Time may fly like an arrow here on Earth, but in a strong gravitational field, it crawls like a turtle, according to Einstein's general theory of relativity. Now astronomers have measured this slowing for the first time in a neutron star. The result will open the way to a better understanding of the physical make-up of these ultradense stellar corpses, and it is already hinting that they might not contain weird brews of quark matter.
Neutron stars are the collapsed cores of exploded giant stars. They pack roughly 1.5 times the mass of the sun into a sphere as small as a city. A rough estimate, however, isn't good enough for astronomers who want to learn about the properties of these balls of nuclear matter and whether some of them consist of even denser and weirder quark matter. The new result is an important first step toward that goal, says astrophysicist Walter Lewin of the Massachusetts Institute of Technology in Cambridge.
The strategy was to examine the so-called gravitational redshift of x-rays from the star. Because extreme gravity slows down time, radiation will shift toward the red part of the spectrum. Using a sensitive spectrometer on the European x-ray satellite XMM-Newton, the astronomers studied bursts of x-rays from the surface of a neutron star known as EXO 0748-676. The spectral lines of highly ionized iron and oxygen turned up at wavelengths that are 35% longer than their laboratory values, Jean Cottam of NASA's Goddard Space Flight Center in Greenbelt, Maryland, and her colleagues report in the 7 November issue of Nature.
According to general relativity, the gravitational redshift is proportional to the mass and inversely proportional to the radius of a celestial body. So with the gravitational redshift in hand, astronomers will be able to calculate the neutron star's radius from its mass, which they hope to determine next month by measuring the orbital motion of its companion star. While the current measurements are only a step toward sizing up the neutron star, they are almost certainly inconsistent with a star made of quark matter, says co-author Mariano Mendez of Space Research Organization Netherlands in Utrecht.
"It's a difficult analysis," Lewin says of the current findings. "But it looks as if they're found the holy grail." Lewin plans to use XMM-Newton next year to measure the gravitational redshift of another neutron star. "So yes, I've been scooped. But no, I'm not sad. It's a great result."
--GOVERT SCHILLING
Related sites
XMM-Newton observatory
More about gravitational redshift and general relativity
Neutron star basics
