STATS ON OUR OWN LIFE GIVING
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The Sun is a normal
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).
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
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 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
The Sun's magnetic field is very strong (by
terrestrial standards) and very complicated. Its
magnetosphere (also known as the
heliosphere) extends well beyond
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
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
Further study of the solar wind will be done by the
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 (left).
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.)
Distance Radius Mass
Planet (000 km) (km) (kg) Discoverer Date
--------- --------- ------ ------- ---------- -----
Mercury 57,910 2439 3.30e23
Venus 108,200 6052 4.87e24
Earth 149,600 6378 5.98e24
Mars 227,940 3397 6.42e23
Jupiter 778,330 71492 1.90e27
Saturn 1,426,940 60268 5.69e26
Uranus 2,870,990 25559 8.69e25 Herschel 1781
Neptune 4,497,070 24764 1.02e26 Galle 1846
Pluto 5,913,520 1160 1.31e22 Tombaugh 1930
More detailed data and definitions of terms can be found on the
More about the Sun
- Is there a causal connection between the Maunder Minimum and the Little
Ice Age or was it just a coincidence? How does the variability of the Sun
affect the Earth's climate?
- Several careful
experiments have failed to detect the expected flux of
neutrinos from the Sun. The explanation will probably turn out to be just
a minor glitch in some esoteric calculation. But that's what they said in 1900
orbit of Mercury.
- Since all the planets except
orbit the Sun within a few degrees of the plane of the Sun's equator, we know
very little about the interplanetary environment outside that plane. The
Ulysses mission will provide information about the polar regions of the
- The corona is much hotter than the photosphere.
Overview ... Sun ...
last updated: 2002 May 01