The Sun is an ordinary G2 star, one of more than 100 billion stars in our galaxy.

            diameter:       1,390,000 km.
            mass:             1.989e30 kg
            temperature:  5800 K (surface)
                                    15,600,000 K (core)

    The Sun is by far the largest object in the solar system. It contains more than 99.8% of the total mass of the
    Solar System (Jupiter contains most of the rest).  The Sun is personified in many mythologies: the Greeks
    called it Helios and the Romans called it Sol.
 
    The Sun is, at present, about 75% hydrogen and 25% helium by mass (92.1% hydrogen and 7.8% helium by
    number of atoms); everything else ("metals") amounts to only 0.1%. This changes slowly over time as the
    Sun converts hydrogen to helium in its core.
 
    The outer layers of the Sun exhibit differential rotation: at the equator the surface rotates once every 25.4
    days; near the poles it's as much as 36 days. This odd behavior is due to the fact that the Sun is not a solid
    body like the Earth. Similar effects are seen in the gas planets. The differential rotation extends considerably
    down into the interior of the Sun but core of the Sun rotates as a solid body.

    Conditions at the Sun's core are extreme. The temperature is 15.6 million Kelvin and the pressure is 250
    billion atmospheres. The core's gases are compressed to a density 150 times that of water.
 
    The Sun's energy output (3.86e33 ergs/second or 386 billion billion megawatts) is produced by nuclear
    fusion reactions. Each second about 700,000,000 tons of hydrogen are converted to about 695,000,000
    tons of helium and 5,000,000 tons (=3.86e33 ergs) of energy in the form of gamma rays. As it travels out
    toward the surface, the energy is continuously absorbed and re-emitted at lower and lower temperatures so
    that by the time it reaches the surface, it is primarily visible light. For the last 20% of the way to the surface
    the energy is carried more by convection than by radiation. It takes 50 million years for a photon to reach the
    surface.
 
    The surface of the Sun, called the photosphere, is at a temperature of about 5800 K. Sunspots are "cool"
    regions, only 3800 K (they look dark only by comparison with the surrounding regions). Sunspots can be very
    large, as much as 50,000 km in diameter. Sunspots are caused by complicated and not very well
    understood interactions with the Sun's magnetic field.

    A small region known as the chromosphere lies above the photosphere.

    The highly rarified region above the chromosphere, called the corona, extends millions of
    kilometers into space but is visible only during eclipses (left). Temperatures in the corona are over
    1,000,000 K.

    The Sun's magnetic field is very strong (by terrestrial standards) and very complicated. Its magnetosphere
    (also known as the heliosphere extends well beyond Pluto.

    In addition to heat and light, the Sun also emits a low density stream of charged particles (mostly electrons
    and protons) known as the solar wind which propagates throughout the solar system at about 450 km/sec.
    The solar wind and the much higher energy particles ejected by solar flares can have dramatic effects on the
    Earth ranging from power line surges to radio interference to the beautiful aurora borealis.

    Recent data from the spacecraft Ulysses show that the solar wind emanating from the polar regions flows at
    nearly double the rate, 750 kilometers per second, that it does at lower latitudes. The composition of the
    solar wind also appears to differ in the polar regions. And the Sun's magnetic field seems to be surprisingly
    uniform.

    Further study of the solar wind will be done by the recently launched Wind and SOHO spacecraft from the
    dynamically stable vantage point directly between the Earth and the Sun about 1.6 million km from Earth.

    The solar wind has large effects on the tails of comets and even has measurable effects on the trajectories of
    spacecraft.

    The Sun's output is not entirely constant. Nor is the amount of sunspot activity. There was a period of very low
    sunspot activity in the latter half of the 17th century called the Maunder Minimum. It coincides with an
    abnormally cold period in northern Europe sometimes known as the Little Ice Age. Since the formation of the
    solar system the Sun's output has increased by about 40%.

    Since the Sun's birth it has used up about half of the hydrogen in its core. It will continue to radiate
    "peacefully" for another 5 billion years or so (although its luminosity will approximately double in that time).
    But eventually it will run out of hydrogen fuel. It will then be forced into radical changes which, though
    commonplace by stellar standards, will result in the total destruction of the Earth (and probably
    the creation of a planetary nebula).
 

Sun Statistics

            Mass (kg)                                                     1.989e+30
            Equatorial radius (km)                                695,000
            Mean density (gm/cm^3)                            1.410
            Rotational period (days)                             25-36*
            Escape velocity (km/sec)                           618.02
            Luminosity (ergs/sec)                                  3.827e33
            Magnitude (Vo)                                           -26.8
            Mean surface temperature                          6,000°C
            Principal chemistry
                Hydrogen                                                   92.1%
                Helium                                                        7.8%
                Oxygen                                                       0.061%
                Carbon                                                       0.030%
                Nitrogen                                                     0.0084%
                Neon                                                           0.0076%
                Iron                                                              0.0037%
                Silicon                                                        0.0031%
                Magnesium                                                0.0024%
                Sulfur                                                          0.0015%
                All others                                                    0.0015%
 

The Sun's satellites

    There are nine planets and a large number of smaller objects orbiting the Sun. (Exactly which bodies should
    be classified as planets and which as "smaller objects" has been the source of some controversy, but in the
    end it is really only a matter of definition.)
 
 
 

Planet	Distance (km)	Radius (km)	Mass (kg)	Discoverer	Date
Mercury	57,910,000	2439	3.30e^23
Venus	108,200,000	6052	4.87e^24
Earth	149,600,000	6378	5.98e^24
Mars	227,940,000	3397	6.42e^23
Jupiter	778,330,000	71492	1.90e^27
Saturn	1,426,940,000	60268	5.69e^26
Uranus	2,870,990,000	25559	8.69e^25	Herschel	1781
Neptune	4,497,070,000	24764	1.02e^26	Galle	1846
Pluto	5,913,520,000	1160	1.31e^22	Tombaugh	1930