SCIENCE

Notebook

X-Ray Reveals More Stars

After scanning the heavens for a decade, a venerable NASA satellite appears to have discovered the origin of the "X-ray Milky Way," the mysterious X-ray counterpart to the profusion of visible stars that make up the galaxy.

The new research suggests that scientists have undercounted the number of stars in the galaxy -- perhaps by billions. NASA said the findings by a team from Germany's Max Planck Institute for Astrophysics and Russia's Space Research Institute would be published in upcoming editions of the journal Astronomy & Astrophysics.

The X-ray Milky Way is a high-energy, invisible background haze that emanates from the Milky Way like the loom of light over a city at night. Only inside the city can an observer see the individual "point sources" -- from streetlamps, signs and headlights.

Scientists for years have known that the galaxy had much more X-ray haze than observable point sources suggested, and theorized that the glow may have been hot, interstellar gas. But by using data from the sensitive X-ray detectors aboard NASA's Rossi X-ray Timing Explorer, launched in 1995, the German-Russian team determined that the Milky Way is rife with very faint, previously undetectable, dim "white dwarf" stars interacting in binary pairs with normal stars, and emitting X-rays as a consequence.

"The XTE was able to count enough X-ray stars that are bright and close," said XTE project scientist Jean Swank of NASA's Goddard Space Flight Center, and use that number to estimate the total point sources in the galaxy.

-- Guy Gugliotta

Original DNA Strand Preservation

One of the many marvels of DNA is that its design allows it to be easily copied. The molecule comes apart into two halves that are complements of each other (although not exact mirror images). Each half can be used as a template to make a new complementary half, which is what happens when one cell divides into two.

But during that process, copying errors can occur and produce mutations in the newly made DNA chains. More than 25 years ago, a scientist theorized that one way to prevent such errors from descending to subsequent generations was to have one of the two "daughter" cells produced by cell division keep an old strand of DNA. They would be preserved over time as a master template.

Research published last week in Science suggests this theory may be true.

Animals have paired chromosomes, one from each parent. (Human beings have 23 pairs). The first time a chromosome pair is copied, each daughter cell gets a pair that contains "original" strands of ancestral DNA. The current view is that in subsequent divisions, however, those original strands will be randomly distributed to descendants. Most of the grand-, great-grand and great-great-granddaughter cells have chromosomes whose DNA strands are copies of copies of copies.