Researchers examine Astro's universe.

Researchers examine Astro's universe

Last December, NASA launched the Astro Observatory, a shuttle-borne laboratory that carried two lutraviolet spectrographs, an ultraviolet camera and an X-ray telescope on a nine-day, Earth-orbiting mission. During Astro's problem-plagued flight, astronomers failed to get about 60 percent of the images and spectra they had hoped to obtain (SN: 12/8/90, p.356). But this week researchers reported that the data Astro did acquire revealed a wealth of new details about the composition and structure of the universe.

Because extreme ultraviolet and X-ray radiation cannot penetrate Earth's atmosphere, these wavelengths are detectable only from space. At the annual meeting of the American Physical Society in Washington, D.C., astronomer Theodore P. Stecher took his audience on an armchair tour of galaxies, supernova remnants and globular star clusters. The vistas and details that Stecher displayed had never before been fully imaged at such short wavelenghts.

Stecher, principal investigator of Astro's only imaging instrument, unveiled the first ultraviolet portrait of the Crab nebula. It displayed regions where the supernova remnant's huge magnetic field accelerates electrons to high speeds, inducing the particles to emit synchrotron radiation. Astro's camera also revealed previously undetected features at the core of teh globular cluster Omega Centauri, including the presence of some 1,300 blue stars. Even in visible light, detectros on Earth cannot image these hot stars. Intense, competing emissions from cooler stars in the cluster's core dominate the visible radiation, explains Stecher, of NASA's Goddard Space Flight Center in Greenbelt, Md. But the cooler stars fade in the ultraviolet, allowing warmer objects to step into the spotlight.

Stecher notes that thousands of hot stars in nearby galaxies such as Andromeda and the Large and Small Magellanic Clouds also made their ultraviolet debut on film from the Astro camera.

Astro's broad-band X-ray telescope found several novel features. The instrument, which cannot image objects but instead serves as a high-resolution X-ray spectrograph, clearly detected iron in hot gas throughout the relatively nearby Perseus cluster of galaxies, reports Peter J. Serlemitsos of Goddard. This refutes studies suggesting the metal concentrates at the cluster's center, he says.

Serlemitsos and his colleagues also determined the redshift of the X-ray spectra, indicating the cluster's distance -- 224 million light-years from Earth. (According to standard cosmology, more distant objects move away at higher speeds and the light they emit gets shifted more toward the red, or longer-wavelength, end of the spectrum.) That measurement represents one of the first times that astronomers have used X-ray spectra, rather than visible-light spectra, to determine a redshift, he says. This method may prove especially useful in determining the distances of very far galaxies, he adds, because the visible light reaching Earth from these objects is often too faint for computing redshifts.

An observation with one of Astro's two spectrographs, the Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE), supports the notion that Seyfert type-I and type-II galaxies are actually a single class of galaxy, even though their unpolarized spectra differ.

Arthur D. Code and his co-workers at the University of Wisconsin-Madison used WUPPE to examine ultraviolet light emitted by the core of NGC 1068, the brightest known type-II Seyfert. They found that the light has a high polarization rate of 15 percent, which remains constant over many wavelengths. This indicates that electron clouds sorround the dust-shrouded care, Code says. The clouds act like reflectors, polarizing radiation escaping from a small hole in the dust and sending some of this core light -- which would otherwise go undetected -- back toward Earth.

Unlike most radiation from type-II Seyferts, this polarized light appears to contain broad emission lines characteristic of type-I galaxies. Code says the new observations lend credence to a theory that the spatial orientation of some Seyferts causes radiation from their cores to travel through ligh-absorbing dust, preventing the light from taking a direct, unpolarized path to reach Earth. The dust would tend to suppress certain telltale spectral fingerprints -- such as broad emission lines -- that all Seyferts might share.