Astronomers have long had a basic understanding of how stars are formed. But observing and proving the theory has proven elusive. Now, a German-American project, complete with an infrared telescope mounted on an old Pan Am 747, is providing new insights.
Stars are heavenly bodies that originated in darkness. All of the twinkling points of light in the night sky were once born in inky black clouds that wander through the vast expanses of the Milky Way.
The maximum temperature in these ghostly clouds is minus 250 degrees Celsius (minus 418 degrees Fahrenheit); indeed the clouds are hardly any warmer than space itself. Furthermore, their density initially is almost as low as that of a vacuum, with a volume the size of the Pacific Ocean containing but a single gram of hydrogen.
These widely dispersed gas atoms are the raw material from which stars are born. But it takes eons for the thin, icy strips of clouds to condense into compact, hot balls of fire. First, clumps must form within the clouds of gases and dust, with particles attracting one another and flying together at supersonic speeds. In the end, extreme pressure builds in the center of the increasingly dense ball of gas, and thermonuclear fusion in the hydrogen core begins. It is the fire of the new star.
This, at any rate, is the theory, based on computer simulations. No one has actually seen the birth and growth of suns. To the regret of astronomers, all of these processes are either invisible in the early stages or are hidden in the late stages. The gas-and-dust clouds surround the growing stars like a protective cocoon, preventing all light from escaping. The precise details of how a star is born remain a mystery.
Now astrophysicists are taking a new approach to gazing at the places where stars are born. Even the icy, dark clouds emit thermal radiation. While such waves are invisible to the human eye, the world's most unusual telescope, which stares into space from an aircraft, is now being used to intercept the infrared signals.
The telescope, called SOFIA (Stratospheric Observatory for Infrared Astronomy), is the largest German-American research project currently running. It took 20 years of development, at a cost about €1 billion ($1.27 billion), before it was ready for prime time -- and the project was repeatedly in jeopardy of cancellation.
The infrared telescope is located on board a converted Boeing 747 that was once used on Pan Am's trans-Atlantic routes. At altitudes of up to 14,000 meters (45,932 feet), high above the flight paths of commercial aircraft, the older jet transforms itself into a convertible. A retractable roof in the fuselage is opened so that SOFIA can gaze into infinity. To offset turbulence and vibration, the telescope, the largest ever to be mounted in an aircraft, floats on a film of oil.
The great effort involved in unavoidable. An earthbound telescope is largely blind to infrared radiation from space, because most of it, with the exception of a few wavelengths, is absorbed by water vapor in the earth's atmosphere. As a result, the cold gas-and-dust clouds can only be studied from the stratosphere on out.
The effort seems to be paying off. "On the first images, it looks as if the dark clouds were ablaze, because of how intensely they glow in the infrared spectrum," says Hans Zinnecker, the deputy director of SOFIA Science Mission Operations. "We can now finally recover an incredibly valuable treasure trove of data."
The clouds of matter aren't just the raw material from which the stars emerge. They also form cosmic factories in which highly complex molecules are incubated, presumably even building blocks of life, like amino acids. "Complex chemical processes are taking place inside the clouds, processes that we haven't even come close to understanding," says Zinnecker.
The astrophysicists are currently evaluating data they have obtained with their flying telescope. And it looks as if SOFIA will indeed provide them with a view into the delivery room of the stars.
Fast-Forward Star Production
A team led by astrophysicist Friedrich Wyrowski of the Max Planck Institute for Radio Astronomy in Bonn, Germany, set its sights on an early phase of star development. The scientists have observed how ice-cold protostars gradually consume the clouds of matter surrounding them. Their appetite seems to be limitless.
The scientists picked up telltale light signals of ammonia molecules rushing toward the protostars "at measured speeds of up to 10,000 kilometers per hour (about 6,200 mph)," says Wyrowski. Their calculations show that, because of its tremendous gravitational force, each of the gas balls absorbs the mass of the earth within only a few weeks.
Because of this high "accretion rate," stars develop at a much faster rate than computer models had predicted. The star embryos will continue to grow in their cocoon for just another 10,000 years or so until some of them will have reached masses 30 times that of our sun. Then, nuclear fusion will suddenly begin deep in their cores, transforming a dim ball of gas into a blazing star within a few years.
The astrophysicists plan to keep sending their flying telescope on trips into the stratosphere until at least 2030. Their infrared observatory will soon be without any competition at all. The only other telescope sensitive to comparable wavelengths is the European Space Agency's Herschel Space Observatory, which will run out of coolant in a few months.
The Herschel spacecraft has recently made some exciting discoveries. Last week, a team headed by Markus Nielbock of the Max Planck Institute for Astronomy in Heidelberg presented a unique 3-D map of a dark cloud, which the scientists created with the help of Herschel data. The 3-D image shows that the cloud of molecules, dubbed Barnard 68 (in the constellation Ophiuchus), has apparently collided with a smaller cloud.
This collision could now trigger the collapse of Barnard 68, leading to the birth of several new stars within the next 100,000 years. The Next Generation
The scientists working on the SOFIA project hope to make such a dramatic event, the ignition of a star, visible within the next few years. "By measuring infrared radiation, we'll be able to observe how a star taps against its eggshell from the inside," Zinnecker hopes. "And we'll also be able to watch it emerge from the egg."
With each birth of a new star, the cosmic cycle of coming into being and passing away begins anew. When a star like our sun is extinguished after a lifetime of several billions of years, it ejects its hot gaseous shell out into empty space. The gaseous remnants of an extinguished sun continue to burn at several thousands of degrees for thousands of years after that. But then the cloud gradually cools off and drifts through the Milky Way, so that the ashes of dead suns furnish the raw material for the next generation of stars.
In the end, perhaps the astronomers will even solve the mystery of why such an astonishingly small number of stars are born in our home galaxy. The Milky Way contains an almost unimaginable number of stars, more than 100 billion, as a photo published by the European Southern Observatory at the end of October impressively illustrates. It shows more than 84 million stars in the center of the galaxy but, oddly enough, hardly any new stars are being added.
The swirling dark clouds should provide enough fuel to allow a new star to flare up in the sky every hour. But star maturation is apparently a sensitive process, one that only begins when the clouds of molecules reach exactly the right temperature and density. This helps to explain why only three new stars are ignited in the Milky Way every year. Translated from the German by Christopher Sultan