Our Sun is a normal main-sequence [url=http://nineplanets.org/help.html#obaf]G2[/url] star,
one of more than 100 [url=http://nineplanets.org/help.html#billion]billion[/url] stars in our galaxy.
[url=http://nineplanets.org/data1.html]diameter[/url]: 1,390,000 [url=http://nineplanets.org/help.html#km]km[/url].
[url=http://nineplanets.org/data1.html]mass[/url]: [url=http://nineplanets.org/help.html#expnot]1.989e30[/url] [url=http://nineplanets.org/help.html#kg]kg[/url]
[url=http://nineplanets.org/data2.html]temperature[/url]: 5800 [url=http://nineplanets.org/help.html#kelvin]K[/url] (surface)
15,600,000 K (core)
The Sun is by far the [url=http://nineplanets.org/datamax.html#largest]largest[/url] object
in the solar system.
It contains more than 99.8% of the total mass of the Solar System
([url=http://nineplanets.org/jupiter.html]Jupiter[/url] 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
[url=http://www.pantheon.org/articles/h/helios.html]Helios[/url] and the Romans
called it [url=http://www.pantheon.org/articles/s/sol_2.html]Sol[/url].
The Sun is, at present, about 70%
[url=http://cst.lanl.gov/CST/imagemap/periodic/1.html]hydrogen[/url]
and 28% [url=http://cst.lanl.gov/CST/imagemap/periodic/2.html]helium[/url]
by mass everything else
("[url=http://nineplanets.org/help.html#metal]metals[/url]")
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
[i]differential rotation[/i]: 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 [url=http://nineplanets.org/overview.html#gas_p]gas
planets[/url]. 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 [b]core[/b] (approximately the inner 25% of its radius) are extreme. The temperature is
15.6 million Kelvin and the pressure is
250 billion [url=http://nineplanets.org/help.html#atm]atmospheres[/url].
At the center of the core the Sun's density is more than 150 times that of water.
The Sun's power (about 386 billion billion megaWatts)
is produced by [url=http://nineplanets.org/help.html#fusion]nuclear fusion[/url] 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 [url=http://nineplanets.org/help.html#convection]convection[/url] than by radiation.
[url=http://www.solarviews.com/cap/sun/sunspot.htm][img(150px,100px)]
http://nineplanets.org/thumb/sun06.jpg[/img][/url]The surface of the Sun, called the [b]photosphere[/b],
is at a temperature of about 5800 K.
[b]Sunspots[/b]
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 [b]chromosphere[/b] lies above the photosphere.
[url=http://www.solarviews.com/cap/sun/eclips94.htm][img(150px,150px)]
http://nineplanets.org/thumb/eclipse77b.jpg[/img][/url]The highly rarefied region above the chromosphere, called the [b]corona[/b],
extends millions of kilometers into space but is
visible only during a total solar eclipse (left).
Temperatures in the corona are over 1,000,000 K.
It just happens that the Moon and the Sun appear the same size in the sky as viewed from the Earth.
And since the Moon orbits the Earth in approximately the same plane as the Earth's orbit around
the Sun sometimes the Moon comes directly between the Earth and the Sun. This is called a solar
eclipse; if the alignment is slighly imperfect then the Moon covers only part of the Sun's disk and
the event is called a partial eclipse. When it lines up perfectly the entire solar disk is blocked
and it is called a total eclipse of the Sun. Partial eclipses are visible over a wide area of the
Earth but the region from which a total eclipse is visible, called the path of totality, is very
narrow, just a few kilometers (though it is usually thousands of kilometers long). Eclipses of
the Sun happen once or twice a year. If you stay home, you're likely to see a partial eclipse
several times per decade. But since the path of totality is so small it is very unlikely that it
will cross you home.
So people often travel half way around the world just to see a total solar eclipse.
To stand in the
shadow of the Moon is an awesome experience. For a few precious minutes it gets dark in the
middle of the day. The stars come out. The animals and birds think it's time to sleep.
And you can see the solar corona. It is well worth a major journey.
The Sun's magnetic field is very strong (by terrestrial standards) and
very complicated. Its [url=http://nineplanets.org/help.html#magnetosphere]magnetosphere[/url]
(also known as the [url=http://nineplanets.org/medium.html#heliopause]heliosphere[/url])
extends well beyond [url=http://nineplanets.org/pluto.html]Pluto[/url].
[img(100px,100px)]
http://nineplanets.org/thumb/xraySunICON.jpg[/img]In addition to heat and light, the Sun also emits a
low density stream of charged particles (mostly electrons and protons)
known as the [b]solar wind[/b]
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
[url=http://nineplanets.org/help.html#aurora]aurora borealis[/url].
Recent data from the spacecraft [url=http://nineplanets.org/spacecraft.html#ulysses]Ulysses[/url]
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,
than 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 [url=http://sci.esa.int/content/news/index.cfm?aid=1&cid=1&oid=24671]intermediate speed[/url].
Further study of the solar wind will be done by the recently launched
[url=http://nineplanets.org/spacecraft.html#wind]Wind[/url],
[url=http://www.srl.caltech.edu/ACE/]ACE[/url] and
[url=http://sohowww.nascom.nasa.gov/]SOHO[/url] 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.
[url=http://www.solarviews.com/cap/sun/sun.htm][img(100px,65px)]
http://nineplanets.org/gif/Sol.jpg[/img][/url]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 [i]the Maunder Minimum[/i]. 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
[url=http://nineplanets.org/twn/types.html#planetary]planetary
nebula[/url]).
[b]The Sun's satellites[/b]
There are eight [b]planets[/b] and a large number of
[url=http://nineplanets.org/smallbodies.html]smaller objects[/url] orbiting the Sun.
(Exactly which bodies should be classified as planets
and which as "smaller objects" has been the source of some
[url=http://nineplanets.org/overview.html#classification]controversy[/url], but in the end it
is really only a matter of definition. Pluto is no longer officially a planet but
we'll keep it here for history's sake.)
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