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The Electromagnetic Spectrum

False color image of coronal loops, eruptive gas fountains formed on the Sunís surface (Courtesy: NASA)
Since the Sun powers all life processes on Earth, the long history of Life on Earth is only possible because the Sun has sufficient energy resources to burn for thousands of millions of years. (In fact, the coal and oil and gas we are burning in such profligate fashion represent solar energy collected by life forms over more than two hundred million years.)

The energy provided by the Sun has to come in the right amount, shape and form to be useful to Life on Earth. Life cannot use X-rays or radio waves as an energy source. Visible light is just right ó the plants use it to make plant matter by photosynthesis, we and many other organisms use it to see by. Likewise, the amount of energy delivered by the Sun to our planet is just right for the hydrologic cycle to work, with water and water vapor changing back and forth, and some minor amount of ice (2 percent of the total water) collecting near the poles. Thus, the climate is between cold and warm, dry and wet ó just about right.

The electromagnetic spectrum.
The Sun sends light and heat rays (called infrared, IR) and some ultraviolet light (UV). The UV is dangerous to living organisms; it damages eyes, human skin and tree leaves, among other things. Fortunately only a few percent of the energy arrives as UV, the rest is half visible light, half (invisible) IR. In addition, the atmosphere takes out most of the UV before it reaches the ground. The ozone layer in the lower stratosphere (just above the highest clouds) is especially important in protecting living things from UV exposure.

The amount and type of energy given off by the Sun corresponds closely to what is expected from its surface temperature (6000 degrees Kelvin). Stars hotter than the Sun are more bluish (and emit a relatively greater amount of UV radiation) and cooler stars are more reddish (with greater amounts of IR radiation). The visible light is made of many colors. We can see three of them (our brain constructs all sorts of color hues from that information). Other organisms with eyes do not necessarily see the world the way we do; some cannot see color, but some can see ultraviolet in addition to color (many insects). Some have organs to sense infrared, for hunting warm prey (snakes).

The energy coming in from the Sun must be returned to space to keep Earth from overheating. In fact, the Earth sends exactly as much heat out to space as it receives from the Sun (plus a tiny bit more corresponding to Earth's own heat production, from radioactive decay). About 30 percent of the incoming radiation is simply reflected. The reflectivity of a planet is called its "albedo". Venus has a very high albedo (that is why that planet is so brilliant), while Earth has an intermediate one. Clouds and snowfields are especially efficient in reflecting sunlight. What is not reflected (70 percent) is absorbed in the atmosphere and on the ground, and is then re-radiated to space by the warmed objects, in the infrared portion of the spectrum (that is, as heat radiation). How is this balance maintained? Earth warms up to exactly the temperature that is necessary to re-radiate exactly the right amount of energy.

Stonehenge during a solar eclipse (Courtesy: NASA)
Many ancient (and not so ancient) cultures revered the Sun as a god (Ra among the Egyptians, Helios for the Greeks, Mithras for the Persians, Apollo for the Romans, Huitzilopochtli for the Inca). Ancient astronomy arose from the necessity to map the path of the Sun through the heavens, to know when to sow and to harvest, and when to sacrifice to the gods so that the harvest should be good. Retaining parts of an old pagan tradition, people in Scandinavian countries celebrate the lengthening of the days at the end of December, that is the return of the Sun, and the maximum length of the day (when the Sun is highest over the horizon). The reason is that the Sun gives warmth and makes the countryside green, which is nowhere more appreciated than in regions with harsh winters.

The Sun is one of 100,000 million stars in the galaxy and about one fourth are much like it. Thus, there is no shortage of sun-like stars. We do not know, however, how many of these stars have a planetary system like ours.

Two thousand and five hundred years ago the Geek philosopher Anaxagoras claimed that the Sun is not a god (who adjusts energy output to human needs) but a huge ball of fire (which doesn't particularly care about humans). This idea did not go over well, and resulted in exile for the philosopher, after a proper trial for impiety.

Internal structure of the Sun. (Courtesy of: UN-ESA)

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