It is often claimed that Einstein’s theory of general relativity and the slow anomalous precession of the perihelion of Mercury’s orbit have passed the first test, i.e. the fact that each of the planet’s orbits loops not around an ellipse but through a point. This orbit closest to the Sun changes every time.

In fact, the reality is less simple because instead of modifying Newton’s law of gravitation we could maintain for the time being that we did not know the size of the Sun well, thereby creating the possibility of different mass distributions. and homogeneous mass distributions producing a distinct gravitational field. This is the case with the Earth, which above all, like the Sun, is rotating so that the Earth’s equilibrium figure under the combined influence of gravity and centrifugal force resembles an ellipsoid of rotation flattened at the poles. This phenomenon also applies to the Sun and its magnetic field has an additional effect on ionized gas.

In the decades following the discovery of general relativity, measurements of the Sun’s size improved and it became clear that Mercury’s precession of perihelion was actually a consequence of Einstein’s theory, once we took into account the usual part of precession. There was gravitational perturbation from other large attractive objects in the Solar System, and we cannot account for it from the Sun’s size.

The gist of this story is that even a minor improvement in determining the size of the Sun can have significant consequences. Let us imagine that the precession of Mercury was actually caused by the ellipticity of the rotation of the solar mass, Einstein’s theory would be refuted, at least in its initial form!

So we understand why it is being put online arXiv An article by two astrophysicists from the University of Cambridge and the University of Tokyo is attracting some attention because it concerns a new determination of the Sun’s size.

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

waves and solar sound waves

According to these researchers, our star is smaller than previously thought. This reduction is small, only a few hundredths of a percent, compared to the radius determination provided by studying the size of the photosphere during a solar eclipse. Remember that the photosphere is the visible surface of the Sun above which the solar plasma is transparent. Below, this ionized gas behaves like a fog that blocks us from seeing the Sun’s interior.

However, for several years already, the transfer of Earth seismological techniques to our star has led to the assumption that the determination of its size based on the study of the photosphere was biased. Technically, the seismic waves used are F waves, which are similar to waves on Earth but at the surface of the photosphere.

The two researchers today confirmed this reduction in size using measurements related to P waves, which are sound waves in the solar plasma, pressure waves inside the Sun, as is the case with so-called seismic waves. Even in the rocks on earth.

The reduction in the Sun’s size may not seem like much, but it places constraints on the details of thermonuclear fusion reactions inside the Sun and its internal structure. However, the Sun is in some ways the main laboratory for testing our ideas about the structure and evolution of stars in the observable universe, stars which in turn influence the evolution of galaxies.

So any better understanding of the Sun affects our understanding of the star universe.

What do we call the laughter of the stars? Observing stars at different wavelengths is not enough to know what formed them. The variations of light observed on their surface are associated with sound waves that propagate inside and cause them to vibrate. By recording these vibrations of light, researchers specializing in asteroid science can understand what is happening in the hearts of stars, their rotation speed, their lifetimes, etc. So, if you look at the Sun Ready to hear the laughs, let’s launch this episode! © CEA

60 years of helioseismology

To understand more precisely on what the researchers have based their work, it is necessary to give some clarifications about helioseismology, which we can call asteroseismology and which can also be applied to other stars and even gaseous ones such as Jupiter and Saturn. Can be applied to planets also. , Taking into account what Futura has already explained on the subject, we can begin to remember that it is a young discipline of astrophysics that was born in the 1960s from a discovery related to the Sun. Its full significance has not only been understood since the 1970s. When astrophysicists tried to uncover the secrets of our star, they showed that they could, by solving inverse problems with measurements of the gravitational field, the magnetic field, but in this case with seismic waves, much like their geophysicist colleagues on Earth. One can play a game in comparison. ,

The general idea is easy to understand. It is equivalent to reconstructing the shape, size and structure of a musical instrument by analyzing as accurately and completely as possible all the frequencies and amplitudes inherent in the sounds emitted by this instrument. Like any inverse problem, this involves returning the signal data to its source and therefore doing the inverse of predicting the signals from the characteristics of this source. This technique is particularly effective for Earthquakes and is obviously more difficult to implement in the case of the Sun.

However, it is possible to do so and to do so requires measuring the spectral shift produced by the Doppler effect coming from material on the Sun’s surface, which vibrates like the membrane of a drum. It was in 1960 that extremely weak pulsations of the Sun with a period of five minutes were first observed by Robert B. It was demonstrated this way by Leighton (whose name is associated with the famous lecture of his colleague Richard Feynman at Caltech). More generally, the vibrations that can be detected are primarily the expression of two types of wave modes, which are themselves generated, in particular, by the turbulent movements of the solar plasma in its convection zone, as The sightings have begun. A decade later, Roger Ulrich, Robert Stein and John Leibacher are involved.

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