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.