The Sun

Distance

d = 1 AU = 150 million km = 500 light seconds
How measured: radar to the planets.
 

Radius

R = 1 R = 700,000 km = 110 Earth radii.
How measured: geometry using angular size and distance.
 

Mass

M = 1 M = 333,000 Earth masses = 2 x 10³⁰ kg.
How measured: from earth's orbit and period using Newton's version of Kepler's 3rd law.

Density

Average = 1.4 x density of water = 1/4 average density of earth.
How determined: by definition, density = mass/volume.
The atmosphere is much less dense than air, while the core is believed to be more than 100 times the density of water.

Luminosity

L = 1 L = 4 x 10²⁶ watts
How determined: from brightness at 1 AU and inverse square law.

Temperature

At surface T = 5800 K = 10,000 °F
How determined: from color, spectrum.
Core temperature is calculated to be about 15 million K.

Composition

Atmosphere is approximately 3/4 H, 1/4 He, 2% "metals," of which the most abundant are O, C, Ne, Fe, N, Si (by mass)
How measured: from absorption lines in spectrum.
Core is thought to be depleted in H, enriched in He

There are many spectacular images and movies of the sun from space at the SOHO, Yohkoh, and TRACE websites. You can find up-to-the-minute news of events on the sun and their effects on the earth at SpaceWeather.com and also at Space Weather Now. The Stanford Solar Center is a very good source of information and up-to-date images.

For some fascinating history see Great Moments in Solar Physics, which is part of the High Altitude Observatory Education Pages, an excellent source of information on the sun.

You can find further information on the sun by following links from the web pages of Bill Arnett or, if you can stand the obnoxious ads, those of Calvin J. Hamilton. (Note that some sources give composition by number of atoms rather than by mass. A collection consisting of 9 hydrogen atoms, each having a mass of 1 unit, plus 1 helium atom, with a mass of 4 units, is 90% hydrogen by number but 9/13 = 69% hydrogen by mass.)