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Structure of the SunIdentifying the Sun's Layers, Temperatures and Densities, and Solar Phenomena
Aug 29, 2010 © Harry P. Schlanger
The Sun is an amazing fiery gas ball with its layers at various temperatures and densities. It has enough energy to last 5 billion years.
Our Sun is thought of as an "energy machine". For
example, the sun's energy released in one second is so large that it could power the U.S. for 9 million years
(Briggs and Carlisle, 1996),
In order to study the sun, physicists usually divide the solar sphere into three conceptual layers,
or zones:
Temperature and Density Structure of the Sun
Figure 1 shows that below the surface, the temperature is at millions of degrees but cools to thousands
of degrees at the surface. Above its surface, the Sun's temperature suddenly rises to 2 million degrees.
Unlike temperature, the Sun's gas density is relatively high and remains fairly constant in the interior.
However, gas density drops sharply in the surface's chromosphere layer and quickly approaches that of a
vacuum when away from the surface and in space.
Figure 1. Temperature/density structure of the Sun
(Modified from Briggs and Carlisle).
Description of the Sun's Layers
The Sun's diameter is estimated to be 1.4 million kilometres, or about 109 times the diameter of Earth.
However, the Sun has no true "surface." There is nothing hard, nor is there any definite boundaries about
the solar disk. According to Briggs and Carlisle (1996), the matter making up the apparent surface is so
rarefied that it is more accurate to think of the Sun’s
boundaries as diffuse and extending far out into the solar system, well beyond Earth.
Figure 2. Schematic of the Sun's structure showing outflow of energy
(Haubold and Mathai).
Sun's Interior Layers
The Sun's interior is considered to be composed of three subregions - the core, the radiative layer and the
convective layer (see Fig 2 above).
- The Core
is the sun's source of energy produced by thermonuclear fusion.
Hydrogen is converted into Helium at a temperature of 15 million ºK. Enormous excess energy is
released as photons.
- The Radiative Layer
contains solar material, hot and dense enough, so that thermal radiation (primarily by photon
diffusion) transfers the intense heat
of the core outward to the cooler regions. The temperature cools
outwards from 7 to 2 million ºK.
- The Convective Layer
contains solar plasma hot and dense enough to transfer heat energy of the
interior outward through thermal convection. Columns are formed that carry hot buoyant
material to the surface where cooling to 5,700 ºK causes material to return down to the base of the
convective layer. Thus are formed giant convective cells.
Sun's Surface Atmospheres
The solar surface atmospheres are composed of two layers.
- The Photosphere
is the part of the Sun we can see from Earth as a yellow-white light. Its temperature is 5,700 ºK.
Below this layer, the Sun becomes opaque to visible light. Above the photosphere, visible sunlight
is free to propagate into space. Huge magnetic-field bundles may break through the photosphere,
creating cooler, darker regions, seen as sunspots that grow and retreat in an
11-year solar cycle.
- The Chromosphere
lies just above the photosphere. The prefix "chromo" refers to color, which can only be
seen when the much brighter light from the photosphere is eliminated, such as at the beginning
and end of a solar eclipse. Many solar features can be seen, for example: cellular
convection patterns, vertical jets of material called spicules, and solar flares.
The Solar Corona
The sun's temperature rises in the corona to around 2 million ºK; the reason for such high temperature is
not well understood.
The inner corona extends more than 1 million kilometres out into space and can be seen when the Moon
blocks the brilliant disk of the Sun during a total eclipse.
The corona consists of a hot gas merging gradually into the
transparent interplanetary medium, and flowing outward from it is the solar wind, which causes disturbances
in Earth's magnetic field.
The inner corona is also a large source of x-rays but which do not penetrate Earth's atmosphere
References:
- "Solar Structure"
in "Encyclopaedia of Planetary Sciences", Hans Haubold and A. M. Mathai,
Pp 786 - 794, Chapman & Hall, ISBN: 0-412-06951-2, 1997.
- "The Structure of the Sun"
in "Solar Physics and Terrestrial Effects - A Curriculum Guide for Teachers",
Roger P. Briggs and Robert J. Carlisle,
Chapter 2, Pp 7 - 15, Space Environment Center, National Oceanic and Atmospheric Administration,
Second Edition, June 1996.
- Sun, Wikipedia.
- "Pictures of the Solar Atmosphere"
Online Astronomy eText: The Sun.
The copyright of the article Structure of the Sun: Identifying the Sun's Layers, Temperatures and Densities, and Solar Phenomena is owned by Harry P. Schlanger. Permission to republish in print or online must be granted by the author in writing.
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