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The Sun - Structure and Characteristics

The Sun - Structure and Characteristics

The Sun is a star around which the Earth rotates.

It has a diameter of approximately 1,400,000 km and a mass of 1,99 × 1033 g.

The Sun revolves around itself, however, since it is constituted by a great mass of gases, the different regions do not revolve solidarily, but they do it at different speeds, that depend on the latitude.

The Sun, and with it the whole solar system, moves to a point of the firmament located in the constellation of Hercules at a speed of about 19 km / s. This is because the Sun has a translational movement around the galactic center, like all the stars of the Galaxy, whose period is 200 million years.

The Sun is formed by a number of concentric layers, but the only ones that can be directly observed are the outer ones, which are called the photosphere, chromosphere and crown, respectively, and constitute the so-called solar atmosphere. Each of these layers has distinctive properties that are its own. The transition from the properties of one layer to those of the next does not take place abruptly, but gradually.

Model of the internal structure of the Sun

Layers of the Sun - Fotosfera, cromosfera, corona Fotosfera, the innermost layer of the Sun

The photosphere is the innermost layer and is responsible for almost all visible light that the Sun emits into space. For its innermost side, it limits with the convective region of the Sun, and its outermost side, with the chromosphere. Its approximate width is 400 km, and the temperature decreases from a value of 7,500 K, in the innermost regions, up to a value of 4,700 K, to the outer ones. The average pressure of the photosphere is only a few hundredths of the atmospheric pressure at sea level, while the density is only one ten millionth of the density of the Earth's atmosphere at sea level. Observed in the telescope, the photosphere does not have a uniform appearance, but appears to consist of small cells of granular aspect, called rice grains, which appear separated by darker regions in which the temperature is smaller.

Chromosphere, intermediate layer of the Sun

The chromosphere is the middle region of the solar atmosphere, and during eclipses of the Sun, in the moments before the plenitude phase, it appears as a very fine reddish arc surrounding the eclipsed disk of the Sun. Under conditions Normal, that is, when there is no eclipse, it is not possible to observe the chromosphere so that light from the photosphere attenuates its much weaker brightness. It is supposed to have a width of 2,000 to 3,000 km, but its upper end is covered by a forest of luminous gas springs, called spicules, making it difficult to determine exactly the true dimensions. The density of the chromosphere decreases from the innermost to the outermost regions, but the temperature increases at the same time from 4,500 K to 100,000 K.

Crown, outer layer of the Sun

The crown is the outermost region of the solar atmosphere, and during total eclipses appears as a whitish halo that surrounds the eclipsed disk of the Sun. Its width is a few million kilometers, but the total luminosity is equivalent only to Half the brightness of the full moon. Analogously to the chromosphere, its view is also not possible outside the total eclipses, because the powerful luminosity of the neighboring photosphere masks the presence. The density of matter at the base of the crown is 109 atoms / cm2, which equals 10-10 times the density of the Earth's atmosphere at sea level.

The temperature of the crown usually oscillates around a million kelvins, but in some of its regions can be much higher. For a long time the cause of these temperatures has been a mystery, but today it is believed that they are the shock waves, originated by certain convective currents that occur in the photosphere, the cause of the warming of the crown and probably also of the chromosphere.

For the study of these two solar regions the coronagraph is used. By analyzing sunlight with a spectrometer, a continuous emission spectrum is obtained on which numerous dark emission lines, Fraunhofer, are superimposed. The continuous component of the spectrum originates in the photosphere, and especially in its more superficial regions.

Indeed, in the photosphere there is at all times a dynamic equilibrium between the rate of creation of negative ions of hydrogen and the rate of destruction of these same ions, so that there is always a negative ion of hydrogen per million atoms of hydrogen. The formation of these ions takes place by means of the absorption of photons that come from the interior of the Sun, whereas their destruction is accompanied by the emission of photons towards the outside of the Sun, which constitute almost all the visible component of Solar radiation.

It is necessary, however, to establish three hypotheses concerning the solar photosphere. According to the first, the photosphere must be in hydrostatic equilibrium; The second assumes that it must also be in thermal equilibrium; And, according to the third, its chemical components require at least 90% hydrogen.

On the other hand, the absorption spectrum of the Sun occurs due to the presence in the outer layers of the solar atmosphere of atoms capable of absorbing the photons of certain well-defined wavelengths; Therefore, some radiation from the interior of the photosphere is selectively absorbed, resulting in the corresponding dark rays of the spectrum. From these, it has been possible to determine the chemical composition of the outer regions of the Sun. It has been found that hydrogen and helium together constitute 96 to 99% of the photosphere, and that the rest is constituted by the other elements Chemical, of which at the moment has been identified about 60. It has been detected also the existence of about 18 types of molecules in the coldest regions of the solar surface.

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Last review: November 9, 2016

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