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Young stars in the Orion nebula emerge from hiding in this superposition of infrared images from
the Spitzer Space Telescope and visible-light images form the Hubble Space Telescope.
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Image courtesy of NASA
When most people gaze up on a clear, dark night, they are struck by how many stars there are. Astronomers have an opposite reaction: They marvel at the stars’ amazing scarcity. Considering the total amount of raw material available in our galaxy for star formation, there should be up to 10 times the current count. Why, then, does the night sky not blaze with starlight?
It is not a bad thing that there are so few stars; quite the opposite. Stars burn gas, mostly hydrogen. All of the hydrogen gas in the universe was formed during the Big Bang, some 13.8 billion years ago. Each galaxy possesses a finite portion of this primordial fuel, and there is no way to make any more of it. If there were more stars, galaxies would burn through their fuel reserves more quickly and would shine more briefly before lapsing into eternal darkness.
Understanding how stars form and why they are so hard to make does much more than just foretell our far-off cosmic future. Star birth also explains where the atoms in our bodies come from and why the universe looks the way it does today. As astronomer John Bally of the University of Colorado puts it, “Star formation is the single most important process for determining the fate and evolution of normal matter in the universe.” Yet until recently, the details of how stars are born were literally shrouded in mystery: Stars form within dense clouds of dust and gas that block visible light.
Now astronomers are parting the veil with telescopes that detect infrared light, the kind of light central to terrestrial night-vision systems. “Seeing in infrared light is important because the diminution of visible light from inside a dusty cloud can be enormous,” says Judy Pipher, a professor of observational and experimental astronomy at the University of Rochester in New York. “This is not a problem when you use an infrared camera because at those longer wavelengths the cloud will be a million times more transparent.”
If galaxies raced through their fuel reserves, creating lots of first-generation stars early on, few stars with rocky planets would be born later. The odds for life would be much worse. Earth might very well not even exist.
The picture of star formation given to us by infrared telescopes is one of unexpected violence, and it is this violence that is the key to understanding why there are so few stars. The birth of one disrupts the formation of others nearby, limiting the rate at which raw hydrogen can be assembled into shining stars.
Efforts to spy on the star-birth process got a huge boost with NASA’s launch of the Spitzer Space Telescope in 2003. Pipher, considered by many to be the mother of infrared astronomy, worked for 20 years with collaborators William Forrest and Dan Watson to develop the detectors that form the heart of this 2,000-pound floating observatory.
Spitzer does not orbit Earth but trails behind us in space, following Earth’s orbit around the sun at a distance of about 56 million miles. Spitzer was sent so far out because its delicate infrared-sensitive instruments must be kept at a frigid temperature just above absolute zero, and it is easier to maintain that temperature by operating far from the heat that radiates from the surface of our planet.
Spitzer’s new views of stellar nurseries as places of chaos and turbulence contrast sharply with astronomers’ old preconceptions. In the absence of the direct view provided by infrared telescopes, scientists spent the bulk of the last century building beautiful theories of individual gas clouds collapsing gracefully under their own gravity to form individual stars. The basic model of star formation was mapped out by British astrophysicist Sir James Jeans a little over 100 years ago. Jeans began with a large cloud of interstellar gas whose inward pull of gravity perfectly balanced the outward push of pressure from its own internal heat. Jeans found that this balance was unstable. With just a nudge—from, say, the remnants of a supernova shock wave—gravity would win the tug-of-war and start the cloud’s collapsing in on itself. At the center of the cloud, matter would pile up to densities and temperatures that (scientists later realized) were high enough to allow hydrogen atoms to fuse into helium. When fusion began, a star was born.
Most of the 20th century was spent filling in the details of Jeans’s story. “You begin with single stars because they are simple,” says Héctor Arce, an astrophysicist at Yale University. “They are kind of a theorist’s dream.” These early models paid scant attention to the way infant stars might influence each other. “Before we could understand how neighbors affect individual star formation,” explains Arce’s former thesis advisor, Alyssa Goodman of the Harvard-Smithsonian Center for Astrophysics, “we had to understand the evolution of stars in isolation. That was pretty complicated in itself.”
During the 1980s and 1990s, Spitzer’s less sophisticated predecessors put astronomers on the trail of a more holistic model of star formation. From hard-won infrared images, a story much more complex than Jeans’s emerged about the birth of stars and planets. Star formation, those first pictures hinted, is distinctly a family affair—with all the turbulence, chaos, and tumult that implies.
