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What Are the Layers of the Sun?

The Sun is made up of 3 layers. From the inside out we have:

What are the layers of the Sun?
  1. The photosphere
  2. The Chromosphere
  3. The Crown

Photosphere, the Innermost Layer of the Sun.

The photosphere is the innermost layer of the Sun and is responsible for almost all of the visible light that the Sun emits into space. On its inner side, it borders the convective region of the Sun, and on its outer side, the chromosphere.

Observed in the telescope, the photosphere does not appear uniform, but appears to consist of small, granular-looking cells called rice grains. These cells appear separated by darker regions where the temperature is lower.

Dimensions and Temperature in the Photosphere

The width of the photosphere is approximately 400 km, and the temperature decreases from a value of 7,500 kelvin, in the innermost regions, to a value of 4,700 kelvin, in the outermost 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.

Chromosphere, Middle Layer of the Sun.

The chromosphere is the middle region of the solar atmosphere.

During eclipses of the Sun, just before the fullness phase, it appears as a very fine reddish arc, which surrounds the eclipsed disk of the Sun. In normal conditions, that is, when there is no eclipse, it is not possible observing the chromosphere so that the light from the photosphere dims its much dimmer brightness.

Dimensions and Temperature of the Chromosphere

The chromosphere is supposed to be 2,000 to 3,000 km wide, but its upper end is covered by a forest of luminous gas springs, called spicules, making it difficult to pinpoint the true dimensions.

The density of the chromosphere decreases from the innermost to the outermost regions, but the temperature increases from 4,500 kelvin to 100,000 kelvin at the same time.

Corona, the Outermost Layer of the Sun

The corona is the outermost region of all the Sun's layers.

During total eclipses, it 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 only half the luminosity of the full moon.

Analogous to the chromosphere, its vision outside of total eclipses is also not possible, because the powerful luminosity of the neighboring photosphere masks the presence. The density of matter at the base of the corona is 109 atoms / cm 2 , a value that is equivalent to 10 -10  times the density of the Earth's atmosphere at sea level.

At What Temperature Is the Sun's Corona?

The temperature of the corona generally ranges around a million kelvins, but in some of its regions it can be much higher.

The cause of these temperatures has long been a mystery, but it is now believed to be shock waves, caused by certain convective currents occurring in the photosphere, the cause of warming of the corona and probably also of the chromosphere.

What Is a Crown Bite For?

The coronagraph is used to study these two solar regions.

Analyzing sunlight with a spectrometer produces a continuous emission spectrum on which numerous dark emission streaks are superimposed, said by Fraunhofer. The continuous component of the spectrum originates in the photosphere, and especially in its shallowest regions.

Hydrogen Ions in the Corona 

In the photosphere there is at all times a dynamic balance between the rate of creation of negative hydrogen ions and the rate of destruction of these same ions, such that there is always one negative hydrogen ion for every million hydrogen atoms.

The formation of these ions takes place by means of the absorption of photons that come from the interior of the Sun, while their destruction is accompanied by the emission of photons towards the exterior of the Sun, which thus constitute almost all of the visible component of solar radiation.

Interaction with the Photosphere

However, it is necessary to establish three hypotheses regarding the solar photosphere:

  1. The photosphere must be in hydrostatic equilibrium.
  2. The photosphere must also be in thermal equilibrium.
  3. Among its chemical components, there must be 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 photons of certain well determined wavelengths; therefore, some radiation from the interior of the photosphere is selectively absorbed, giving rise to the corresponding dark streaks of the spectrum.

From these concepts it has been possible to determine the chemical composition of the external regions of the Sun.

It has been seen that hydrogen and helium together make up 96 to 99% of the photosphere, and that the rest is made up of the other chemical elements, of which about 60 have currently been identified.

The existence of some 18 types of molecules in the colder regions of the solar surface has also been detected.


Published: April 11, 2020
Last review: April 13, 2020