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Why are red stars cooler than white stars? I thought red things were hot.
Question Date: 2000-02-14
Answer 1:

Red things are hot, but white things are hotter! Here's why. When a hot object starts emitting light, it starts by giving off the lowest energy light, which is red light. As it gets hotter, it then gets enough energy to emit yellow and eventually blue light, while at the same time still emitting the red light. The reason is that yellow and blue light takes more energy to emit than does red light, so a higher temperature is needed. So, as an object heats up, it first turns red, then orange (red+yellow light), and finally white (red+yellow+blue looks white to the eye).

You might want to think about what the sequence of colors would be if our eyes could see more than just visible light.

Answer 2:

The hotter a material is, the more average energy there is in the light it emits.Room temperature objects emit light of relatively low energy in the infrared. This is invisible to your eye. Objects heated in a flame can become red hot, and this is probably where you have learned to associate red with hot.

In the visible spectrum, light increases in energy from red, through green to blue and into the ultraviolet (invisible to your eye). Cooler stars emit much of their light in the red part of the spectrum, so you see them as red. Somewhat hotter stars, like our Sun, emit much of their light in the green (or yellow), with smaller amounts of red and blue. This balance of colors your eye sees as white. Even hotter stars emit most of their light in the blue and ultraviolet, so you see them as blue-white.

On a clear night, how many of the bright stars you see are reddish, how many yellowish and how many bluish? Which ones are more common?

Answer 3:

Most stars are the color they appear because of their temperature. All objects have thermal energy. Temperature is a measure of the amount of this thermal energy. Every object radiates thermal energy to its surroundings (and the surroundings radiate their thermal energy to the object. Question - what happens when you put a cold pan in a hot oven?) So everything around you is radiating thermal energy - why can't you see it? Because you can only see a narrow range of frequencies of light, from red to purple. Objects at room temperature are radiating at much lower frequencies. The hotter an object is, the higher the frequency is in which it radiates most of its thermal energy. If you watch a toaster or electric stove burner warm up, you can feel the thermal radiation it's giving off, but you can't see it glow until it gets hot enough to emit red light. Your toaster and stove don't get hot enough to emit much more than a red-orange. Stars, though, can get much hotter, so, the hotter stars can emit large amounts of their thermal radiation in yellow and blue frequencies. What color do you think the hottest stars would look? Why do you think some stars look white, and why don't you see green stars?

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