It's hard to be modest when you live in the Milky Way.
Our galaxy is far larger, brighter, and more massive than most other galaxies. From end to end, the Milky Way's starry disk, observable with the naked eye and through optical telescopes, spans 120,000 light-years. Encircling it is another disk, composed mostly of hydrogen gas, detectable by radio telescopes. And engulfing all that our telescopes can see is an enormous halo of dark matter that they can't. While it emits no light, this dark matter far outweighs the Milky Way's hundreds of billions of stars, giving the galaxy a total mass one to two trillion times that of the sun. Indeed, our galaxy is so huge that dozens of lesser galaxies scamper about it, like moons orbiting a giant planet.
As a result of its vast size, the Milky Way can boast at least one planet with intelligent life. Giant galaxies like the Milky Way and the nearby, even larger Andromeda galaxy possess the power to create and retain a rich supply of iron, oxygen, silicon, magnesium, and other elements heavier than helium. Forged by the Milky Way's abundant stars, such heavy elements are the building blocks of terrestrial planets.
Heavy elements are equally essential for life: Witness the oxygen we breathe, the calcium in our bones, the iron in our blood. When a star explodes in a lesser galaxy, this raw material for life shoots out into space at millions of miles an hour and is lost. But in the Milky Way, the elements encounter interstellar gas and dust and are restrained by the strength of the galaxy's immense gravitational field. These impediments slow their speed, so they can enrich star-forming gas clouds with the ingredients for new generations of stars and planets. That's what happened 4.6 billion years ago, when the sun and the Earth emerged from a now-vanished interstellar nebula.
Because we reside within the Milky Way, we actually know less about its overall appearance than we do about distant galaxies—just as absent a mirror, you know more about your friends' faces than your own. Nevertheless, in the past decade astronomers have made numerous new discoveries about our galaxy, beginning with revelations about the huge black hole at its heart.
Every star in the Milky Way revolves around this black hole, named Sagittarius A* (abbreviated "Sgr A*" and pronounced "Sagittarius A-star"). The sun, 27,000 light-years away, completes a revolution once every 230 million years. Within just a light-year of the black hole swarm more than 100,000 other stars caught far more firmly in its grip. Some take only a few years to complete their orbits. These paths reveal that Sgr A* is four million times the mass of the sun, somewhat more massive than had been thought a decade ago.
Every now and then, the black hole swallows a bit of gas, a wayward planet, or even an entire star. Friction and gravity heat the victim to such high temperatures that it lets out a scream of x-rays. These light up nearby gas clouds, preserving a record of the black hole's past feasts. For example, in 2004 scientists reported an x-ray echo in a gas cloud some 350 light-years from the black hole. Since x-rays travel at the speed of light, the echo indicates that an object fell into the black hole around 350 years ago. The x-ray intensity suggests it had the mass of a small planet. Another object took the plunge as recently as the 1940s.
Surprisingly, the black hole also catapults stars away. In 2005 astronomers reported an extraordinarily fast-moving star some 200,000 light-years from the galactic center. "It was serendipitous," says Warren Brown at the Harvard-Smithsonian Center for Astrophysics. He was searching for "star streams"—remnants of small galaxies the Milky Way's gravitational pull has torn to shreds—when he found a star in the constellation Hydra racing away from the galactic center at 709 kilometers a second, or 1.6 million miles an hour. At that speed, it will escape the galaxy's grasp and sail off into intergalactic space. By 2010 Brown and other astronomers had discovered 15 more of these hypervelocity stars.
In a remarkable display of prescience, Jack Hills, while at the Los Alamos National Laboratory in New Mexico, had predicted just such a phenomenon. "I was actually rather surprised that the discovery had taken so long," says Hills, "but I was certainly delighted." In a 1988 paper, Hills wrote that if a binary star—two stars orbiting each other—ventured too close to Sgr A*, one star of the pair could fall toward the black hole and go into a tighter orbit around it, thereby losing an enormous amount of energy. Since the laws of physics dictate that energy be conserved, the other star would gain an equally large energy boost, flying away at tremendous speed. Over the Milky Way's lifetime, says Brown, the black hole may have flung a million stars out of the galaxy in this fashion.
Despite the violence around the black hole, the galactic core is a fertile place. Stars congregate most tightly at the galaxy's center, so the life-giving heavy elements they create are most plentiful there. Even near our sun—a bright yellow star halfway between the black hole and the edge of the starry disk—many newborn stars possess orbiting disks of gas and dust that survive millions of years, long enough to give birth to planets.
In contrast, prospects for planets at the galaxy's edge are bleak. Last year Chikako Yasui, now at the National Astronomical Observatory of Japan, and her colleagues reported on 111 newborn stars in a Milky Way exurb, over twice as far out as the sun. These youngsters had low supplies of heavy elements—for example, their oxygen content was only 20 percent of the sun's. Although the stars were just half a million years old—still in their infancy in stellar time scales—most had already lost their planet-forming disks of gas and dust. No disks, no planets; and no planets, no life. Quipped science writer Ian O'Neill on his astronomy blog Astroengine, "Life is grim on the galactic rim."
Stars with even lower amounts of oxygen and iron offer insight into the birth of the galaxy itself. Residing in the stellar halo extending above and below the galaxy's disk, these stars are so old that they formed before earlier generations of stars had much of a chance to produce heavy elements. Thus, the typical halo star has only 3 percent of the sun's iron content.
Astronomers traditionally date the stellar halo, and hence the age of the entire galaxy, by studying globular clusters—bright, tightly packed conglomerations of stars so old that their shorter lived stars have died. But estimates of their ages depend on theories of how stars live and perish.
Fortunately, there is another way to measure the galaxy's age. While still a graduate student at the Australian National University, Anna Frebel began looking for individual stars in the halo. "I want to find these stars because I want to look back in time," says Frebel, now at the Harvard-Smithsonian Center for Astrophysics. In 2005 she discovered a halo star in the constellation Libra with just 1/1000 of the sun's iron content—low, even by halo standards, indicating it is so pristine it probably arose from gas enriched by a single supernova. Unlike most supernovae, this one spewed lots of elements far heavier than iron, including radioactive thorium and uranium.
For Frebel, this was a lucky star indeed. Since these radioactive elements decay at a steady rate, comparing their abundance in the star today allowed her to estimate its age: around 13.2 billion years old. Although that figure is uncertain by two or three billion years, it agrees with the ages derived from studies of globular clusters, and it suggests that the Milky Way is only slightly younger than the universe itself, which is 13.7 billion years old. The mighty galaxy whose countless stars would later make life possible on Earth didn't waste any time being born.