'Musical stars' that help scientists search for new life
Stars may be many light-years away but the sounds they produce can give scientists insights into their size, age and whether habitable Earth-like planets are nearby.
The study of star sounds, called asteroseismology, helps scientists working with the Kepler Space Telescope in their hunt for life in the depths of the Universe.
The "music" inside stars makes them resonate just like a musical instrument, said professor of astrophysics William Chaplin, who analyses data on the internal structure of pulsating stars obtained from Kepler.
When air is blown over the reed of an oboe, the reed vibrates and creates a change of pressure in the air inside it. The resultant note is characteristic of the space in which the pressure waves were generated.
"The same thing happens within a star. Sound is made in the outer layers of stars like the Sun. Because a star is a ball of ionised gas, the trapped sound inside makes it gently breathe in and out, or oscillate," said Prof Chaplin.
By observing the oscillations, it is possible for scientists to "listen" to the sound waves.
"As the star gently breathes in and out, we are able to detect the effect of the sound indirectly. As it breathes in, gas is compressed and it gets hotter. This results in the star becoming brighter. As the star breathes out and relaxes, the gas gets cooler and the star gets a bit fainter," added Prof Chaplin.
Variations in the brightness of the star can then be turned back into sound, although for a star like the Sun, the variations must be sped up around 100,000 times before sound can be heard.
By detecting and analysing the oscillations, Prof Chaplin and his colleagues are able to measure the properties of the stars, for example their size, mass and age. They can even do the equivalent of a CT scan to pick apart the structure within the star.
The pitch produced by a star informs scientists of its size - in the same way a large instrument like a tuba produces a low pitch, whereas the smaller piccolo trumpet produces a higher pitch.
These measurements are important for Kepler's goal of detecting Earth-like planets in the habitable zones of other stars.
This so-called "goldilocks" zone must be the right temperature for liquid water to be present on the surface of the planet.
Kepler has received extra funding from Nasa to continue until 2016 , even in the face of severe budget cuts in some other areas of the agency's activities. The mission was otherwise due to finish towards the end of 2012.
By studying the sounds of the stars, Prof Chaplin and his colleagues were able to confirm that the recently discovered planet Kepler 22-b is close enough to its host star to be classified as habitable. It orbits a star about 600 light-years from Earth and has a temperature of about 22C.
The Kepler telescope measures the change in the brightness of a star, caused by a planet passing in front of it. By observing the amount of light that has been blocked, it gives an indication of the size of the planet relative to the star.
Once an Earth-sized planet is detected, questions about its age and size become important. Prof Ronald Gilliland, a science co-investigator with Kepler, said scientists would then be able to ask whether other planets like Earth exist.
"There is great public interest in learning whether other planets like our own are common in the galaxy, and most importantly being able to speculate with good founding whether or not conditions are conducive to life on other planets.
"What we hope to achieve with Kepler in the next few years are important steps towards the ultimate goal to see if there is evidence for intelligent life."
But he said that it would be impossible to travel between stellar systems, due to the limitations of the current rocket and spacecraft technology.
Even if another civilisation was as little as four light-years away, it would take eight years to hear an answer to a question over the radio.
In order to discover whether new life does exist, the next step would be to analyse a new planet's atmosphere, said Douglas Gough, emeritus professor of astrophysics at Cambridge.
"If you look at the spectrum of a star, you can see all the colours of a rainbow. Thin dark lines between the colours are produced by the interaction of light with chemical elements of the atmosphere of the star, or the planet around the star."
It is these patterns of elements that will allow scientists to establish the atmosphere of new planets, in a similar way that helium was discovered in the Sun. Only then will scientists further their understanding of whether life exists on other planets.