|Motion | Pages 8-9 ||
Everything moves. From the molecules of air around us to distant islands of stars, nothing sits still. We hear of comets crashing into planets, black holes gulping streams of gas, and space itself expanding like some vast balloon. And yet the night sky cloaks these cosmic motions. Apart from the wandering Moon and planets and an occasional meteor, the heavens don't seem to change.
Indeed, our eyes cannot see stars moving relative to each other from night to night, or even from one generation to the next. The constellations look essentially the same as those described by astronomers thousands of years ago. The sky's steady patterns make it easy to understand why our ancestors thought the stars were fixed on a giant black sphere that revolved around Earth. Telescopes also reveal countless galaxies suspended in space beyond our home galaxy, the Milky Way, but they seem motionless as well. How can we reconcile the motion we know with the stillness we see?
We can start by looking at our own world in a different way. Many things on our planet appear locked in place even though they move relentlessly. With patience we can see the hour hand swivel around a clock's face or a new rose open its petals to the Sun. If we could somehow watch for millions of years, we would see continents drift across the globe like slabs of ice on a partly frozen lake. Other things seem to move slowly, but only because they are far away. A plane 35,000 feet overhead appears to pass lazily above the clouds, but we know that it speeds 10 times faster than a car zipping past us on the street. Satellites show a hurricane swirling with a grace that belies the fury of its impact on a shoreline. Time and distance hide the true nature of these motions. We see them only when we learn to transcend those barriers, to look beyond the usual rhythm and scope of our world.
Some of the greatest triumphs of scientific inquiry have come from such leaps in vision. In the mid-1950s, geologists learned to read the history book of motion on Earth by finding ancient magnetic imprints preserved in rocks. The imprints revealed the past travels of continents and the constant birth of new crust at the bottom of the sea. With that discovery our planet became a dynamic world. Earthquakes and volcanoes suddenly made sense as the results of those churnings. In a similar way, astronomers learned to break the barrier of time in the cosmos by using telescopes to expose patterns hidden in the light from distant stars. A man named Edwin Hubble studied those patterns to unveil an astonishing fact: Our entire universe is expanding, and at a phenomenal speed. When we look today at the silent night sky, Hubble's discovery remains as awe inspiring as when he announced it in 1929.
Hubble had access to the 100-inch telescope on Mount Wilson, California, the biggest in the world at the time. During the 1920s, he and astronomer Vesto Slipher of Lowell Observatory in Arizona measured the speeds at which scores of galaxies move through space. They calculated the speeds by detecting subtle shifts in the colors of light emitted by stars in each galaxy. Such shifts reveal a galaxy's motion toward or away from Earth (page 137). One of the first galaxies Slipher studied was the great nebula in Andromeda, which we can see as a fuzzy patch of light with our unaided eyes. He was shocked to find it streaming toward the Milky Way, closing the gap between the two galaxies by 186 miles every second. Most other galaxies analyzed by Hubble and Slipher displayed the opposite motion, receding from us at even higher rates--up to nearly 700 miles per second.
By itself that research was noteworthy. But Hubble's great contribution was to combine the speed measurements with distances to the galaxies. That critical link was provided by rare giant stars that grow brighter and dimmer in regular cycles every few days. Henrietta Swan Leavitt, an astronomer at the Harvard College Observatory, studied these stars, called Cepheid variables, in a pair of galaxies close to the Milky Way. By 1912 she had demonstrated that the length of a Cepheid's cycle depends only on the star's luminosity, or inherent brightness. Bright Cepheids flicker more slowly than faint ones, Leavitt found. It was as though she had discovered that all 60-watt lightbulbs flicker at a certain rate, while all 100-watt bulbs flicker more slowly. Each Cepheid variable has a characteristic "wattage" that astronomers can deduce by observing how long it takes for the star to pulsate.
The Mount Wilson telescope gave Hubble the light-gathering power he needed to spot Cepheid variables in galaxies much farther away than those studied by Leavitt. The first such star he spotted in a distant galaxy was in 1923, in the great Andromeda nebula itself. He marked the discovery with an excited notation of "VAR!" on a photographic plate. During the next several years, Hubble measured the flicker rates (continued)