Journal 3/10

I've still had my mind stuck on color and also on stars. I was wondering why if you stare at the stars for a long time that they sometimes change color (or at least look like they do). This is what I found:



Stars That Change Colors

I've noticed that a few bright stars near the horizon seem to be very rapidly cycling through colors. They'll change from red to green to blue very quickly, it's almost like watching a Christmas light. There are only a few stars that do this, but it's the same ones every night (I know Arcturus is one for sure). All of the stars are close to the horizon and very bright. However, there are some close to the horizon bright stars which do not exhibit this behavior. Why does this happen?

1. The effect you're seeing is almost certainly due to refraction in the atmosphere. The probable reason that only certain stars seem to change color is that it requires a certain light level before the human eye can distinguish colors (lower than that, you only see a grayscale). So only the brightest stars (Arcturus is the fourth brightest, fifth including the Sun) will exhibit this type of sparkling. Dimmer stars only twinkle (move around and change brightness).

   Dr. Eric Christian

Doppler Shift Vs. Intrinsic Color of Stars

A distant star might appear blue, due to its temperature. If it is travelling away from you, you could use its Doppler shift to determine what is going on. As I understand it, an object approaching is blueshifted, and departing it is redshifted. How could a blue star be red, or a red star be blue?

There are two processes involved here. One is the "intrinsic" color of the star, which is the color someone would see if they were not moving towards or away from the star. That color is completely determined by the temperature of the star. Blue and white stars are hotter than our Sun, red and orange stars are cooler. Each star actually emits a broad range of colors, but the peak emission (the color that has the most light) is what determines the star's color.

An observer sees all of the colors that the star emits shifted either towards the red or the blue depending upon whether the observer is moving away or towards the star. An intrinsically red star moving very quickly towards the observer "would" appear blue, and an intrinsically blue star away very fast "would" appear red. Most stars are not moving quickly enough for them to shift from red to blue or vice versa and we observe them at nearly their intrinsic color. But because of narrow "emission lines" that stars give off (very specific colors from one element), scientists can measure small red and blue shifts even when its overall color doesn't change.

Dr. Eric Christian
(March 2003)

I got this info here: http://helios.gsfc.nasa.gov/qa_star.html