The star would therefore appear white — a combination of all colors. Earth's sun emits a lot of green light, but humans see it as white. Purple stars are something the human eye won't easily see because our eyes are more sensitive to blue light. Since a star emitting purple light also sends out blue light — the two colors are next to one another on the visible light spectrum — the human eye primarily picks up the blue light. Reference here. How about a star that is violet, when seen from some viewpoints, via the doppler shift?
If a blue-dominant star say peak emission nominally nm were coming toward an observer fast enough, the light -- including that dominant blue peak -- would decrease in wavelength. Enough speed, and the peak would sit in the violet band of the spectrum. A back of the envelope calc. A cooler longer wavelength star would need a greater approach speed. Seen from behind, the star would appear red shifted. To appear violet, the star doesn't have to be coming straight at the observer.
But, what if it is headed right at you? That gorgeous lavender you see in pictures of massive stars and the nebulae around them is hydrogen. The Balmer alpha line Hydrogen alpha photos of the sun, for instance, are true-color pictures. But the Balmer delta line The physics to understand what I just said is in Wikipedia, so I won't bother to explain.
On a world in a starburst galaxy, you could be close enough to something like the Tarantula Nebula or 30 Doradus to see that glowing hydrogen with the naked eye. A number of kinds of star are actually colored ultraviolet. Only a small part of the light they emit is reddish enough for us to see.
Central stars of planetary nebulae, for example, are usually hotter than K, which we see as blue-white. If you could shift such a star's spectrum until its peak emission was near nm, like hydrogen's Balmer epsilon, you would see those stars as distinctly blue, probably with some purple thrown in.
Assuming human eyes. Our own world has creatures which see well into the ultraviolet. A green star is radiating right in the center of the visible light spectrum, which means it is emitting some light in all the possible colors. The star would therefore appear white — a combination of all colors. Earth's sun emits a lot of green light, but humans see it as white. Purple stars are something the human eye won't easily see because our eyes are more sensitive to blue light.
Since a star emitting purple light also sends out blue light — the two colors are next to one another on the visible light spectrum — the human eye primarily picks up the blue light. Live Science. The brightness measured through each filter is usually expressed in magnitudes. The difference between any two of these magnitudes—say, between the blue and the visual magnitudes B—V —is called a color index.
By agreement among astronomers, the ultraviolet, blue, and visual magnitudes of the UBV system are adjusted to give a color index of 0 to a star with a surface temperature of about 10, K, such as Vega. Why use a color index if it ultimately implies temperature? Because the brightness of a star through a filter is what astronomers actually measure, and we are always more comfortable when our statements have to do with measurable quantities.
Stars have different colors, which are indicators of temperature. The hottest stars tend to appear blue or blue-white, whereas the coolest stars are red. A color index of a star is the difference in the magnitudes measured at any two wavelengths and is one way that astronomers measure and express the temperature of stars.
Skip to main content. Analyzing Starlight. Search for:. Colors of Stars Learning Objectives By the end of this section, you will be able to: Compare the relative temperatures of stars based on their colors Understand how astronomers use color indexes to measure the temperatures of stars.
Key concepts and summary Stars have different colors, which are indicators of temperature.
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