An artist's conception of the jets that will shoot from the center of the Milky Way during its next starburst period. In about 300 million years, material in a ring around the galactic center will collapse inward, causing rapid star formation and feeding the central black hole. Credit: David Aguilar, Harvard-Smithsonian Center for Astrophysics

A team of astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) led by Chris Martin and Antony Stark has mapped molecular clouds near the center of our galaxy in unprecedented detail at submillimeter wavelengths. Their data was presented this week at a meeting of the American Astronomical Society in Albuquerque, NM. Their results suggest that we are headed for some celestial fireworks. Sometime in the next 300 million years, the galactic center will experience a dramatic burst of star formation and will shine with the light of thousands of newborn suns.

The effects of these starbursts will be dramatic. "Many of the stars that form will be very massive and short-lived," says Stark. "They'll quickly use up their fuel and explode as supernovae. Right now, we see one supernova in our galaxy about every 100 years. When the starburst happens, we'll see one supernova every year."

The team mapped the molecular clouds using submillimeter emissions from carbon monoxide molecules. Submillimeter radiation has wavelengths of less than one millimeter and lies between the infrared (heat) and radio portions of the electromagnetic spectrum. The map spans an area of the sky about 3/4 of a square degree in size, or about three times the area covered by the full moon. At the galactic center, this corresponds to a map about 400 light-years on a side. Using this submillimeter map, astronomers now can determine the temperature and density of material near the center of the Milky Way in great detail.

"Most of the matter in these molecular clouds is very cold, at temperatures just a few tens of degrees above absolute zero. At those low temperatures, gas emits subillimeter radiation rather than visible light," says Martin. "We can combine our map with the recent X-ray map of the galactic center taken by NASA's Chandra X-ray Observatory as well as with observations at other wavelengths of light. Together, the data allow us to build a complete picture of the environment near the center of our galaxy."

The observations were made using the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO). AST/RO is a 67-inch (1.7-meter) diameter telescope operated by CfA in collaboration with the University of Arizona, Caltech, and the University of Cologne, Germany. AST/RO is located at the National Science Foundation's Amundsen-Scott Station at the South Pole. This telescope offers unique capabilities unmatched by other submillimeter observatories around the world. The air at the South Pole is very dry and cold, so radiation that would be absorbed by water vapor at other sites can make it to the ground and be detected.

AST/RO operates in a strange and forbidding environment where temperatures can plummet to minus 100 degrees F. The South Pole sees only one sunrise and one sunset each year, so visitors experience six months of constant daylight followed by six months of darkness. Yet the darkness offers delights of its own, such as aurorae that paint the sky with dancing green fire and the opportunity to observe the same area of sky 24 hours a day, 7 days a week.

"There's a certain irony to our work," says Martin. "We point a giant thermometer into space and measure the temperature of the galactic center from the coldest place on Earth."

The observations also show that the gas clouds are near a critical density beyond which they will collapse into the center of the galaxy. This infall of material will lead to a great burst of star formation, with stars being born at 10-100 times the current rate. The process will then begin again as more gas collects around the outskirts of the galactic center, gradually accumulating in a smooth ring. When that ring reaches a critical density, it coagulates into one or two giant molecular clouds, each one millions of times more massive than the Sun. These clouds fall inward to the galactic center where another starburst takes place. The cycle repeats approximately every 500 million years.

Astronomers see starbursts in other galaxies such as the nearby irregular galaxy dubbed M82. Studying a similar process in our home galaxy offers a unique advantage. The Earth's distance from the galactic center of 25,000 light-years enables astronomers to observe the process in much greater detail than can be seen in galaxies millions of light-years away.

The infalling gas will also feed the black hole at the center of the Milky Way. As the matter spirals around the black hole, some material will be ejected outward in two long jets toward the north and south galactic pole.

"We're lucky that the energy from these jets is directed out away from the plane of the Milky Way. If it weren't, the Earth might be periodically sterilized of all life," says Stark.

Other CfA researchers who contributed to this work are Adair P. Lane and Sungeun Kim.

Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists organized into seven research divisions study the origin, evolution, and ultimate fate of the universe.