Kepler and the Convection, Rotation and Planetary Transits (COROT) are measuring short 5-15 minute changes in the brightness of a star. These brightness changes are due to the ringing or oscillation of the star's material. "Triggered by the turbulent rise and fall of hot gases on the star's surface, the vibrations penetrate deep into the stellar interior and become resonating tones that reveal the star's size, composition and mass".
"The sounds are internal vibrations that reveal themselves as a subtle, rhythmic brightening and dimming of a star, explains Chaplin, an astrophysicist at the University of Birmingham, UK, and a specialist in asteroseismology." [aforementioned Nature article]
What we call 'sound' is a pressure wave of energy moving through matter. There are lots of types of pressure waves besides 'sound'-- earthquakes, for example, are pressure waves moving through rock, water waves are energy moving through water. Waves can be up-and-down transverse waves or compression-like longitudinal waves, as this excellent primer from Kettering University shows:
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| Transverse wave |
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| Longitudinal wave |
Wave waves have both transverse and longitudinal motion. The key with any wave is that it's transferring energy, not material. The above images are not of material moving from one end to the other, but of the wave moving while the material largely stays in the same place. This is a key concept with waves-- they require some matter to move through, but they don't transfer the matter from one end to the other, they transfer energy from one to the other via the matter.
With astroseismology, we can figure out the waves from the brightness changes. It's not as direct as earth seismology, where we can actually measure the ground vibration of the wave directly, but it is both distinct and accurate enough to understand the wave's energy and how long it takes the wave to get from one side of the star to the other.
Knowing this, we can then deduce what's inside the star-- stuff we can't see-- based on how the wave going through it behaves. Imagine being in a pool with a blindfold and sending a wave out with a push of your hand, then feeling the returning wave. You would feel a difference if the pool was filled with water (weak wave return), with jello (very bouncy wave return), or with gravel (no wave comes back). The wave gave you information about the material you can't see.
Many of us use seismology at home. Ever tap on a wrapped present to try and figure out what is inside? Knock on a wall while hanging a picture to see if it's hollow or has a support you can drive a nail into? Pick up a drink container and shake it to hear how full it is? Our sense of touch in part uses intuitive seismology. If you had big enough hands and could survive high temperature, pinging a star to hear how it rings and resonates would give you good information on its contents. For now, we have to be content with their own natural ringing due to self-generated activity on their surfaces.
Aside from the science value, it's rather neat that stellar objects having their own inherent ringing and acoustics. I have discussed sonification-- the conversion of electromagnetic data to sound in order to discern patterns. It's a neat technique I'm using with my own Project Calliope satellite. These Kepler and COROT (as well as earlier solar observations of our own sun's helioseismology) are not sonification, they are truly sounds produced in stars. You can cue overused cliches like 'celestial music' or 'music of the spheres', but even those do not diminish the coolness of realizing that giant sun-sized or larger objects are pulsating with sound.
The sounds do not travel off the stars, because sound cannot travel through a vacuum. Remember, waves are energy traveling through matter. No matter = no wave = no sound. But even if a star makes a sound no one hears, the sound still exists-- and gives us information. That's science that's worth listening too.
Until next time,
Alex
working on a new schedule for 2012
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