vendredi 7 septembre 2018




Although it was not until the seventeenth century that astronomers finally realized, the Sun is not a particular star of the Universe, but simply a star like the others.

The only thing that sets it apart from other stars is its proximity. The Sun is thus the only star sufficiently close to the Earth to be studied in detail, the only one from which we can observe the surface and the close environment with precision. The study of the Sun is therefore a fundamental step in our general understanding of the stars.

A gigantic ball of gas

The Sun is a relatively simple body, a gigantic ball of gas 1.4 million kilometers in diameter, 110 times the size of the Earth. Its mass is about 2000 billion trillions of kilograms, 330,000 times that of the Earth. About 75 percent of this mass is composed of hydrogen, 25 percent helium and the rest (0.1 percent) consists of heavier elements

The interior of the Sun being inaccessible to observation, it is principally necessary to resort to the theory to describe the phenomena that occur there and to determine its internal structure.
This interior is divided into three zones: the nucleus, the radiative zone and the convective zone.

The core of the Sun

The nucleus is the part in which the energy of the Sun is created by nuclear reactions. The temperature is extremely high, about 15 million degrees. This region represents 25 percent of the Sun's diameter and, because of its high density, contains nearly 60 percent of the total mass of our star.

The radiative zone of the Sun

Around the nucleus comes the radiative zone, which represents 45 percent of the Sun's radius. In this region, the energy created in the nucleus is transported to the outside by the photons.

This mode of transport is very slow because the photons are constantly absorbed and then re-emitted by all the particles present. It is estimated that the "time" put by a photon to get out of the sun is several hundreds of thousands of years, while it would take a few seconds if there were no obstacles on the way.

The convective zone of the Sun

Finally, we arrive at the outer layer, the convective zone, which represents 30 percent of the solar diameter and where the temperature drops below one million degrees.

In this layer, the energy transport is done by convection, that is to say by movements of all the material present. The hot gas of the depths thus rises to the surface, releases energy by cooling, then plunges inwards and so on.

As we continue to move away from the Sun, we arrive at what we can consider as its surface, although it is not really a well-defined limit. This region of a few hundred kilometers thick is called the photosphere. The temperature decreases only slightly, from 6000 to 4000 degrees, but the density decreases very rapidly.

For this reason, all the layers of gases located beyond are very tenuous and therefore transparent. Thus, the photosphere is the last opaque and shiny layer and it is it that we see when we look at the Sun. Moreover, as the density drop is very fast, the contours of this region are well defined, which explains why the solar disk has a clear outline rather than fuzzy boundaries.

The surface of the Sun

The Sun's surface is far from uniform. High resolution observations show that the photosphere has a grainy appearance. At any moment, millions of grains are visible on the solar disk, with an average size of a thousand kilometers. Successive images show that the appearance of the surface varies very rapidly because each grain lives only a few minutes.

Thanks to spectral analysis, astronomers have shown that these grains are linked to convection in layers near the surface. The hot gas rises from the depths and reaches the surface at the center of the grains, then spreads as it cools, before plunging inward to the edges of the grains. Thus, the gas that springs in the center of the grains has a temperature 300 degrees higher than the one that plunges to the edges, and it is this difference of temperature, and therefore of luminosity, which gives rise to the granular aspect of the solar disk.

Note that more in-depth occur other gas movements on a larger scale. These movements define huge cells up to 30,000 kilometers in diameter and have a lifespan of the order of a day.


Other phenomena affect the photosphere more transiently. Sunspots are the best-known example since Chinese astronomers already observed them more than a thousand years ago. These are small, dark areas that vary in size from a few thousand to a hundred thousand kilometers and last from a few days to several months.

There are also bright regions, called facula, which appear a little before the spots and persist several weeks after the disappearance of these.

The solar cycle

The continuous observation of the Sun has shown that the number of spots is not constant but varies strongly with time. It oscillates between zero and a maximum value with a cycle that lasts 11 years. The last maximum date of the year 2000 and the next one will occur at the beginning of 2012.

Sunspots are regions of the photosphere where the temperature is slightly lower than average, about 4000 degrees instead of 6000. They emit a little less light than their neighborhood and appear dark in contrast. Their spectral analysis revealed the presence of a very intense magnetic field. This is most likely the cause of the difference in temperature, although the exact mechanism is not yet very clear.

Several hypotheses have been put forward. In particular, it is possible that the magnetic field prevents ascending hot gas currents from reaching the surface, but it is also possible that intense magnetic waves are emitted at the spots, which would imply a loss of energy, therefore a cooling.

Solar magnetism

The 11-year cycle of sunspots is related to the presence of a magnetic field combined with two other phenomena: the differential rotation of the Sun and the convection movements near its surface. By differential rotation, it must be understood that our star does not turn on itself as a rigid body. On the contrary, each zone of given latitude turns at a speed different from the others. For example, near the poles, a complete turn takes place in 35 days, while it lasts only 25 days near the solar equator.

To explain how the 11-year cycle is produced, use the concept of field lines. These are imaginary lines that indicate the direction of the magnetic field at all points and are very useful as a means of representation.

In a period of calm, when there is no visible spot, the field lines simply connect the two poles of the Sun to each other, following more or less the axis of the latter. It is then the differential rotation that begins to disturb things. Because of it, in fact, the field lines turn faster at the equator than at the pole. This forces them to curl up on themselves and get closer to each other.

After a large number of rotations, the field lines finally resemble spirals strongly coiled on themselves and very concentrated in the equatorial regions, which results in a very intense magnetic field.

In the meantime, convective movements near the surface also affect the field lines by deforming and twisting them. It is then possible from time to time for a very twisted field line to emerge from the convective zone and to form a loop outside the Sun. It is at the foot of this loop, where the line crosses the photosphere, that two sunspots appear. It is thus that the spots, paired by two, are gradually born, and that the sun is covered with dark spots.

Finally, in the middle of the cycle, the multiplication of loops causes interactions between the different magnetic regions. These lead to a general decrease in intensity and a redistribution of field lines between different spots. When this recombination step is complete, the field lines have resumed the spiral appearance strongly wound, but in the opposite direction to the previous one. It only remains for the differential rotation to unroll the lines so that they return to their original appearance and the Sun returns to a calm period without sunspots.

The chromosphere

Leaving the photosphere, we enter a very thin layer called the chromosphere. This layer has a thickness of a few thousand kilometers and the temperature goes up from 4000 to 10,000 degrees. Due to its very low density, one millionth of that of the photosphere, this layer is almost transparent and therefore invisible in daylight. It is nonetheless observable during solar eclipses and then appears as a very thin reddish ring that surrounds the lunar disk.

A relatively simple way to study the chromosphere without waiting for an eclipse is to observe the Sun in a wavelength corresponding to a line of hydrogen called H alpha. In this wavelength, the hydrogen atoms of the chromosphere absorb light from the photosphere and re-emit it to the outside. By thus observing the Sun, the photosphere is invisible and only the chromosphere appears.

This type of observation has shown that the chromosphere is far from uniform. Its outer border is surmounted by a multitude of vertical peaks, called spicules, which live on average for about ten minutes. These are jets of gas ejected from the chromosphere at twenty kilometers per second and which penetrate the outer region over several thousand kilometers.

The solar corona

Continuing to move away from the Sun we reach the outer limit of the chromosphere, a few thousand kilometers from the surface. After this limit, the temperature suddenly starts to increase in a dizzying way to reach very quickly a few hundreds of thousands of degrees: we entered the solar corona.

This region stretches for millions of kilometers and is highly variable. It is even less dense than the previous one, of the order of one-ten-billionth of the density of the photosphere. Its temperature is extreme and reaches up to a few million degrees.

Solar protuberances

One of the most spectacular phenomena in the crown is the formation of protuberances. These are gigantic columns of gas, warmer but denser than that of the crown, which are born close to the surface and can extend over hundreds of thousands of kilometers.

Some protuberances, called quiescent, take on an arch form and can last for several months. Others, described as eruptive, are rather vertical and evolve quickly in a few minutes.

The protuberances are observable either beyond the solar disk, in the form of long, shiny flames, or on the disk, where they appear very dark in contrast to the brilliant background, and are then also called filaments.

Solar flares

The crown is sometimes agitated by even more violent phenomena called solar flares. In a few minutes, small areas of the inner crown rise to five million degrees and remain at that level for nearly an hour. In this short time, these highly localized regions can release a significant fraction of the energy emitted by the entire Sun.

In addition, eruptions are often accompanied by coronal mass ejections. Billions of tons of material are then projected to the interplanetary medium at speeds of several hundred kilometers per second.

Sun observations in X-rays

Further details of the processes involved in the corona have been brought to us by observations in the X-rays. Indeed, since the coronal gas is at a temperature of several millions of degrees, it is in this domain of length wave that emits the most radiation.

Such observations can only be made from space. Several space-based instruments were launched to carry them out, with the Skylab US station in the mid-1970s, the SMM satellite in the 1980s and the European SOHO spacecraft in 1995.

X-ray observations have shown that the distribution of gas in the corona is very inhomogeneous. In particular, they identified two particular types of regions. First the active regions, very bright areas in X-rays that are subject to an intense magnetic field and are probably related to sunspots in the photosphere. Then coronal holes, areas of low light in X-rays, in which the density and temperature of the gas are lower than average. It is through these coronal holes that most of the energetic particles transit before leaving the Sun.

The solar wind

As the temperature is extremely high in the crown, the particle's stirring speed is so great that it can escape the attraction of the sun. Even in times of relative calm, a large amount of electrons, protons, and other energetic particles - about two million tons of matter per second - escape from the Sun and get lost in the interplanetary environment.

As we move away from our star, the corona looks less and less like an atmosphere and transforms into a continuous stream of particles called the solar wind. As the density and pressure of the gas decreases with the distance to the Sun, the particles gradually gain in speed, to far exceed that of sound. At the level of the Earth, their speed is of the order of 500 kilometers per second, with a density of ten particles per cubic centimeter.

THE HISTORY OF ASTRONOMY : THE SUN Reviewed by INVENTION on septembre 07, 2018 Rating: 5 THE SUN Although it was not until the seventeenth century that astronomers finally realized, the Sun is not a particular sta...