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Above: The Greek sign for the Sun.
Our Sun is an ordinary G2 star and is 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 has been 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.
 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 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, 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 planetary nebula).
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.) Below is a table listing: all the planets, their distance from the Sun, their radius, their mass, and who discovered the planet and when.
| Planet |
Distance
(000 km)
|
Radius
(km)
|
Mass
(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 |
Here you will find all the figures for the Sun. This includes everything from its escape velocity needed to leave its surface to temperature values at the center of the Sun! All these figures will be compared with Earth if possible...
|
Sun
|
Earth
|
Ratio (Sun/Earth)
|
| Mass (1024 kg) |
1,989,100 |
5.9736 |
332,950 |
| GM (x 106 km3/s2) |
132,712 |
0.3986 |
332,950 |
| Volume (1012 km3) |
1,412,000 |
1.083 |
1,304,000 |
| Volumetric mean radius (km) |
696,000 |
6371 |
109.2 |
| Mean density (kg/m3) |
1408 |
5520 |
0.255 |
| Surface gravity (eq.) (m/s2) |
274.0 |
9.78 |
28.0 |
| Escape velocity (km/s) |
617.7 |
11.2 |
55.2 |
| Ellipticity |
0.00005 |
0.0034 |
0.015 |
| Moment of inertia (I/MR2) |
0.059 |
0.3308 |
0.178 |
| Visual magnitude V(1,0) |
-26.74 |
-3.86 |
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|
| Sidereal rotation period (hrs)* |
609.12 |
23.9345 |
25.449 |
| Obliquity to ecliptic (deg.) |
7.25 |
23.45 |
0.309 |
| Speed relative to nearby stars (km/s) |
19.4 |
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|
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|
| Absolute magnitude |
+4.83 |
|
|
| Luminosity (1024 J/s) |
384.6 |
|
|
| Mass conversion rate (106 kg/s) |
4300 |
|
|
| Mean energy production (10-3 J/kg) |
0.1937 |
|
|
| Surface emission (106 J/m2s) |
63.29 |
|
|
| Spectral type |
G2 V |
|
|
|
Values At The Center Of The Sun
|
| Central pressure |
2.477 x 1011 bar |
| Central temperature |
1.571 x 107 K |
| Central density |
1.622 x 105 kg/m3 |
**Don't understand these measurements? Click here to get help with the definitions and notes**
*This is the adopted period at 16 deg. latitude - the actual rotation rate varies with latitude L as:
( 14.37 - 2.33 sin2 L - 1.56 sin4 L ) deg/day.
- 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 about the orbit of Mercury.
- Since all the planets except Pluto 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 Sun.
- The corona is much hotter than the photosphere. Why?
All photos below were taken using the Extreme-Ultraviolet Imaging Telescope (EIT) at different wavelengths giving them different colors. The bottom four are enhanced pictures that show off many solar flares, a must see!
Click on any image to view it full size!
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SUN
