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A constellation is a group of stars with a name assigned to it; the name is usually from Greek mythology. The stars of a constellation are located close to each other and form a pattern, although in most cases it is pretty much hopeless to try to picture a constellation looking like its namesake. Imagine the constellation’s picture being formed by straight lines drawn between its stars. The lines form geometric shapes, which are easy to remember.
The official definition of a constellation differs from the intuitive one described above. Technically, a constellation defines not a group of stars, but an area of the sky. There are 88 recognized constellations that cover the entire sky.
Every star within the constellation’s border officially belongs to that constellation. If you draw lines between all the stars in a constellation area, the result will be a clutter of lines with no easily definable shape. Therefore we tend to consider only the brightest stars as full-fledged members of the constellations. Many of the visible stars of each constellation have been assigned Greek letters. The brightest star usually has the first letter in the Greek alphabet: alpha; the next brightest receive the second letter: beta, then gamma, delta, epsilon, and so on. Some of the bright stars also have names. For instance, the brightest stars of the constellation Bootes is alpha Bootis, or Arcturus.
| Alpha |
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Beta |
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Gamma |
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Delta |
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| Epsilon |
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Zeta |
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Eta |
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Theta |
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| Iota |
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Kappa |
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Lambda |
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Mu |
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| Nu |
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Xi |
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Omicron |
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Pi |
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| Rho |
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Sigma |
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Tau |
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Upsilon |
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| Phi |
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Chi |
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Psi |
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Omega |
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Every star is a member of one, and only one, constellation; nevertheless we still look only at certain stars, usually the brighter one, as creating the shapes of the constellations.
You might think of a constellation as one of those pictures you created a child by "connecting the dots." After all, the stars are white, sparkling, unchanging, solitary dots set in the black sky. Some are brighter than others are, but all are more or less the same. Or are they? Let's take a look at the constellation of Orion. Orion appears as a large trapezoid with a short belt of three stars in its middle. Orion's two brightest stars are Betelgeuse and Rigel. Betelgeuse is found on the trapezoidal side facing Polaris, while Rigel is placed on the corner crosswise from Betelgeuse. Both stars are very bright, with a magnitude between 0.1 and 0.4 and are easy to pick out. First observe Rigel on the side away from Polaris. Its color is not pure white but a bluish white. Betelgeuse is also not really white, but quite reddish. In fact, almost all the stars have some color. Our eyes can only detect the grosser shades of stellar colors, while scientific instruments can determine if a stars is blue, bluish-white, white, yellow, yellowish-white orange or red. Do not expect to see a red stars a traffic light red. I find that there is always white mixed in with the color of a star. The varying colors of the stars are caused by their different surface temperatures. Blue stars are the hottest. Rigel has a temperature of about 20,000 Fahrenheit. The surface of the Sun, a yellow star, 11,000 degrees, while the surface of red Betelgeuse is a cool 6,000 degrees.
Is there a connection between a star's brightness and its color? Although there are many exceptions, the hotter a star is generally the brighter it is. Hot blue stars such as Rigel are extremely bright compared to most other stars. We are talking about absolute inherent brightness, not just the way the star appears from the Earth, which is indicated by its apparent magnitude. Cool, red stars are usually ver dim. So why does Betelgeuse appear so bright? Betelgeuse is one of the sky's red giants. These are stars that have reached the end of their lives. A red giant has used up almost all of its fuel. It has booth cooled down and expanded. Betelgeuse appears bright to us because of its enormous size.
Now take a look at the sky’s other bright constellation, the Big Dipper, or as it more correctly called, Ursa Major - the Great Bear. The Big Dipper looks like a square scoop attached to a handle. The star at the bend of the handle is Mizpar. If you look carefully, you will see another dimmer star next to Mizpar called Alcor. The early Arabs used these two stars as an eye test, the criterion of the test being to see both stars as separate points of light. Nowadays, it is one of the easiest double stars to spot. Any set of two stars that appear very close together are called a star pair. A few pairs may be the result of two widely separate both happening to be in the same line of sight from the Earth, while most star pairs are physical pairs whose members are actually revolving around each other. Occasionally, there are multiple stars, with trios, or sextets of stars revolving around some point. It is very likely that our solitary Sun is the exception and not the rule. Most of the stars very close to us seem to have at least one companion star.
Many stars are neither colorless nor solitary and some are not unchanging either. There are stars whose brightness periodically changes -- the variable stars. Some variable stars are actually binary stars with the smaller member of the pair being too faint to see. When both stars in line of sight with the Earth, the system appears brighter than when the smaller star is off to the "side" and we only see the larger star. Other variable stars are inherently variable and they actually change their brightness periodically. Its period (the time between brightness peaks of a variable star) can be from a few hours to several months.
The sky is not as unchanging as it may appear at first glance, even though most stars are not variable and will have the same magnitude for millions of years. The constellations do not consist of static monochromic dots, but of multi-colored lights, some of them in pairs and others variable, set in the heavens.