4. Pulse of the Sun
Sounds inside the Sun
Astronomers peel back the outer layers of the Sun, and glimpse inside it by examining sounds with different paths within the Sun. They use the term helioseismology to describe such investigations of the solar interior. It is a hybrid name combining the Greek word helios for Sun or light, the Greek word seismos meaning quake or tremor, and logos for reasoning or discourse. So literally translated helioseismology is the logical study of solar tremors. Geophysicists similarly unravel the internal structure of the Earth by recording earthquakes, or seismic waves, that travel to different depths; this type of investigation is called seismology. The techniques resemble the way that Computed Axial Tomography (CAT) scans derive views inside our bodies from numerous readings of X-rays that cross them from different directions.
The photosphere’s oscillations are the combined effect of about 10 million separate notes. Each of these notes travels along a unique path inside the Sun, and sounds of different frequency or pitch descend to different depths (Fig. 4.2). Some of them stay within the convective zone, while others travel to the very center of the Sun. To trace our star’s physical landscape all the way through – from its churning convective zone down into its radiative zone and core – we must determine the precise pitch (frequency) of all the notes.
The combined sound of all the notes reverberating inside the Sun has been compared to a resonating gong in a sandstorm, being repeatedly struck with tiny particles and randomly ringing with an incredible din. The Sun produces order out of this chaos by reinforcing certain notes that resonate within it, like the plucked strings of a guitar. They are called standing waves.
Scientists have examined the oscillation power in the various surface oscillations, or how often each and every note is played, confirming that the power is concentrated into such resonant standing waves. Instead of meaningless, random fluctuations, orderly motions are detected with specific combinations of size and period, or wavelength and frequency (Fig. 4.3). Destructive interference filters out all but the resonant waves that combine and reinforce each other. Yet, there are still millions of such notes resonating in the Sun, so prolonged observations with high spatial resolution and detailed computer analysis are required to sort them all out.
The dominant factor affecting each sound is its speed, which in turn depends on the temperature and composition of the solar regions through which it passes. The sound waves move faster through higher-temperature gas, and their speed increases in gases with lower than average molecular weight. So, by investigating many different waves we can build up a very detailed three-dimensional picture of the physical conditions inside the Sun, including the temperature and chemical composition, from just below the surface down to the very core of the Sun. The observed frequencies are integral measures of the speed along the path of the sound wave; helioseismologists have to use complex mathematical techniques and powerful computers to invert this measured data and get the sound speed.
Small sound-speed discrepancies between measurements and theory are significant (Fig. 4.4). Just below the convection zone, there is an increase in the observed sound speed compared to the model, suggesting that turbulent material is mixing in and out within this base layer. Since the speed of sound depends on temperature as well as composition, the temperature might also increase in this place. Without additional information, scientists cannot distinguish between temperature and compositional mixing as causes of discrepancies from standard models. There is a sharp decrease of the observed speed relative to theoretical expectations at the boundary of the energy-generating core, hinting that either cooler material or turbulent churning motions might occur there. Scientists speculate that they could be due to unstable nuclear burning processes. If substantiated by further studies, this could be very important for studies of stellar evolution; they usually assume that nuclear reactions proceed without any mixing of fresh material into the core.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University