The Sun

16.3 Solar Activity


The Sun is the star at the center of the Solar System and is by far the most important source of energy for life on Earth.

Layers of the Sun

The layers are

  • Core

  • Radiative Zone

  • Convection Zone

  • Photosphere

  • Chromosphere

  • Transition Region

  • Corona


The core is the central area of the sun that helps convert hydrogen to helium when it consumes the gas. The temperature of the core is about 27,000,000° F, with a density of about 150 g/cm³. As one moves away from the core, the temperature and density steadily decrease.

Radiative Zone

The layer of the Sun's interior where energy is usually transported toward the outside by means of radiative diffusion and thermal conduction (instead of convection). Energy travels through here in the form of electromagnetic radiation as photons.

Convection Zone

A region of turbulent plasma between a star's core and its visible photosphere at the surface, through which energy is transferred by convection. In the convection zone, hot plasma rises, cools as it nears the surface, and falls to be heated and rise again.


The photosphere is a star's outer shell from which light is radiated. It is a star's outer shell from which light is radiated. The photosphere is typically used to describe the Sun's or another star's visual surface.


The chromosphere ("sphere of color") is the second of the three main layers in the Sun's atmosphere and is roughly 2,000 kilometers deep. It sits above the photosphere and below the solar transition region. The density of the chromosphere is very low and it is normally invisible and can only be seen during a total eclipse, where its reddish color is shown.

Transition Region

The transition region is a thin and very irregular layer of the Sun's atmosphere that separates the hot corona from the much cooler chromosphere. Heat flows down from the corona into the chromosphere and in the process produces this thin region where the temperature changes rapidly from 1,000,000°C (1,800,000°F) down to about 20,000°C (40,000°F). Instead of hydrogen, the light emitted by the transition region is dominated by such ions as carbon, oxygen, and silicon (each with three electrons stripped off).


A corona is an aura of plasma that surrounds the sun and other celestial bodies. The Sun’s corona extends millions of kilometres into space and is most easily seen during a total solar eclipse, but it is also observable with a coronagraph.


Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They correspond to concentrations of magnetic field flux that inhibit convection and result in reduced surface temperature compared to the surrounding photosphere.


A prominence is a large, bright, gaseous feature extending outward from the Sun's surface, often in a loop shape. Prominences are anchored to the Sun's surface in the photosphere, and extend outwards into the Sun's corona.


A solar flare occurs when magnetic energy that has built up in the solar atmosphere is suddenly released. Radiation is emitted across virtually the entire electromagnetic spectrum, from radio waves at the long wavelength end, through optical emission to x-rays and gamma rays at the short wavelength end.


An aurora is a natural light display in the sky, predominantly seen in the high latitude (Arctic and Antarctic regions. Auroras are produced when the magnetosphere is sufficiently disturbed by the solar wind that the trajectories of charged particles in both solar wind and magnetospheric plasma mainly in the form of electrons and protons, precipitate them into the upper atmosphere, where their energy is lost.