What happens when neutron stars meet?

What happens when two neutron stars meet? They don't exactly shake hands and talk about the weather. Scientists say that a merger of two neutron stars results in the creation of a black hole along with a powerful, short burst of gamma-ray light.

On May 9, the NASA Swift satellite detected and analyzed a so-called short gamma-ray burst. Scientists have been waiting for this opportunity for about 30 years, since the discovery of the gamma-ray burst phenomenon.

Scientists think they know a thing or two about the "long" gamma-ray bursts -- those lasting more than two seconds. These seem to be from the explosion of massive stars. As the star explodes, it creates a black hole in its place and sends a beam of gamma rays racing across the Universe. Scientists have studied thousands of long gamma-ray bursts in the past three decades.

Short gamma-ray bursts -- less than two seconds and sometimes only a few milliseconds -- are harder to study. As you can imagine, they are too quick to catch. Gamma-ray bursts in general are random and fleeting. But imagine trying to study something that's here and gone in a few milliseconds, occurring somewhere "out there" in the vast Universe. It's a daunting task. The Swift satellite was designed to catch these short bursts. Each burst triggers the satellite to turn automatically to the origin to search for clues.

That's what happened, for the first time, on May 9. Swift detected a 50-millisecond burst and turned fast enough to see the afterglow, which itself faded in five minutes. Many short bursts have been detected before, but never has a short burst afterglow been seen. From the clues left in the May 9 afterglow, scientists found evidence for a neutron star merger.

The "neutron star merger" theory has been around for several decades. All the evidence from the May 9 event strongly supports the theory. For example, the explosion appears to have taken place in the outer reaches of a close, old galaxy. Old galaxies contain old stars that are more likely to have evolved into neutron stars, compared to young galaxies with young stars. Also, massive star explosions tend to occur in crowded, dusty regions filled with star birth. Most observations of long gamma-ray bursts support this. Yet this short gamma-ray burst on May 9 is not from a region of new star creation.

The May 9 burst is not the clincher for the merger theory, however, even with more analysis. What is needed is more short burst afterglows. Seeing how Swift caught this one after only a few months on the job, chances are that a neutron star meeting will be seen soon enough.

This week's question comes from Christopher Wanjek. Mr. Wanjek is a science writer supporting the Beyond Einstein initiative, a roadmap to understand the forces of nature beyond General Relativity and Quantum Mechanics through the study of the Universe from the Big Bang to black holes.