Green Stars
Published on Aug 8, 2006 at 12:02 am.
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Go out this time of year a bit after sunset and look south. You’ll see a bright reddish star named Antares. The name Antares actually means “rival of Ares”, or “rival of Mars”, to use the Latin equivalent. That is because this star is similar in brightness and color to Mars when that planet is near opposition. Then, look high in the sky towards the east, and you’ll see a bluish star named Deneb. That name actually means “tail” because it is the tail of Cygnus, the Swan. Most people who haven’t looked carefully at the sky think of stars as all being little white dots. Well, many of them have hues that give them some color. Few are very red or very blue. The shades are often pretty subtle, but if you look carefully, then you definitely can see that many stars do have different colors.
The color of a star comes from its temperature. Any object not at the absolute zero of temperature emits radiation. This electromagnetic radiation actually covers a range of wavelengths. Most objects emit most strongly in the infrared. However, the warmer that an object is, the shorter the wavelength most strongly emitted. However, the object still emits the wavelengths emitted by cooler objects, and emits more of them. The hotter than an object is, the brighter that it is, too. If the temperature of the object is gradually raised, then eventually it emits most strongly in the near infrared, but it emits some visual light, too. The object then appears to glow a dull red color.
Different colors of light are different wavelengths. In order from longest to shortest wavelengths are red, orange, yellow, green, blue, indigo, and violet. Infrared light is longer wavelength than red light, and ultraviolet is shorter than violet light. An object hot enough to emit in the near infrared still emits some red, though not nearly as much red as infrared light. So, the object glows a dull cherry red. But increase the temperature, and it begins to glow a brighter red, and then an orangish red, and finally reddish-orange. Then, it glows a yellow-orange, and then yellow. Keep heating it, and it glows yellow-white, then white, and then blue-white. While blue-white, the object is really emiting more ultraviolet light than visual, but it emits more blue visual than other colors, so it appears blue-white. A yellow or yellow-white hot object emits pretty much across the visual spectrum equally. Your eye is most sensitive to the yellow-green part of the spectrum, so that is why the Sun looks yellowish. But, you never get to see a green star, or an indigo star. Even the blue-white stars are more white than blue. Some of the really hot stars emit definitely strongest in the ultraviolet, and they emit far more violet and indigo light than they do other colors, but they emit the other colors, too. Your eyes are not so sensitive to the violet, so that part is downplayed, and the stars appear blue. in theory, you could do this with objects on Earth, but to get them that hot would mean that they would be vaporized.
So, there are no green stars. However, if you look up Antares in most reference guides for amateur astronomers, you will see that it is a double star with a dim companion. Then, the shocker: that companion’s color is normally listed as green! How can that happen? How do you get a green star?
Well, there are two ways that a star can appear green. As one of the last stages in the life of a star like the Sun, it sheds its outer layers into space. The gasses of these shed layers form a shell around the star that expands to be a roundish nebula that we call a planetary nebula (The name comes from the fact that the nebula is round, like a planet. It does not in any way mean that the nebula has anything at all to do with planets!). One of the fusion products of a solar mass or heavier star is oxygen. This oxygen in the gas has a greenish glow. While the expanding nebula is still close to the star, it is hard to distinquish it without very powerful magnification. So, the nebula imparts some of its color to the star. But, this is only a very faint shade of green, so that does not account for a clearly green looking star like Antares B.
To explain this, we need to understand how we see light. Your eye has four different color receptors. One type, called rods, detects light across many wavelengths, and is basically good for seeing in black and white. The rods also work to see in dimmer light than do the other receptors, so on a dark night, you don’t really see colors. The three other receptors are collectively called cones, and they see colors. There are three types of cones, each tuned to see different colors. Two types, the types that see red and green light, see only slightly different spectral ranges. So, you distinguish long wavelength colors by light causing one type of cone to be more active than a neighboring cone of another type. You learn that certain ratios of activity are called certain colors. Problems with these two cones can lead to a difficulty in distinguishing between colors on the long wavelength end of the visual spectrum (red, orange, yellow, and green). Because these cones are so close to one another in wavelengths detected, juxtaposition of an object of one color near another of the other color makes both seem brighter. These are complementary colors.
Color perception involves the brain resolving signals from multiple cones firing. So, as you might suspect, this means that color perception is not just physical, but also psychological. Studies have indicated that you are born with the ability to see color, but you have to learn what the different inputs mean. Animals raised in monochromatic environments have no experience with different colors, even though they may have a full set of cones, so they don’t immediately recognize the concept of different colors once they are exposed to them. But, they do learn. If you get a group of people together and show them color slides, they will correctly identify the colors shown. And they do this even though different people have different densities of the different types of cones in their eyes. From this, we gather that we learn our own individual eye’s responses to colors. This means that color perception is partly psychological. This explains in part some of the optical illusions in color perception that colors can play one you.
You sometimes perceive colors differently if they are placed on different color backgrounds. Also, because color peception is to a large degree mental, you can still see colors correctly even if the ambient illumination isn’t right. You go from outside, where sunlight is yellow-white, to indoors where incandescent lights are orange or fluorescent lights can be blueish. Because the light shining on objects is different, they reflect different colors, and you see them differently. However, after a few minutes, your brain reinterprets the input from your eyes, and you begin seeing the actual colors of the objects, not the colors that they are actually reflecting. This is why photographs don’t always show the same colors that the photographer sees.
So, what does this have to do with star colors? Well, Antares is nearly 50 times brighter than its companion star. The companion star is blue-white. However, it is close enough to white that you perceive it as red’s complementary color: green. Yes, the blue-white star looks green! Some references designed for professional astronomers list the correct color, but many written for amateur astronomers list “green” as the companion star’s color, since that is what it looks like to the eye. But, if you take a photograph, the companion is definitely not green in a photograph (actually, it is really tough to take a photo of such a dim star next to such a bright one).
So, if you’ve got a telescope, go look at Antares, and see what color you think the companion looks like to you!
-Astroprof
