It may seem unbelievable, but sound waves travel through the surface of the stars with which astronomers are able to intuit what is happening in the interior areas, hidden from their telescopes. This peculiar branch of modern astrophysics is known as asteroseismology . In the same way that geophysicists use seismic waves to study the interior of the Earth and unravel the structure of our planet, asteroseismologists do the same with the stars.
Evidently, the first star where these very weak oscillations were discovered was our Sun. In the 1960s, solar telescopes revealed thatacoustic waves similar to those of our earthquakes ran through its surface, only with a totally different origin and that has to do with the way in which the star transports the energy produced in the nucleus to the outside. In stars like the Sun, the energy produced in the central nuclear furnaceis transmitted to the surface by convection, the mechanism used by fluids to heat up: when boiling water in a pot, it bubbles because the hot material from the bottom rises and the cold material from the surface falls. It is curious that this mechanism of energy transmission was discovered in the kitchen. In fact, its discoverer was an adventurous scientist and schoolteacher named Benjamin Thompson, later Earl Rumford of the Holy Roman Empire. Married to the widow of the great French chemist Lavoisier, one day he wondered why his applesauce took longer to cool than his soup. We now know that a viscous liquid takes longer because it moves with more difficulty, which is precisely the liquid’s way of transporting energy from one point to another. In the Sun the gas is virtually boiling and bubbles of hot gas rise to the surfaceat a speed close to the speed of sound. Just as we can hear water boiling in a pot, turbulent convection on the Sun creates a well-known type of noise, random noise. Unfortunately, it is foundbelow the threshold of human hearingeither. The set of these sound waves propagates through the solar interior and is reflected by the surface, causing it to oscillate like a bell. This beating is what provides, through the amplitude and frequency of the oscillations, vital information to understand the physical conditions of the solar interior.
the star bell
Now, if we want to imagine the tolling of these immense gas bells, let’s forget about the clapper’s blow. A much more accurate image is that of thousands of grains of sand crashing against a church bell . Our Sun behaves, then, like the resonant cavity of a violin or a guitar, and what physicists know as standing waves are produced on its surface. A curious name to describe a type of waves that appear when, for example, we fix a string at both ends and pluck it this is what happens in all stringed instruments. In the case of the Sun, the resulting standing waves are of the same type as those produced inside organ pipes.
Obviously, if the Sun is a normal star, the rest must also show these oscillations. The problem with observing them is that they are very weak and, therefore, it is necessary to achieve enormous precision in the measurements. It was in 2001, thanks to the efforts of Swiss astronomers François Bouchy and Fabien Carrier of the Geneva Observatory, that it was first observedthe ringing of another star, Alpha Centauri A, the main component of the closest star system to the Sun composed of three stars.Alpha Centauri Ais the main component of the system, a yellow dwarf very similar to the Sun, both in its size (its radius is 1.23 times that of theSun), its mass (10% higher) and its temperature (between 5,790 and 5,850 ºK, compared to 5,780 for theSun). Nor is its luminosity excessively greater: only one and a half times more than that of our star. The observations ofAlpha Centauri AThey were conducted in May 2001 over five nights at the Swiss Leonard Euler telescope at the La Silla Observatory in Chile. To do this, they used the CORALIE spectrograph of this telescope, which had proved tremendously useful in detecting exoplanets: the spectrograph was capable of identifying the almost imperceptible oscillating movement of the star caused by the gravitational pull of the planet. Thus, the same technique can be used to study the oscillations on the surface of stars caused by acoustic waves, which cause it to periodically pulse in and out. The oscillations that the Swiss astronomers detected were minuscule, with speeds of up to 35 cm per second, which means thatthe surface of Alpha Centauri A rises and falls only 40 meters. Taking into account that the radius of the star is 875,000 kilometers, we can understand the exquisiteness and finesse of this detection. Bouchy and Carrier’s measurements showed that this star, very similar to our own, pulsates following a 7-minute cycle.
The passage of time is not only marked by the clock of the village church…
