Editing Standard Candles (section)

== Why It Is ==

Light from point sources, such as stars, spreads out into space spherically, in all directions, following an inverse square law (see diagram). The surface area of a sphere is 4*pi*r^2. As light travels away from the source, it fills a larger and larger surface in space that is proportional to the distance from the source squared. As the light gets spread our over this larger area, it appears to decrease in brightness proportionally.

Many astronomical objects serve as standard candles. We will mention just a few here. Perhaps the most famous are Cepheid variable stars, whose brightness oscillates with time at a rate which is related to the average brightness. Main sequence stars (meaning stars in the same stage of life as our sun) have a color which is related to their average brightness because they are essentially blackbody radiators. At very large distances, astronomers may take the brightest galaxies in a cluster of galaxies to be at a certain typical brightness.

Interstellar extinction is caused by dust and gas in the ISM. This is the same dust and gas that stars are eventually born from. Much of the ISM was actually contributed by stars that have died and expelled their contents into the galaxy. The ISM is mostly made of hydrogen gas, but it is the small percentage of dust (microscopic particles made from carbon, silicon, and other components) that does most of the absorbing of light.

Why does the dust redden light? This has to do with the size of the dust particles. When the dust grains are about the same size as the wavelength of the light or smaller, then the dust absorbs the light very well (see e.g. the Mie Theory of interstellar extinction). Think of this like waves in the ocean. These waves have wavelengths of many meters - much larger than a grain of sand. Therefore sand does not block ocean waves. However, waves hitting a big island will simply stop. The only way for waves much smaller in wavelength than the island to pass through is to avoid the island altogether! Similarly, long-wavelength light like red or infrared light can pass through the ISM without too much trouble. Short wavelength light like blue light has trouble passing through. Extremely short wavelength light like X-Rays might pass through very well, if they simply do not hit any of the dust.