From Core to Corona

Layers of the Sun

Image Credit: p. 110,125, Kaler

The Core

The innermost layer of the sun is the core. With a density of 160 g/cm³, 10 times that of lead, the core might be expected to be solid. However, the core's temperature of 15 million kelvins (27 million degrees Fahrenheit) keeps it in a gaseous state.

In the core, fusion reactions produce energy in the form of gamma rays and neutrinos. Gamma rays are photons with high energy and high frequency. The gamma rays are absorbed and re-emitted by many atoms on their journey from the envelope to the outside of the sun. When the gamma rays leave atoms, their average energy is reduced. However, the first law of thermodynamics (which states that energy can neither be created nor be destroyed) plays a role and the number of photons increases. Each high-energy gamma ray that leaves the solar envelope will eventually become a thousand low-energy photons.

The neutrinos are extremely nonreactive. To stop a neutrino, one would have to send it through a light-year of lead! Several experiments are being performed to measure the neutrino output from the sun. Chemicals with which neutrinos react are put in large pools in mines, and the neutrinos' passage through the pools can be measured by the chemical changes in the pools. For example, perchloroethane contains some isotopes of chlorine with 37 particles in the nucleus (17 protons, 20 neutrons). These Cl³⁷ molecules can take in neutrinos and become radioactive Ar³⁷ (18 protons, 19 neutrons). From the amount of argon present, the number of neutrinos can be calculated.

Solar Envelope

Outside of the core is the radiative envelope, which is surrounded by the convective envelope. The temperature is 4 million kelvins (7 million degrees F). The density of the solar envelope is much less than that of the core. The core contains 40 percent of the sun's mass in 10 percent of the volume, while the solar envelope has 60 percent of the mass in 90 percent of the volume.

The solar envelope puts pressure on the core and maintains the core's temperature.

The hotter a gas is, the more transparent it is. The solar envelope is cooler and more opaque than the core. It becomes less efficient for energy to move by radiation, and heat energy starts to build up at the outside of the radiative zone. The energy begins to move by convection, in huge cells of circulating gas several hundred kilometers in diameter. Convection cells nearer to the outside are smaller than the inner cells. The top of each cell is called a granule. Seen through a telescope, granules look like tiny specks of light. Variations in the velocity of particles in granules cause slight wavelength changes in the spectra emitted by the sun.

Image Credit: p. 369, Chaisson

"Convective cells are arranged in tiers containing cells of progressively smaller size as the surface is neared. This is still a highly simplified diagram, however. There are many different cell sizes, and they are not so neatly arranged." (369, Chaisson)

Photosphere

The photosphere is the zone from which the sunlight we see is emitted. The photosphere is a comparatively thin layer of low pressure gasses surrounding the envelope. It is only a few hundred kilometers thick, with a temperature of 6000 K. The composition, temperature, and pressure of the photosphere are revealed by the spectrum of sunlight.

Chromosphere

In an eclipse, a red circle around the outside of the sun can sometimes can be seen. This is the chromosphere. Its red coloring is caused by the abundance of hydrogen.

From the center of the sun to the chromosphere, the temperature decreases proportionally as the distance from the core increases. The chromosphere's temperature, however, is 7000 K, hotter than that of the photosphere. Temperatures continue to increase through the corona.

Sunspots

Sunspots are dark spots on the photosphere, typically with the same diameter as the Earth. They have cooler temperatures than the photosphere. The center of a spot, the umbra, looks dark gray if heavily filtered and is only 4500 K (as compared to the photosphere at 6000K). Around it is the penumbra, which looks lighter gray (if filtered). Sunspots come in cycles, increasing sharply (in numbers) and then decreasing sharply. The period of this solar cycle is about 11 years.

The sun has enormous organized magnetic fields that reach from pole to pole. Loops of the magnetic field oppose convection in the convective envelope and stop the flow of energy to the surface. This results in cool spots at the surface which produce less light than the warmer areas. These cool, dark spots are the sunspots.

Corona

The outermost layer of the sun is the corona. Only visible during eclipses, it is a low density cloud of plasma with higher transparency than the inner layers. The white corona is a million times less bright than the inner layers of the sun, but is many times larger.

The corona is hotter than some of the inner layers. Its average temperature is 1 million K (2 million degrees F) but in some places it can reach 3 million K (5 million degrees F).

Temperatures steadily decrease as we move farther away from the core, but after the photosphere they begin to rise again. There are several theories that explain this, but none have been proven.

Photo Credit: Astronomical Society of the Pacific; used with permission.

The picture of the sun that looks smooth (above) was taken with heavily filtered visible light.

Solar Flares

In the corona, above sunspots and areas of complex magnetic field patterns, are solar flares. These sparks of energy sometimes reach the size of the Earth and can last for up to several hours. Their temperature has been recorded at 11 million K (20 million degrees F). The extreme heat produces x rays that create light when they hit the gasses of the corona.

Prominences

Prominences are generally less violent than solar flares. They are "cool sheets of gas that condense out of the corona above the active regions. Some are quiet and hang there for weeks, others rain matter down on the photosphere, still others literally explode into space, pushing the corona out in front of them in a great burst that carries the gas off the sun altogether." (113, Kaler)

Solar Wind

The solar corona is constantly losing particles. Protons and electrons evaporate off the sun, and reach the earth at velocities of 500 km/s. Most of the mass of the sun is held in by magnetic fields in the corona, but particles slip through occasional holes in the fields. Solar wind affects the magnetic fields of all the planets in the solar system. When the solar wind hits the Earth's magnetic field, the wind compresses the field and creates a shock wave called the Bow shock. Closer to the Earth are the Van Allen radiation belts where solar particles are trapped due to magnetic forces. Still closer are huge rings of electric current around the poles, formed by the influence of the solar wind on the magnetic field. Earth, Jupiter, Saturn, Uranus, and Neptune have magnetotails where the wind extends their magnetic field.

The heliopause is the boundary where the sun's solar wind hits the gasses of interstellar space. The sun's particles flow at least to Neptune, and probably farther. That means that we're inside the sun!

See also The Solar Wind

Computer Model of the Sun at 4.5 Billion Years

% radius	Radius (10⁹ m)	Temperature (10⁶ K)	% Luminosity	Fusion Rate (joules/kg-sec)	Fusion Power Density (joules/sec-m³ )
0	0.00	15.7	0	0.0175	276.5
0.09	0.06	13.8	33	0.010	103.0
0.12	0.08	12.8	55	.0068	56.4
0.14	0.10	11.3	79	.0033	19.5
0.19	0.13	10.1	91	.0016	6.9
0.22	0.15	9.0	97	0.0007	2.2
0.24	0.17	8.1	99	0.0003	0.67
0.29	0.20	7.1	100	0.00006	.09
0.46	0.32	3.9	100	0	0
0.69	0.48	1.73	100	0	0
0.89	0.62	0.66	100	0	0

(Stromgrew, 483)

Bibliography and Additional References:

Bahcall, J.N. Neutrino Astrophysics. Cambridge University Press, 1989.

Chaisson, Eric and Steve McMillan. Astronomy Today . New Jersey: Prentice Hall, 1993.

Kaler, James B. Stars . New York: Scientific American Library, 1992.

B. Stromgrew (1965) reprinted in D. Clayton Principles of Stellar Evolution and Nucleosynthesis. New York: McGraw-Hill, 1968.

Page written by Hannah Cohen.
Last Revised 15-Oct-97 by Robert Heeter.