Space physics--the role of the Sun

(under construction)

by Wm. Robert Johnston
last updated 2 February 2005

The Sun is a star. Most stars visible in the night sky are much brighter intrinsically than the Sun, but most stars in the universe are dimmer than the Sun.

The Sun is a sphere of hydrogen (78%), helium (20%), other elements (2%), all plasma or gas. Its tendency to collapse inward from its own gravity is balanced by energy from thermonuclear reactions in core: pressure and temperature in the center are high enough that hydrogen atom nuclei fuse (through several steps) into helium nuclei, releasing much energy.

Energy release is related to the lower mass of the resulting helium nuclei by E=mc2. About 0.5% of the mass is converted to energy in the reactions, so every second 4.3 million (metric) tons of the Sun's mass is converted into EM radiation.

Energy is carried 70% of the distance towards the Sun's surface by radiation, most of the remaining distance by convection, and from the Sun's surface by radiation.

The visible surface of the Sun is the photosphere (temperature 5600° C). Sunspots here are transient regions of cooler (3000° C) gas, which appear dark because they give off less light.

The Sun's magnetic field is complex, since the fluid Sun distorts this field. Results include sunspots, prominences, flares.

Above the surface is the corona, which is very low density very hot gas. Charged particles stream away from the Sun as the solar wind; this interacts with the Earth's magnetic field.

Diameter of Sun: 1,392,500 km (109 times the Earth)
Mass: 332,900 times the Earth's mass
Distance: 149,600,000 km average

© 2005 by Wm. Robert Johnston.
Last modified 2 February 2005.
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