What IS the sun?
"The Sun Is A Mass Of Incandescent Gas..."
"The sun is a mass of incandescent gas, a gigantic nuclear furnace, where
hydrogen is built into helium at a temperature of millions of degrees."
So begins a song written in 1956, originally recorded to promote nuclear
power. In actuality, the sun is a ball of luminescent hydrogen,
helium, and other gases, already burning itself away for billions of years.
- In a strictly astronomical sense, the Sun is an "average-sized" star,
the center of a small solar system on the outer edge of a spiral arm of the
- Compared to the Earth, however, the Sun is incredibly large: the Sun has
the diameter of 109 Earths, or about 1.35 million kilometers.
- The Sun has the volume of 1.25 million Earths.
- The surface temperature of the Sun is about 6,000 Kelvins, nearly 10,500
degrees Fahrenheit. (For comparison, the average temperature in Los Angeles
is about 72 degrees Fahrenheit, about 295 Kelvins.)
- Oddly enough, its density, because of its gaseous composition, is close
to that of water, about 1000 kg per cubic meter.
- The Sun sits about 150 billion meters from the Earth.
- Since light travels at about 300 million meters per second, it takes
light about 8 minutes to get from the Sun to the Earth.
A diagram of the sun showing (1) the Photosphere, (2) the Chromosphere,
and (3) the Corona.
For the reason of temperature alone, it is fortunate that Earth only
receives one two-billionth of the total energy radiated by the sun. Most
of the visible energy, light, is radiated from the photosphere
(1, above), the outside shell of the sun. The incandescent gases here
measure a depth of about 300 kilometers (200 miles), but unlike the surface
of Earth, the pressure here is only about 10 millibars (1000 Pascals), a mere
1/10 of the pressure at the surface of Earth.
The chromosphere (2, above) is above the photosphere, and its
luminescence is rarely visible, despite the fact that these gases also glow.
During the correct moments of a solar eclipse, however, the overpowering glow
of the photosphere is hidden, and, for a few minutes, one can see the
chromosphere as a thin red circle about the sun using special filter. The
chromosphere is only a few thousand kilometers thick, and it is almost
amazing that it can be detected at all at this great distance.
Outside the chromosphere, at a depth of about a million or so kilometers,
are the ionized gases of the corona (3, above). This gossamer
layer of the Sun is only visible during a solar eclipse, and it emits light
about half as bright as that of a full moon. (Note that care should be
used when viewing the corona; cobalt welder's glass is recommended.)
The outermost particles of the corona , mainly ions, move so fast that
they are able to escape the gravity of the Sun, and these charged atomic
particles form the solar wind, traveling away from the sun;
much of the solar wind is lost to the vacuum of space, but some particles
travel far enough to affect planets and their satellites. An increased solar
wind can also interact with the Earth's magnetic field and ions in our upper
atmosphere, and we see this interaction as the Aurora Borealis in the
northern hemisphere (often called the "Northern Lights"), and the Aurora
Australias in the southern hemisphere.
The Sun itself operates on an eleven-year cycle, its activity ebbing and
growing in a steady rhythm. This cycle was calculated after much observation
in the early 1800s; the main object of examination in this experiment was the
frequency of sunspots, dark spots visible on the surface of the
sun. (Again, special filters are required.) There have been many attempts
at connecting solar activity to climate on Earth, but there is no conclusive
evidence that sunspot activity is related to terrestrial climate.
A solar flare occurs when the area above a sunspot brightens
and releases huge amounts of energy in the forms of ultraviolet, radio, x-ray
radiation, and high-speed atomic particles. These particles intensify the
solar wind and yield auroras, while the radiation may interfere with radio
and television reception.