On the trail of missing matter
X-ray astronomy opens up some of the universe's most violent and creative processes.
GREENBELT, MD. — At first glance, the narrow, 8-meter-long truss resting on its side in an assembly hall here at NASA's Goddard Space Flight Center hardly looks like a telescope - no highly polished glass mirror to focus starlight. And in an age when pixels have replaced pupils, certainly no eye-piece.
Yet the instruments mounted on the truss represent key stepping stones toward an orbiting observatory that astronomers say is critical to helping them pick up the trail of the universe's missing matter, observe the origin and evolution of chemical elements, and unravel the mysteries behind black holes - objects with gravity so intense that light fails to escape.
Known as Constellation-X, the observatory is designed to study X-ray emissions from some of the most energetic - and bizarre - objects and processes in the universe.
"The nice thing about X-ray astronomy in general is that virtually every object we study is weird," says Robert Petre, who heads up the X-ray astrophysics branch of Goddard's Laboratory for High Energy Astrophysics and is deeply involved in the Constellation-X project.
Yet, Dr. Petre adds, understanding the processes driving such objects - from neutron stars cannibalizing the matter of companion stars to the highly energetic cores of distant galaxies - is vital to understanding how the universe evolved since it burst into existence 15 billion years ago. Nor is he alone in that view.
Last May, the National Research Council published a report laying out astronomy and astrophysics projects for the next decade. Constellation-X was high on the list of "major initiatives."
Astrophysicists certainly don't lack highly capable orbiting X-ray telescopes. In 1999, the National Aeronautics and Space Administration launched the Chandra X-Ray Observatory, one of its Great Observatory series, which includes the Hubble Space Telescope.
Chandra has been a stunning success so far. For example, it has helped determine the source of X-ray emissions from the Crab Nebula, a supernova remnant in our own galaxy. And just last week, researchers using Chandra announced that they've bagged the most distant cluster of galaxies ever seen via X-rays. The cluster lies 10 billion light years from Earth. According to Harvey Tanabaum, director of the Chandra Observatory Center at the Harvard-Smithsonian Center for Astrophysics and chairman of Constellation-X's science team, Constellation-X will serve a different purpose.
More light, please
Chandra, he says, is analogous to the Hubble Space Telescope, which "does an amazing job of providing highly detailed, beautiful images from space." Yet at 2.4 meters across, Hubble's light-gathering mirror fails to capture enough light to allow astronomers to conduct detailed studies of the distant objects it images. That task falls to large ground-based "light buckets" such as the twin 10-meter Keck telescopes on Hawaii's Mauna Kea.
Constellation-X is designed to be the X-ray version of these light buckets. And its spectrograph will be able to pick more details out of the emissions it detects than can Chandra's.
As currently envisioned, the observatory would consist of four spacecraft equipped with identical X-ray telescopes whose mirrors measure 1.6 meters across. The four would orbit around a point in space 1 million to 1.5 million kilometers from Earth. By combining the images from the telescopes, which themselves would be separated by hundreds of miles, astronomers can get light-gathering results that would match a single telescope with a 6.4 meter mirror.
Dr. Tanabaum cites last week's galaxy-cluster observation to illustrate the differences. "Chandra detected about 100 X-rays from that cluster," he says, enough to allow researchers to tease an iron-like signature from Chandra's X-ray spectrograph. (The abundance of iron is a key indicator of the age of stars in a galaxy or galaxy cluster.) With the proper exposure time, Constellation-X could collect 10,000 X-rays - enough to uncover significant details in the iron signature, help establish its distribution around the cluster, and reveal information about other elements there. By looking at the ratio of iron to oxygen, for example, astronomers can reconstruct key details of the supernovae that have detonated within the galaxies, Tanabaum says.
Indeed, the metal content of distant galaxies has presented astrophysicists with a puzzle, and represents one of Constellation-X's key themes - how matter is cycled through stars, into the interstellar and intergalactic medium, then back.
"Over time, as galaxies age, heavier elements should build up" as stars' nuclear furnaces forge them from lighter elements such as hydrogen and helium, Petre explains. "But as we look farther back at more-distant clusters of galaxies, we don't see the dramatic evolution we'd expect." The problem, he continues, is that "we see little evolution" in galactic chemistry, "at least back to the point where we can look now. This suggests that all the evolution we see must have occurred earlier."
Weirder and weirder matter
Such observations have set theorists to scratching their chins and looking to Constellation-X observations for clues to the puzzle as it peers deeper into space and thus farther back in time in the hunt for chemically younger galaxies.
Goddard astrophysicist Kim Weaver adds that another theme involves the interaction of matter with intense gravitational fields, such as those presented by black holes, particularly the billion-solar-mass objects thought to lie in the heart of energetic galaxies.
These objects are known to trigger X-ray emissions as matter falls into them. "But we don't know what the ultimate source of the X-rays is," Dr. Weaver says. A leading explanation is that falling gas and dust form an accretion disk around the black hole, and as the material gets closer and speeds up, friction heats it to the million-degree temperatures needed to generate X-rays.
If all goes well, researchers hope to begin launching the Constellation-X telescopes in 2008.
(c) Copyright 2001. The Christian Science Publishing Society