This might be a dumb question but why isn't there other colour stars like purple giants or green giants?

Answer 1

Stars are blackbody radiators. That means they put out a continuous spectrum of light, which peaks at a certain wavelength depending on temperature. None of them put out the kind of narrow spectrum of light needed to make them strongly colored. What happens is that relatively cool stars, that put out most of their energy in the red and infrared, look reddish to us. Slightly hotter stars look yellowish or orange. Stars like our Sun whose light output peaks in the middle of the visual, look white. Stars that peak in the green part of the spectrum put out equal parts of red and blue, because green is right in the middle. So these stars look white, not green. Extremely hot stars put out most of their light in the blue and beyond, and so appear bluish. None of these colors is very pronounced, because all colors of the spectrum are present. The only strongly colored stars are the carbon stars, cool red stars where carbon in their outer layers further absorbs blue light, making them look very red.

Giant stars are mostly reddish, because they swell up so large their energy is spread over a large surface, making them cool and reddish. Only very massive stars manage to become blue giants.

Answer 2

Not so dumb actually. I think it is a good question.

It´s because of the nature of light from a blackbody (and stars correspond quite well to a black body). A blackbody is a hypothetical object that has no energy. And so it is black. When you heat it eventually it gets so hot it will radiate off its energy. Infrared is in the lower part of the spectrum. Eventually we come into the red and it begins to glow. As the temperature increases the color temp goes up. Yellow, green, blue, violet etc. All the colours of the spectrum. But the blackbody does not shine green at any point. It might emit green photons but as the blackbody emits photons of all energies the blend. As you may know if you mix red light, green light and blue light you get white light. So because green is in the middle region of the spectrum it isn´t seen. There is no green hot, only red hot. The dominant wavelenght the sun emits is actually green.

Really hot stars shine blue hot. An extremely hot star do put out violet photons as violet is more energetic than blue light but since the light again goes into the part of the spectrum that is invisible to us the violet is weaker than the blue photons. And so blue is more dominant to our eyes.

Giant stars are cold stars which is why they are red. Red for a black body is cold while blue is hot. In our real world experience we feel the opposite. Red is warm and blue is cold. But the stars give little regard to that...

Answer 3

It has to do with a combination of light radiated from a black body and how the human eye determines color.

First the human eye:
The human eye has three different types of cells called "cones" that are sensitive to three different wavelengths of light. The wavelengths of each of these cones correspond to what our brain interprets as "red",'green", and "blue". incoming light excites these cells and our brain interprets the color. If the brain receives much more from the red cones than from blue or green, then the brain sees red. If it gets roughly an equal amount of red and green it interprets the color as yellow. If the brain get a lot of signal from all of the cones, it interprets the color as white.

Now black body radiation.

All objects radiate light depending on their temperature. Objects also reflect light. An object that is emitting light based upon its temperature, but not reflecting light it is called a black body. There exists an equation that predicts how much light from each wavelength is given off from an object at a given temperature called the blackbody equation. Objects give off light in all wavelengths, but depending on how much is from each wavelength, our brain will interpret the color differently. Here are a few examples. A human body gives off most of its radiation in the infrared wavelength, which is invisible to the naked eye. It gives off a negligible amount of light in the visible. Hence, human bodies do not glow. An object at about 700 degrees Centigrade just begins to glow. Its peak wavelength is still in the infrared, but enough red light is given off that it appears red. At about 1700 degrees Centigrade it is giving off a lot of red, but enough light in the green wavelength, that the green cones in the eye are activating, so the object will appear red/orange. At about 3000 centigrade the red and green cells are both activating alot and the object appears yellow. At 5600 degrees Celsius (the temperature of the sun) the enough light is being given off in the blue wavelengths that the blue cones in the eye are activating. This object appear white. Now we all know the sun looks yellow. This is due to the fact that the atmosphere scatters blue light better than green or red. So more green and red gets to your eye than blue, and hence the yellow color. When an object gets to be around 7500 degrees centigrade, it is giving off a lot of light in many wavelengths including UV ( the eye cannot see UV), but now that there is so much blue light compared to the other wavelengths, that it starts to appear blue.

This explanation is rather crowded. It helps to see the blackbody radiation curve.

This website has a Java applet that shows the blackbody curve for a given temperature, and the color of an object emitting blackbody radiation at that temperature.

http://webphysics.davidson.edu/alumni/MiLee/java/bb_mjl.htm

Answer 4

In reality, all stars are white because they put out light of all wavelengths.

When astronomers mention a star's "color", they are actually referring to its temperature. Red stars are relatively cool, yellow stars warmer, green warmer still, and so on.

And when they talk about giant stars, the terms red and blue are again just to separate the two classes of giants. Red giants are cooler giants that are former main sequence stars swelling up at the end of their lifespan. Blue giants are young stars that burn hot, bright and fast because of their heavy mass. But red and blue don't literally refer to the color you would see if you could see them up close.

Answer 5

Red, yellow, white and blue are the most popular colors.

This is a fact of life.

Answer 6

Because the chemical reactions necessary for the formation/existence/destruction of a star do not produce light waves in those areas of the visible spectrum.

Answer 7

http://acept.la.asu.edu/PiN/rdg/color/color.shtml check it out it interesting http://coolcosmos.ipac.caltech.edu/cosmic_kids/AskKids/starcomp.shtml