to comb through handbags, searching for suspicious packages or powdery substances.

In one of the first-floor exhibition halls, it's also a different world, but an uplifting one. An ambitious display has replaced a long-time exhibit about the nature of stars.

"I was never really thrilled with it," says curator David DeVorkin. "It was OK. It told why we go into space, but it was outdated quickly. And it wasn't inspirational."

In developing a new astronomy exhibit for the 4,600-square-foot gallery, DeVorkin hit upon a unifying theme: the instruments built to observe the cosmos. With the Smithsonian's vast collection of astronomical equipment, both old and new, "we can show the real thing," says DeVorkin.

"Unlike most museums that are dedicated to the process of science, we're interested in the products, the instruments that form the legacy of the discoveries," he adds.

Although it was a theorist, Copernicus, who made the revolutionary claim in the early 1500s that the sun, not Earth, was at the center of the solar system, the idea didn't catch on for decades, notes DeVorkin. Only late in the 16th century, when the Danish astronomer Tycho Brahe developed planet-tracking tools and made the critical observations, was Copernicus proved right.

"When you build new tools of perception, you discover new universes," notes DeVorkin. "It's always been that way, it's always going to be that way. And it also means that there's no definitive answers [to the nature of the universe].

"So, what I hope is that people who come through, rather than accumulating book knowledge by memorizing tables and facts, will get an impression of what it was like to observe the heavens [at a particular moment in history], what the problems were facing the observers, and how [they went] from asking one set of questions about the universe to the next."

Here's a sampling of the exhibit, which opened in September and will remain permanently at the Air and Space Museum:

Looking with the naked eye

Before 1609, when Galileo began using a brand new invention called the telescope, humankind's perception of the cosmos was limited to what could be seen with the naked eye. It was natural to perceive Earth as the center of the universe, with a transparent, starry sphere rotating around it.

The earliest astronomers used several tools to chart the position of objects in the sky and to predict where the sun, moon, and certain stars would move. With the heavens serving as both timekeeper and navigational aid, such knowledge was of much more than scholarly interest.

One measuring device, the astrolabe, had two parts. Its back contained a moveable sighting arm and a scale for measuring altitude, while the front had a flattened map of the heavens that helped to calculate the future position of objects.

An astrolabe featured in the exhibit dates from A.D. 1090 and has several interchangeable plates, each engraved with the celestial coordinates for a different latitude. Pointers on the top plate indicate the locations of 22 bright stars. Like a modern star-finder chart, the top plate rotates to show where these stars would lie at different times of the year.

With this device, astronomers and others could predict when the sun and certain bright stars would rise or set on any given day.

As measuring devices became more and more precise, old notions about the universe began to crumble. For example, Brahe's measurements--even though they were made with the naked eye--were fine enough to reveal that comets move through the same region of space as the planets. That destroyed the idea that planets occupied a special place that no other object could penetrate.

Telescopic view

Soon after Brahe's death, Galileo introduced the telescope to astronomy, leaving behind naked-eye observations. In time, investigators built bigger and better telescopes. A 20-foot telescope and mirror, dating from 1783, marked the birth of modern cosmology, says DeVorkin. "It's the most significant single piece of astronomical equipment we have in here that's complete," he notes.

Built by English astronomer William Herschel, the telescope enabled him to peer deeper into space than anyone had before. Using the data to develop theories about the size and structure of the universe, Hershel was the first observational cosmologist.

"At the time, people were only looking at the dynamics of the solar system, but Herschel had another agenda," says DeVorkin. "He wanted to know what the rest of the universe was like."

Herschel's telescope consisted of a single mirror, mounted at the bottom of the instrument's long, wooden tube, that bounced light up to an eyepiece on the opposite end. He peered into the eyepiece from a platform that could be raised or lowered. "It almost looked like he was speaking into a trashcan," says DeVorkin.

With the telescope sitting just outside his house near Windsor, Herschel would determine the positions of stars and call out the data to his sister Caroline, who sat by an open window and diligently recorded them.

During Herschel's time, astronomers considered the Milky Way to be just a band of light--formed by faint stars and dust--that stretched across the sky. Observing as much of the heavens as he could see from England, Herschel produced a map of the Milky Way.

Carefully tracking stars for months to years, he discovered double stars--two stars that orbit each other, locked in a gravitational embrace. The finding demonstrated that the pairs of stars move in accordance with Newton's law of gravitation, "extending [the law] farther into space than ever before," notes DeVorkin.

Mysterious smudges of light, known as nebulas, held a particular fascination for Herschel. He wondered if they were part of the Milky Way band, perhaps solar systems in the making, or objects far beyond the Milky Way band. Before Herschel's work, only about 100 of these fuzzy patches of light were known; he found another 2,400. He guessed the nebulas might be clouds of material that would condense to form stars or were themselves clumps of stars.

In 1833, a few years after William Herschel died, his son John took the telescope to South Africa. There he completed his father's work by mapping the distribution of stars and nebula over the Southern Hemisphere.

But it would take several decades, and a more powerful telescope, to determine that the nebulas are spiral shaped, and more than a century to figure out that they reside far beyond the Milky Way.

The expanding universe

Seated in the observing cage of the 100-inch Hooker telescope at Mount Wilson Observatory in Pasadena, Calif., Edwin Hubble was the first to realize just how expansive the universe is.

In 1923, while observing the Andromeda nebula, he found that it contained a Cepheid variable, a star that regularly varies in brightness. Hubble and his colleagues were elated by the finding because they knew it would enable them to accurately measure the distance to Andromeda.

How so? Earlier in the century, another American astronomer, Henrietta Swan Leavitt of the Harvard College Observatory, had shown that the brighter the Cepheid, the longer its period of pulsation. By measuring the duration of its pulse cycle, researchers could determine the star's true brightness, which is akin to determining the wattage of a light bulb. Comparing a Cepheid's true brightness with its dim brightness in the sky, they could then find how far away the star must lie.

By studying the Cepheid in Andromeda, Hubble found that the nebula was so distant that it must lie outside our Milky Way. That suggested that Andromeda, as well as other spiral nebula, were themselves galaxies. The Milky Way is not alone, he realized, but one of countless galaxies in a cosmos far vaster than most astronomers had ever imagined.

As a cosmic yardstick for measuring the distances to galaxies, Cepheids have proven invaluable. Launched in 1989, the Hubble Space Telescope, which was named in honor of Edwin Hubble, has detected Cepheids farther away than ever before and has provided better estimates of galaxy distances and the age of the universe.

In 1929, Hubble made a second revolutionary finding, this time with the help of the relatively new science of spectroscopy. Just as motion changes the pitch of a sound wave, it can change the length of a light wave. Light emitted by an object moving away from Earth appears to have its wavelengths stretched, or lengthened, as indicated by its spectrum. The faster the object moves the greater the shift.

By examining the shifts in spectra from a great many galaxies, Hubble discovered that galaxies were fleeing from each other, with the farthest ones receding the most rapidly. Now known as Hubble's law, this finding provided the first observational evidence that the universe isn't static, but instead expands over time.

Pigeons and the Big Bang 

In the late 1940s, the Russian emigre George Gamow and two younger physicists developed a theory about the origin of the chemical elements and how the universe began. Gamow and his colleagues, Ralph Alpher and Robert Herman, calculated that there should be some latent heat left over from the explosive event, now known as the Big Bang, that sparked the expanding universe and forged several of the lightest elements.

The team predicted that a sensitive enough telescope should be able to detect that relic heat, which the researchers calculated to have a temperature of about 5 kelvins.

Gamow adopted the word ylem (pronounced eye-lem), an ancient word meaning primordial substance, for the stuff from which the universe was made. Known for his sense of humor, Gamow slapped a new label on a bottle of Cointreau liqueur, rechristening it ylem.

By the 1960s, most researchers had forgotten about Gamow's prediction that a tiny amount of heat left over from the Big Bang should still be aglow in the universe today. But Robert Dicke of Princeton University and his colleagues arrived at a similar conclusion and in the early 1960s began searching for the background glow using a radio-wave detector mounted on the roof of a Princeton building.

At about the same time, Arno Penzias and Robert Wilson of Bell Laboratories in Holmdel, N.J., were cataloguing all known sources of radio emissions from space in an effort to improve satellite communications. They soon encountered a puzzle, however. Some radio static persisted no matter where in the sky they pointed their radio antenna, which was shaped like a large horn. Pigeons had taken to roosting in the antenna, and Penzias and Wilson thought that heat from the bird droppings might be the source of the static. Trapping the pigeons didn't solve the problem, however.

Ultimately, the researchers were forced to conclude that a faint microwave hiss bathed the entire sky. By accident, Penzias and Wilson had discovered the cosmic microwave background--the Big Bang's latent heat. The energy of the radiation is equivalent to that emitted by a material at about 3 kelvins.

"Boys, we've been scooped!" said Dicke when he heard the news.

For their landmark finding, Penzias and Wilson won a Nobel Prize in Physics. The discovery, which helped to confirm the Big Bang model, spurred a new era in which astronomers have probed the microwave sky in finer and finer detail in their effort to understand the origin of the universe.

The digital era

The proliferation of tools for observing the heavens has itself reached astronomical proportions. Various telescopes, spectrometers, particle detectors, and other gadgetry--some earthbound and some on spacecraft--can measure virtually any wavelength of the electromagnetic radiation and most of the other physical or chemical phenomena the cosmos can muster. Computer-controlled telescopes automatically record terabytes of data from the sky.

Some of the newest instruments are revealing a cosmic puzzle that astronomers are still trying to comprehend: Not only is the universe expanding, as Hubble showed in the 1920s, but it's doing so at an everfaster rate. As Smithsonian curator DeVorkin likes to remind visitors, every instrument enlarges our view of the universe.