The sun is smaller than we thought!

It is often claimed that the first test that Einstein’s theory of general relativity passed was to explain the slow anomalous precession of the perihelion of Mercury’s orbit, that is, the fact that each orbit of the planet does not pass through an ellipse, but that it point of this orbit closest to the sun shifts every time.

In fact, the reality is less simple, because instead of changing the law of NewtonNewton of the gravitygravity It could be maintained for some time that the shape of the sun was not well known, leading to the possibility of the spread of massesmasses other than that of a perfectly spherical and homogeneous mass, where distributions produce a field of gravitygravity different. This is the case with the Earth, which, like the Sun, is mainly in rotation, so that the equilibrium figure of the Earth under the combined effect of its gravity and the centrifugal forcecentrifugal force is that of a rotational ellipsoid flattened at the poles. The phenomenon also applies to the Sun, with the additional influence of its magnetic field on its ionized gas.

In the decades following the discovery of general relativitygeneral relativityimproved the measurement of the Sun’s shape and it became clear that the precession of Mercury’s perihelion was indeed a consequence of the theory ofEinsteinEinsteinafter taking into account the normal part of the precession due to the gravitational perturbations of the other large attracting bodies of the Solar systemSolar systemand that this could not be explained by the shape of the sun.

The moral of this story is that even a trivial improvement in determining the shape of the sun can have important consequences. Let us imagine that Mercury’s precession was indeed caused by an ellipsoid of solar mass rotation; Einstein’s theory, at least in its original form, would have been refuted!

We understand why we put it online arXiv of one article, of two astrophysicistsastrophysicists from the University of Cambridge and the University of Tokyo, is attracting some attention because it represents a new determination of the size of the sun.

Jean-Pierre Luminet talks to us about helioseismology. © Jean-Pierre Luminet, YouTube

Waves and solar sound waves

According to these researchers, our starstar is smaller than we thought. The reduction is small, only a few hundredths of a percent compared to the radius determination obtained by studying the size of the photospherephotosphere in the case that solar eclipsessolar eclipses. Recall that the photosphere is, in a sense, the visible surface of the Sun above which the solar plasma resides transparenttransparent. Below this gasgas ionized behaves like one fogfog so we cannot see the interior of the sun.

However, the conversion to our star of techniques has been in progress for several years now seismologyseismology on Earth had led to the belief that the determination of its size based on the study of the photosphere was biased. Technically the seismic wavesseismic waves f-waves are used, which are the analogues of waveswaves on Earth, but right on the surface of the photosphere.

The two researchers confirmed this reduction using current measurements of P waves sound wavessound waves in the solar plasma, waves of BusyBusy in the sun, as in the case of seismic waves, also called P on Earth in rocks.

The reduction in size of the sun hasskysky You’re welcome, but it imposes limitations on the details of thermonuclear fusion reactions in the Sun and on its internal structure. However, the Sun is in some sense the most important laboratory for testing our ideas about the structure and evolution of stars around the world. cosmoscosmos observable stars which in turn influence the evolution of galaxiesgalaxies.

Any better understanding of the Sun therefore influences our understanding of the stellar world.

What do we call the laughter of the stars? Observing stars at different wavelengths is not enough to know what they are made of. The light variations observed on their surface are associated with sound waves that propagate inside and make them vibrate. By transcribing these light vibrations, researchers specializing in asteroseismology can understand what’s happening in the hearts of stars, by inferring their rotation rates and their lifespans… So, if you’re ready to hear the sun laugh , then start this episode! © CEA

60 years of helioseismology

To understand more precisely what the researchers based their work on, it is necessary to provide some explanation about helioseismology, a applicationapplication special aspect of what we can call asteroseismology and which can be applied to other stars and even to gas planetsgas planets as JupiterJupiter And SaturnSaturn. By including what Futura has already explained on this topic, we can begin to remember that it is a young disciplineastrophysicsastrophysics which arose from a discovery involving the Sun in 1960. Its full importance was not understood until the 1970s, when astrophysicists trying to unravel the secrets of our star showed that they could play a game similar to that of their geophysical colleagues on Earth, with the resolutionresolution inverse problems with gravitational field measurements, magnetic fieldmagnetic field but precisely in this case with seismic waves.

The common idea is easy to understand. It corresponds to reconstructing the size, shape and composition of a musical instrument by analyzing all its elements more and more accurately and completely. frequenciesfrequencies and amplitudes in the sounds this instrument can emit. As with any inverse problem, it involves returning signal data to the source and thus doing the opposite of predicting signals based on the characteristics of this source. The technique is particularly effective at earthquakesearthquakes that occur on Earth and are obviously more difficult to implement in the case of the Sun.

However, it is possible to do this and to do this it is necessary to measure the spectral shifts produced by theDoppler effectDoppler effect coming from the mattermatter on the surface of the sun, which vibrates like the membrane of a drum. It was in 1960 that extremely weak pulsations from the Sun with a period of five minutes were first demonstrated in this way by Robert B. Leighton (whose name is associated with the famous lecture by his colleague Richard Feynman at Caltech). More generally, the moodmood that can be detected are mainly the manifestation of the modes of two types of waves, which are themselves produced, especially by the movementsmovements turbulent solar plasma in its convective zone, as Roger Ulrich, Robert Stein and John Leibacher came to understand ten years later.

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