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The Sun is an ordinary G2 star, one of more than 100 billion stars in our galaxy. 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 (approximately the inner 25% of its radius) 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.
A small region known as the chromosphere lies above the photosphere.
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, ACE 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%.
The Sun's satellitesThere 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.)
Bulk Parameters
Model values at center of Sun: Central pressure: 2.477 x 1011 bar Central temperature: 1.571 x 107 K Central density: 1.622 x 105 kg/m3 Rotational and Orbital Parameters
*This is the adopted period at 16 deg. latitude - the actual rotation rate
varies with latitude L as: Sun Observational ParametersApparent diameter from Earth
At 1 AU (seconds of arc) 1919.
Maximum (seconds of arc) 1952.
Minimum (seconds of arc) 1887.
Distance from Earth
Mean (106 km) 149.6
Minimum (106 km) 147.1
Maximum (106 km) 152.1
Solar Magnetic FieldTypical magnetic field strengths for various parts of the Sun Polar Field: 1 - 2 Gauss Sunspots: 3000 Gauss Prominences: 10 - 100 Gauss Chromospheric plages: 200 Gauss Bright chromospheric network: 25 Gauss Ephemeral (unipolar) active regions: 20 Gauss Solar AtmosphereSurface Gas Pressure (top of photosphere): 0.868 mb
Effective temperature: 5778 K
Temperature at bottom of photosphere: 6600 K
Temperature at top of photosphere: 4400 K
Temperature at top of chromosphere: ~30,000 K
Photosphere thickness: ~400 km
Chromosphere thickness: ~2500 km
Sun Spot Cycle: 11.4 yr.
Photosphere Composition:
Major elements: H - 90.965%, He - 8.889%
Minor elements (ppm): O - 774, C - 330, Ne - 112, N - 102
Fe - 43, Mg - 35, Si - 32, S - 15
Open Issues
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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.
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.
Spectacular loops and prominences are
often visible on the Sun's limb (left).
The Sun is about 4.5 billion years old.
Since its 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).
