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."