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.