After years of silence, star finally 'sings' to astronomers
As cosmic virtuosos go, a swollen star in the constellation Hydra clearly qualifies as a sub-basso profundo.
For billions of years, however, this "voice" has gone unheard until now.
A team of astronomers from the European Southern Observatory's La Silla facility in Chile is reporting the detection of seismic "oscillations" in xi Hya, a sun-like star 130 light-years away that has expanded into a red giant. The observations represent the first time such signals have been detected in such a highly evolved star, the team says.
Those signals, first detected in our sun 30 years ago, open a unique window for astronomers on a star's structure and composition.
Typically, researchers start with only two characteristics as they study a star: its luminosity and its temperature, notes Fabien Carrier, an astronomer at the Observatoire de Geneve, in Sauverny,
Switzerland, and one of the leaders of the team reporting the results. But luminosity and temperature "trace only the physical conditions prevailing in a thin layer" known as the photosphere, a star's external envelope, he adds.
Just as geologists can learn about the Earth's structure by studying the way seismic waves from earthquakes pass through the planet, astronomers can use a star's seismic signals to probe its interior, he says.
The sun has been under systematic seismic observation since 1995, when the US-led Global Oscillation Network Group, or GONG, finished installing special cameras at six sites around the world to collect data on the sun's "star quakes." These quakes, or oscillations, are triggered as the star's nuclear furnace and convection bring its luminous gas to a state akin to a spaghetti pot's rolling boil.
The ESO team used a highly stable spectroscope to gather information on the star's oscillations, collecting data every two hours over the course of a month whenever the star was visible.
The oscillations up-and-down movements in the star's outer layer appear as slight shifts in the spectrum of chemical elements the star emits. These Doppler shifts indicate movement toward and away from a viewer as well as the speed of the movement. A shift toward one end of the spectrum and back represents one cycle.
Compared with the scales humans are used to, stellar oscillations occur at a lethargic rate. On a piano, the string for middle C oscillates 261.63 times, or cycles, each second when struck. Xi Hya's oscillations, which appear at its surface, occur at an average rate of one cycle every three hours. These rates are akin to measuring a car's gas mileage in terms of gallons per mile rather than miles per gallon.
To bring xi Hya's voice within the range of human hearing, the ESO team had to raise the star's "pitch" by a factor of 1 million, yielding sounds more like the thundering approach of the Four Horsemen of the Apocalypse than the music of the spheres.
The team used a relatively modest telescope, whose light-gathering mirror measures 1.2 meters (3.9 feet) across. This would allow the study of stars out to about 200 light years, Dr. Carrier notes.
But a new spectrograph is being built for a 3.6 meter telescope, he adds, that will allow the team to study the oscillations of stars as far as 1,800 light years away, giving researchers a larger number of stars to work with at different stages of evolution. Those distance ranges are expected to expand with the launch of orbiting telescopes by Canada and the European Space Agency over the next several years.