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# Characteristics of the Sun

The table below gives the basic characteristics of the Sun. Four observed quantities, the value of GMs, the mean radius, the solar constant, and the value of the astronomical unit, are used to derive other quantities in the table. These observed quantities along with the adopted sidereal period and the obliquity to the ecliptic are taken from Yoder (1995).[1] The last-two quantities are taken from Norton's 2000.0 Star Atlas.[2] Go to table of characteristics.

## Characteristics of the Sun

 GMs 1.327124399(45)l026 cm3 s?2 Mass 1.9881033 g Mean Radius 6.9601010 cm Surface Gravity 2740 cm s?2 Escape Velocity 6.175107 cm s?1 Average Density 1.408 g cm?3 Adopted Sidereal Period 25.38 d Obliquity to the Ecliptic 7 15′ Luminosity 3.8461033 ergs s?1 Solar Constant 1.3676106 ergs cm?2 s?1 Effective Temperature 5778K Absolute Visual Magnitude 4.83 Spectral Type G2V

The value of GMs is determined from the relationship of orbital radius to period of the planets, asteroids, and artificial satellites orbiting the Sun. The Sun's mass is derived using the most recent value for the gravitational constant: G = 6.67(4210)l0?8 cm3 g?1 s?2. The Sun's mean radius is the mean radius of the Sun's photosphere, which is the region emitting most of the Sun's optical radiation into space. The acceleration of gravity, the escape velocity. and the average density are calculated directly from the mass and radius.

The adopted sidereal period is an average rotation period?the rotation rate of the Sun's surface is dependent on solar latitude. The obliquity to the ecliptic is the tilt of the Sun's rotation axis relative to the ecliptic.

The solar constant is the energy flux at 1 AU. Despite its name, the solar constant is not constant; the flux averaged over a year can vary by 0.04% over the course of the 11-year solar cycle, and at solar maximum, sunspots can cause 0.25% changes in the solar flux over the one-month rotation period of the Sun. The Sun's luminosity is the total power emitted by the Sun as electromagnetic radiation at all frequencies. The black-body temperature is the temperature a thermal source the size of the Sun must have to produce an equivalent luminosity; the solar spectrum is close to a black-body spectrum.[3]

The absolute visual magnitude of a star is a logarithmic measure of a star's flux at visible frequencies for observers 10 parsecs away. A star's spectral type is a classification that indicates a star's surface temperature and brightness.[4]

[1] Yoder, Charles F. ?Astrometric and Geodetic Properties of Earth and the Solar System.? In Global Earth Physics: A Handbook of Physical Constants, edited by T.J. Ahrens, 1?31. AGU Reference Shelf, No. 1. Washington: American Geophysical Union, 1995.

[2] Ridpath, I., ed. Norton's 2000.0 Star Atlas and Reference Handbook. 18th ed., 81. New York: John Wiley & Sons, 1989.

[3] A black-body spectrum, which is also called a Planck spectrum, is the spectrum found when light is in thermal equilibrium with its environment. For example, if one had a hollow metal sphere of a single temperature, with the metal able to absorb and emit light, then the distribution of light frequencies inside the cavity is described by a black-body spectrum with the temperature of the sphere. The black-body spectrum is a function of frequency and temperature only.

[4] A star's spectral type is an indication of the color, and therefore the temperature, of the star. Running from blue (high temperature) to red (low temperature), the standard classes are O, B, A, F, G, K, and M. There are other letters in the system that are used for unusual classes of stars. A number from 0 to 9 after the letter indicates in tenths how close the star is to the next lower (redder) class. The luminosity of the star is indicated by the roman numeral. From brightest to dimmest, these run from I to VII. In this system, the Sun is cool and dim.