Video of the Day: The Search for Killer Asteroids

Bay Area astronomers police the skies for earth killing asteroids.

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1st North American Antenna Enables ALMA Observatory to Do Its Thing

Astronomers today celebrated the formal acceptance of the first North American antenna by the Joint ALMA Observatory. ALMA, the Atacama Large Millimeter/submillimeter Array, is a gathering armada of short-wavelength radio telescopes whose combined power will enable astronomers to probe with unprecedented sharpness phenomena and regions that are beyond the reach of visible-light telescopes. The observatory is being assembled high in the Chilean Andes by a global partnership.

With ALMA, astronomers will study the universe, the molecular gas and tiny dust grains from which stars, planetary systems, galaxies and even life are formed. ALMA will provide new insights into the formation of stars and planets and will reveal distant galaxies in the early universe, which we will see as they were over 10 billion years ago.

The 12-meter-diameter antenna delivered today is the first of 25 being provided by North America's ALMA partners, whose efforts are led by the National Radio Astronomy Observatory and supported by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada and the National Science Council of Taiwan. The antenna was manufactured by General Dynamics SATCOM Technologies.

The acceptance comes just weeks after the first ALMA antenna--produced under the direction of the National Astronomical Observatory of Japan on behalf of ALMA's East Asian partners--was handed over to the observatory.

"These ALMA antennas are technological marvels," says ALMA Director Thijs de Graauw. "They are more precise and more capable than any ever made. Their performance in the harsh winds and temperatures of our high-altitude site bodes well for the observatory's future."

A single 12-meter antenna's dish is bigger than the largest optical telescope's reflective mirror, but to match the sharpness achieved by an optical telescope, a millimeter-wavelength dish would have to be impossibly large, miles across. ALMA will combine signals from dozens of antennas spread across miles of desert to synthesize the effective sharpness of such a single, gigantic antenna. The process, called "interferometry," involves analysis of the ways in which the signals coming from each antenna interfere with one another.

"This is a major milestone for the ALMA project," says Philip Puxley, NSF's ALMA program manager. "With two antennas now on site, we begin the real work of combining signals from them. We are advancing toward ALMA's ultimate goal of surpassing by tenfold existing technology in this area for sharper resolution, sensitivity and image quality."

ALMA officials expect the pace of antenna acceptance to accelerate. "We have nine North American antennas on site already," said Adrian Russell, NRAO's ALMA project director. "Following handover of Number Three, we plan to get one through the test procedure each month. Additional North American antennas will be arriving in Chile at a rate of one every two months, and General Dynamics is on track to complete delivery of these systems within days of the original schedule."

The antennas, which each weigh about 100 tons, can be moved to different positions in order to reconfigure the ALMA telescope. This repositioning will be carried out by two custom-designed transporters, each of which is some 33 feet wide, 66 feet long, and has 28 wheels.

When completed early this decade, ALMA will have a total of 66 antennas, with an option for further expansion, provided by partners in North America, Europe and East Asia. The first European antennas, produced under the auspices of the European Organization for Astronomical Research in the Southern Hemisphere are scheduled to begin arriving early this year.

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Milky Way's Giant Black Hole Awoke From Slumber 300 Years Ago

Using NASA,Japanese, and European X-ray satellites, a team of Japanese astronomers has discovered that our galaxy's central black hole let loose a powerful flare three centuries ago.

The finding helps resolve a long-standing mystery: why is the Milky Way's black hole so quiescent? The black hole, known as Sagittarius A* (pronounced "A-star"), is a certified monster, containing about 4 million times the mass of our Sun. Yet the energy radiated from its surroundings is billions of times weaker than the radiation emitted from central black
holes in other galaxies.

"We have wondered why the Milky Way's black hole appears to be a slumbering giant," says team leader Tatsuya Inui of Kyoto University in Japan. "But now we realize that the black hole was far more active in the past. Perhaps it's just resting after a major outburst."

The new study, which will appear in the Publications of the Astronomical Society of Japan, combines results from Japan's Suzaku and ASCA X-ray satellites, NASA's Chandra X-ray Observatory, and the European Space Agency's XMM-Newton X-ray Observatory.

The observations, collected between 1994 and 2005, revealed that clouds of gas near the central black hole brightened and faded quickly in X-ray light as they responded to X-ray pulses emanating from just outside the black hole. When gas spirals inward toward the black hole, it heats up to millions of degrees and emits X-rays. As more and more matter piles up near
the black hole, the greater the X-ray output.

These X-ray pulses take 300 years to traverse the distance between the central black hole and a large cloud known as Sagittarius B2, so the cloud responds to events that occurred 300 years earlier. When the X-rays reach the cloud, they collide with iron atoms, kicking out electrons that are close to the atomic nucleus. When electrons from farther out fill in these gaps, the iron atoms emit X-rays. But after the X-ray pulse passes through, the cloud fades to its normal brightness.

Amazingly, a region in Sagittarius B2 only 10 light-years across varied considerably in brightness in just 5 years. These brightenings are known as light echoes. By resolving the X-ray spectral line from iron, Suzaku's observations were crucial for eliminating the possibility that subatomic particles caused the light echoes.

"By observing how this cloud lit up and faded over 10 years, we could trace back the black hole's activity 300 years ago," says team member Katsuji Koyama of Kyoto University. "The black hole was a million times brighter three centuries ago. It must have unleashed an incredibly powerful flare."

This new study builds upon research by several groups who pioneered the light-echo technique. Last year, a team led by Michael Muno, who now works at the California Institute of Technology in Pasadena, Calif., used Chandra observations of X-ray light echoes to show that Sagittarius A* generated a powerful burst of X-rays about 50 years ago -- about a dozen years before
astronomers had satellites that could detect X-rays from outer space. "The outburst three centuries ago was 10 times brighter than the one we detected," says Muno.

The galactic center is about 26,000 light-years from Earth, meaning those on Earth see events as they occurred 26,000 years ago. Astronomers still lack a detailed understanding of why Sagittarius A* varies so much in its activity. One possibility, says Koyama, is that a supernova a few centuries ago plowed up gas and swept it into the black hole, leading to a temporary feeding frenzy that awoke the black hole from its slumber and produced the giant flare.

Launched in 2005, Suzaku is the fifth in a series of Japanese satellites devoted to studying celestial X-ray sources and is managed by the Japan Aerospace Exploration Agency (JAXA). This mission is a collaborative effort between Japanese universities and institutions and NASA Goddard Spaceflight Center.

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