The Sun is a normal G2 star, one of more than 100 billion stars in our
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
It is often said that the Sun is an "ordinary" star. That's true in the sense
that there are many others similar to it. But there are many more smaller stars
than larger ones; the Sun is in the top 10% by mass. The median size of stars in
our galaxy is probably less than half the mass of the Sun.
The Sun is personified in many mythologies: the Greeks called it Helios and
the Romans called it Sol.
The Sun is, at present, about 70% hydrogen and 28% helium by mass everything
else ("metals") amounts to less than 2%. 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 the 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. At the center of the core the Sun's density is more than 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.
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 rarefied 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
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 during the minimum of the
solar cycle 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.
During the solar maximum, however, the solar wind moves at an intermediate
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.
Spectacular loops and prominences are often visible on the Sun's limb
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 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
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.)
Bill Arnett; last updated: 2003 Oct 26