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Galex: seeing starbirth, near and far: NASA's plucky GALEX spacecraft is chronicling starbirth throughout space and time.

Only a smattering of the ultraviolet photons emitted by astronomical objects can get through Earth's atmosphere. That's just as well, your dermatologist will tell you. But our planet's sunscreen blinds astronomers to light from hot, young stars--those that have had the least time to wander from their birthplaces.

That, in a nutshell, is why Caltech astrophysicist D. Christopher Martin and his colleagues designed and built the 280-kilogram (617-pound) Galaxy Evolution Explorer, or GALEX. Launched into low Earth orbit on April 28, 2003, GALEX is building upon earlier ultraviolet forays by surveying most of the sky, imaging hundreds of nearby galaxies and tens of millions of very distant ones in the process. If NASA extends its soon-to-end mission, GALEX also will acquire ultraviolet spectra of tens of thousands of stars and galaxies and thousands of distant quasars.

Ask insiders like Martin what GALEX's most surprising discovery has been thus far and most point to galaxies whose otherwise dark outer disks are aglow with ultraviolet light from stars that are less than 100 million years old (S&T: November 2005, page 40). Visible-wavelength photographs show little or no "regular" starlight from these galaxies' outer disks. Since only massive, short-lived stars give off most of their light at ultraviolet wavelengths, this means that the newfound stellar adolescents seen by GALEX must have been born in star factories that began production only recently. Why those factories have done so remains a mystery.

Less mysterious--but no less fascinating--is recent star formation in systems of colliding galaxies such as Stephan's Quintet in Pegasus (S&T: November 2004, page30). Galaxy collisions compress molecular clouds, igniting the raw fuel for starbirth. Since galaxies collided frequently in the young, crowded cosmos, "that kind of star formation was probably going on in spades in the early universe," says Martin. But it can't be studied in the kind of detail that GALEX provides on cosmologically nearby systems like Stephan's Quintet (a "mere" 300 million light-years from Earth).

Next on Martin's list of surprises in our cosmic backyard: galaxies that emit unusually large amounts of ultraviolet light relative to their overall luminosities. Earlier visible-light surveys showed that such galaxies were prevalent just 2 billion years after the Big Bang. But "people didn't think they existed" in today's mature cosmos, says Daniela Calzetti (Space Telescope Science Institute), since dust formed by the galaxies' earliest stellar generations presumably would absorb later generations' ultraviolet emissions. However, "GALEX has proven this is a misconception," says Calzetti--possibly because the most rapidly evolving stars in ultraviolet-luminous galaxies disperse dust when they explode as supernovae.

Although GALEX can map starbirth regions in detail only within a few hundred million light-years, its designers hope to use its unique spectral sensitivity to quantify how rapidly galaxies have formed stars throughout much of cosmic history. GALEX codesigner David Schiminovich (Columbia University) and his colleagues have taken a first stab at this problem, showing that, when the universe was 6 billion years old, ultraviolet-luminous galaxies typically formed stars 5 to 10 times more rapidly than they do in today's 14-billion-year-old cosmos.

Some of GALEX's biggest lessons about star formation ultimately may come from quasars whose distinguishing signature is a lack of discernible ultraviolet light at key wavelengths. The ultraviolet spectra of these hyperluminous galaxy nuclei are deeply notched at particular wavelengths, a consequence of the earthbound light passing through hydrogen gas clouds in intervening galaxies. If these gas clouds are the raw material from which galaxies build new stars, then probing these clouds by using the spectra of fortuitously placed background quasars might enable astronomers to determine whether or not star-making material thins out as galaxies age. "It would be a neat story," says team member Todd Small (Caltech), if gas supplies declined in lockstep with the dwindling starbirth rates that GALEX already has documented.

Hardly any space mission goes off without a hitch. But even by the industry's standards, GALEX's road to launch was particularly rocky. A patent-infringement lawsuit deprived the spacecraft of its first gyroscopes, the maker of its radio transmitter went bankrupt, and the first grooved prism (or "grism") intended for GALEX's spectroscopy was destroyed in a flooded basement.

Since launch, however, "our in-orbit performance [has] exactly matched our calibrations," says Martin. That is, with one exception: the spacecraft's far-ultraviolet detector nearly failed last March, and as of this writing (October 2005) it was being used for only about half of the observing time available during each 96-minute orbit.

In any case, "we're wading in oceans of data already," says Martin, whose $100 million mission will still achieve most of its original goals. "There are so many things we've found in the data that we haven't had time to follow up."

GALEX at a Glance

NAME: Galaxy Evolution Explorer

LAUNCH: April 28, 2003

ORBIT: Nearly circular, 700-kilometer (420-mile) altitude

TELESCOPE: 0.5-meter f/6.0 Ritchey-Chretien telescope

INSTRUMENTS: Two circular 65-millimeter-diameter (1.2[degrees] field of view) ultraviolet light detectors (used simultaneously at different wavelengths) for imaging and spectroscopy

URL: www.galex.caltech.edu

DURATION: Funded through September 2006; spacecraft can operate far longer (no expendables)

SCIENCE GOALS:

* Map star formation in nearby galaxies

* Census star formation throughout cosmic history

* Image entire sky (except for fields containing bright stars that can damage detectors) to about 20th magnitude, and selected regions much more deeply

* Build atlases of star, galaxy, and quasar spectra shit Before joining Sky & Telescope's staff in 1995, senior editor Joshua Roth studied cosmology and gamma-ray astronomy.
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Title Annotation:National Aeronautics and Space Administration
Author:Roth, Joshua
Publication:Sky & Telescope
Geographic Code:1USA
Date:Jan 1, 2006
Words:908
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