The Sun

Sol

Sun Facts

• The Sun is an ordinary G2 star, one of 100 billion or more stars in our galaxy.
  • diameter: 1,390,000 km.
  • mass: 1.989e30 kg
  • core temperature: 15,000,000 K.
  • surface temperature: 5800 K

• 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, 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 core of the Sun rotates as a solid body.

• Conditions at the Sun's core are extreme. The temperature is 15 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. It takes 50 million years for a photon to reach the surface.

• The surface of the Sun, called the photosphere, is at a temperature of about 5800 K. Sunspots (picture 10) 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 rarified region above the chromosphere, called the corona, extends millions of kilometers into space but is visible only during eclipses (picture 11). 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 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 spacecraft from the dynamically stable vantage point directly between the Earth and the Sun about 1.6 million km from Earth.

• The solar wind is responsible for the ion tails of comets and 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 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).

Pictures

1. (above) A huge eruptive prominence 274k gif; 203k jpg; 6000k tiff

2. small image in visible light 10k gif

3. Sun visible light 158k gif
4. X-ray image of the Sun 172k gif
5. X-ray image 174k gif

6. X-ray image from Yohkoh 168k gif

7. Sun x-ray light 177k gif
8. Sun hydrogen alpha image 149k gif; 34k jpg
9. Solar flare in H-alpha 147k gif; 38k jpg

10. region around a sunspot 336k gif

11. total eclipse (1977) 322k gif

12. 1991 eclipse (Steve Albers) 431k gif; 143k jpg (caption)

13. "Hedgerow" prominence 49k gif
14. a prominence with loops 76k gif
15. solar magnetic fields 170k gif
16. X-ray images of the Sun from Yohkoh ftp directory
17. Current solar images updated daily html; html
18. A diagram of the Sun 233k gif; 108k jpg; 5000k tiff
19. ...more Sun images

Movies

1. Solar corona and a prominence 174k quicktime
2. The xray Sun over 27 days flc
3. Nov 94 Eclipse 15000k quicktime; 3100k mpeg
4. Video clip from Our Star the Sun - The Sun's past - 5000k AVI
5. Video clip from Our Star the Sun describing the Sun - 14000k AVI
6. Corona Animation showing solar prominences 1200k AVI
7. 3d movie of convection 1000k mpeg

The Sun's satellites

            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 about the Sun

• from ASU
• from LANL
• from RGO
• from NASA Spacelink
• Today's Space Weather and index of solar images
• Solar Data Analysis Center (Eclipse info and much more)
• National Solar Observatory Exhibit
• Slide Set Introduction from Sacramento Peak Observatory
• Information and oddities about the Sun by Paolo Amoroso
• Virtual Tour of the Sun by Michiel Berger
• The Sun: a Pictorial Introduction, a slide set by P. Charbonneau and O.R. White
• an article about the Northern Lights
• The HK Project
• Ulysses South Polar Pass
• Spartan 201, NASA's mission to explore the Sun's corona
• IACG Campaign IV: including lots of good references
• The Sun and Stellar Structure; notes by Nick Strobel of the University of Washington
• ESA/NASA's SOHO - Solar and Heliospheric Observatory home page
• Interview with Sol by Robert J. Nemiroff

Open Issues

• Is there a causal connection between the Maunder Minimum and the Little Ice Age or was it just a coincidence? How does the variablility 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?