Listening to stars twinkle
We’re bombarded by sounds that tell us about the world around us. From ringing phones through wailing sirens to birdsong, the slight difference in time it takes for sounds to reach each of our ears reveals the direction sounds are coming from and helps us make sense of the world around us. Now scientists are starting to make sense of the universe by using sounds to find out what’s going on inside distant stars.
Peering inside a star
The inner workings of stars were a mystery until scientists noticed sound waves travelling through the Sun affected the way its surface moved. According to a new field of study, asteroseismology, regions of expanding and contracting plasma quite high in a star’s atmosphere generate sound waves that can be used to reveal the secrets of the stars.
The moving regions of plasma create sound waves with a range of frequencies. Most of these waves die out but particular frequencies create so-called standing waves which persist within the star. However, the temperature and composition of the star change the way waves oscillate, altering the frequencies that can create standing waves.
The standing waves create patterns on the surface of a star. Each pattern is known as a mode and relates to a specific frequency.
Looking for a beat
Our Sun is close enough to observe individual regions of its surface moving, but with distant stars observing this fine detail isn’t possible. Fortunately, the standing waves change the shape of the star slightly, thereby altering its brightness.
Asteroseismologists, like Chris Copperwheat, watch for periodic changes to the brightness of a star to deduce valuable information about the star; “Different modes penetrate to different depths, by identifying these different modes you can essentially map a temperature and composition profile as we go down into the star. The more modes you have, the more detailed map you can create,” says Chris.
These profiles also give clues about the star’s history, something that is not possible from just looking at the star. “The temperature and composition of a star change with age, so asteroseismology deepens our understanding of stellar evolution”, explains Chris.
NASA’s Kepler telescope, although intended to spot exoplanets, has been used for asteroseismology. “Kepler is a game changer, it’s been fantastic with some amazing data published but asteroseismology is always moving on, aiming for more sensitive instruments to pick out lower amplitude signals”, Chris says.
With more new telescopes planned, such as the E-ELT (European Extra Large Telescope), astroseismologists are sure these revealing stellar sounds won’t fall on deaf ears.
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