Re: How come the stars arnt differnt colors like blue or purple or green ?

Question:
How come the stars aren't different colors like blue or purple or green? When we look at the stars here from earth they are mostly seen as white. How come they aren't bright and strange colors like blue or green. What makes them the colors they are?

Answer:
The answer to the first part of your question is that stars ARE bright and strange colors. How you see them has more to do with how the human eye works than the stars themselves.

The human eye has different kinds of light sensitive ("photoreceptive") cells inside the back, grouped into two categories (named after their shape): 'rods' and 'cones'.

A quick search on www.google.com for 'eyes rods cones', and I got a few sources for you:

Borrowing some info from http://www.innerbody. com/text/nerv08.html:

Rods are sensitive enough to respond to a single photon, the basic unit of light, but together they create only one coarse, gray image, which is just adequate for seeing in poor light. Fine detail and color come from the cones, but they need a lot more light and work best in broad daylight. These are arranged in such a way as to produce the best possible combination of night and day vision.

From http://www.howstuffworks. com/eye2.htm:

The outer segment of a rod or a cone contains the photosensitive chemicals. In rods, this chemical is called rhodopsin; in cones, these chemicals are called color pigments. The retina contains 100 million rods and 7 million cones. (93% rods, 7% cones). The retina has a central area, called the macula, that contains a high concentration of only cones. This area is responsible for sharp, detailed vision.

So basically, the cells in your eyes that see the dimmest light, the rods, cannot identify color, and there are far more of them than there are of the other kind, the cones. Light from a star makes up such a tiny dot in the field of vision of the eye, that only a very few (perhaps only a few thousand) of those rod and cone cells are able to detect that light. Since the cones (which see color) are concentrated around the center, you will probably not be able to see the star color unless you look straight at it, and even then, only faintly.

Using a little math for estimations, if only 1000 cells in your eye see the star, 930 of them will be rods, allowing you to tell how bright the star is, and only 70 of them will be cones if you?re not looking straight at it. And even if you are, and even all of the cells that see the star are cones, 1000 out of 7 million cone cells does not register a very strong signal to your brain.

This is why telescopes come in handy. By gathering a larger quantity of light, a telescope allows more of the eye to see the star, and the color is more apparent.

To answer the second part of your question, it is important to understand that when almost any object in the universe gets hot, it gives off radiating energy in a range of wavelengths. The hotter the object gets, the shorter the wavelengths (higher frequency) of energy. When the temperature of an object begins to get up to around 2500 C or warmer, the wavelength of the energy is short enough for us to begin to see the energy as visible light, beginning as a deep red. As an object continues to get hotter, and the wavelength gets shorter, the more the color of the object shifts up the spectrum toward blue. Objects hotter than that are radiating energy at a higher frequency than the human eye can see (ultraviolet, x-rays, gamma rays). (Light bulb manufacturers classify the color of their light bulbs by the temperature of the energy.)

So, a 'cool' star, like Betelgeuse, in the constellation of Orion, appears red because it burns at a temperature around 3000 C. A hotter star, like Rigel , also in Orion, appear a more white-ish blue because it burns a great deal hotter than our Sun.

I hope this is helpful.

For more information, see:
http://cassw ww.ucsd.edu/public/tutorial/Planck.html
http://imagine.gsfc.nasa.gov/docs/science/know_l2/emspectrum.html
for some more detailed explanations and pictures.