The Sun

Overview

"The sun is only a star / around that star we spin" goes the words of a song "Child of the Universe." The sun is only a star, and an average one at that. It is unimportant in the grand scheme of the universe. However, it is special to us, for without the sun, we would not exist. This page discusses the specifics of the sun as it relates specifically to our star.

What We See

The sun has an average distance of 149.6 million km from Earth, and this gives it an apparent diameter of 1919 arcsec in the sky. When Earth is at perihelion, it is 147.1 million km from the sun, making the sun appear to be 1952 arcsec in diameter. At an aphelion of 152.1 million km, the sun is only 1887 arcsec across.

The Light We See

The light that is currently reaching the Earth was generated in the sun approximately 100,000 years ago. It takes that long to get to the surface of the sun because the sun is so dense, and so the escaping energy has a much harder time escaping. This is analogous to walking down a hallway that was crowded with people. You couldn't just run right on through, for you would be shoved back by people and be bounced around. A photon of light encounters this same resistance when it tries to escape from the sun's core. The other people are the atoms that make up the sun and other photons generated by nuclear fusion. This is called a "random walk."

Once light leaves the sun's photosphere, it takes approximately 8 minutes and 26 seconds to reach Earth.

The light from the sun is made up of many colors, called the visible spectrum, and many shorter and longer wavelengths of light. These other wavelengths are invisible to humans, but they can be measured with special detectors. The diagram below represents the electromagnetic spectrum, with the scale being in Hz - oscillations per second.

These other wavelengths consist of Infrared (IR), Ultraviolet (UV), Micro, Radio, X, and Gamma. (IR light rays can be produced by specially-modified light bulbs, and are used in many places that sell food.) IR rays heat up matter. Our atmosphere acts as an "infrared shield," and keeps this light from reaching the surface. UV light has become an increasing concern over the past few years. It is a form of radiation, and the hole in the ozone layer is allowing some of the normally blocked UV light to get through. UV light causes tans, sunburns, and skin cancer. Micro waves are put to use in most people's kitchens in the aptly named "microwave." They are used to heat foods quickly, and are more effective at doing so than IR. Radio waves are used in a whole branch of astronomy, for they can penetrate clouds of gas and dust that visible light can't. They are also used for transmitting radio and television shows - television having a slightly higher frequency. X-rays are a form of radiation that are more powerful than UV, and are normally blocked by our atmosphere. X-rays are mainly used for medical purposes. Since they are a form of higher energy, they can penetrate denser objects than visible light can. Gamma rays are the most energetic form of radiation, and can pass through the human body. In cells, they can cause mutations and other severe damage. Luckily, they are blocked by our atmosphere. If they weren't, human life would be impossible.

The sun does not emit all of these wavelengths evenly. The light emitted follows what is known as a "black body spectrum." This follows the Planck Curve:

The Planck curve peaks at a certain wavelength for any given temperature. The sun's photosphere, 5780 K, is what radiates its light, so this is the temperature that is used to generate the sun's Planck Function graph (right). The point of maximum intensity is given by Wien's Law:

For the sun, this works out to 5014 Å, which is in the green part of the visual spectrum. Therefore, the sun's true color is green. However, if you look at it (which you should never do, for it can cause severe eye damage and in some cases blindness), the sun appears yellow. No one has yet successfully explained why the sun appears to be yellow instead of green. However, an interesting fact is that our eyes are best able to see green light at approximately that wavelength.

The total luminosity of the sun is given by the Stefan-Boltzmann Law (above), which for the sun works out to 3.85x10²⁶ J/s (approximately what is given on the opening page of this section, which is a value taken from NASA).