Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
Every culture has told stories about the stars, and grouped the stars intoConstellations. These differ radically from one society to the next; traditional Chineseconstellations bear almost no resemblance to Western constellations. Most of theconstellations now accepted in astronomy are based on traditional Western and Near Easternstar lore, with a few 17th and 18th-century additions. At one time, a few stars were partsof two constellations, and about a quarter of the stars in the sky were"unformed"; they belonged to no constellation. Now the constellations have fixedboundaries that preserve, as much as possible, the traditional constellation figures.There are 89 constellations that cover the entire sky. Every star belongs to one, and onlyone, constellation. The constellation names are Latin.
The constellations we see from Earth are patterns of stars more or less in the samedirection as we see them from Earth. Most of the objects in a constellation are at widelydifferent distances, and not physically connected with each other. For example,"in" the constellation Virgo, we might find the Moon (240,000 miles away),Jupiter (480 million miles), the bright star Spica (220 light years) and the Virgo Clusterof galaxies (40 million light years). If we were to travel to a distant star, none of theconstellations would be recognizable any more.
Only a few of the brightest stars have proper names. Each culture has its own startraditions, and most of our star names were invented by the Arabs. They came into Englishwhen Arabic learning came to Europe in the Middle Ages, often with interestingmisspellings. For example, the Arabs called a bright red star in Orion Yad-al-Jawza, theShoulder of Orion, but the Arabic characters for Y and B look much alike. A medievalscholar mistook the Y for a B when transcribing the name into Roman letters, andBetelgeuse it has been to this day. A few star names originated in other ways. Regulusmeans "little king" in Latin. Cor Caroli, Latin for "heart of Charles"was named in honor of the deposed and executed Charles I of England.
Most stars are designated in accordance with some star catalog. A common system is thatof Johannes Bayer, who in 1603 used Greek letters for the stars in each constellation,usually, but not always, starting with alpha for the brightest star and proceeding roughlyin order of decreasing brightness. Since Bayer estimated brightness by eye, his system isonly approximate. When he ran out of Greek letters, he used lower-case Roman letters andthen capitals. In Bayer's system, Sirius, the brightest star besides the Sun, is AlphaCanis Majoris, the brightest star in the constellation Canis Major. Later on, in 1725,John Flamsteed employed a system that numbered stars in the order in which they crossedthe meridian. In Flamsteed's system Sirius is 9 Canis Majoris, the ninth star in CanisMajor. Fainter stars are listed in great catalogues like the Bonn Durchmusterung (Siriusis BD -16 1591 in this catalog) or the Palomar Sky Survey. Usually each star designationhas an abbreviation for the catalog and then the catalog number of the star.
The overwhelming majority of very faint stars have no designation, unless they happento be of unusual interest. At times various organizations have offered to name stars afterpeople, in return for a fee. Some such enterprises have been run in jest, to raise moneyfor charity; others are merely aimed at the gullible, but none have any official standing.
When we look at the night sky, the stars vary in color and brightness. They vary incolor from blue-white through white, yellow, and orange to red. The colors are actuallyvery pale; what we call a red star is actually a pale pinkish-orange. Color in stars isdue mostly to differences in temperature: blue-white stars are hottest and red stars arecoolest.
Stars vary in brightness for two reasons. First, they are at varying distances from us,so that distant stars appear fainter. Second, stars actually do give off different amountsof light. Two bright stars in the summer sky, Altair (16 light years) and Deneb (about1,600 light years) appear about equally bright, even though Deneb is 100 times fartheraway. Light intensity drops off in proportion to the square of the distance, so Deneb mustbe about 100 x 100 = 10,000 times as luminous as Altair. Deneb is a stupendous star; itslight left before the fall of Rome, yet it is still a bright star in our sky. It is about70 million kilometers in diameter (half as big as the orbit of Mercury) and about 50,000times as luminous as the Sun. Stars that big don't last long despite their huge mass -Deneb did not exist in the days of the dinosaurs and probably won't last more than a fewmillion years longer.
Astronomers describe the brightness of objects in the sky in terms of Magnitude. The 20or so brightest stars in the sky (like Deneb and Altair) are Magnitude 1. Stars justvisible to the unaided eye are Magnitude 6. This scale was originally devised by the Greekastronomer Hipparchus over 2,000 years ago, and the modern version is largely an extendedand more accurate version of the ancient scale. Faint objects have large magnitudenumbers, bright objects have small numbers. Each step of one magnitude corresponds to anincrease or decrease of about 2.5 times in brightness; a five-magnitude step means a100-fold increase or decrease. Unfortunately, very bright objects can have zero or evennegative magnitudes, and scales with zero and negative numbers are often very confusing tobeginners. The nearest star beyond the Sun, Alpha Centauri, has a magnitude of zero.Sirius, the brightest star besides the sun, has a magnitude of -1.4. Venus at itsbrightest has a magnitude of -4, the full Moon about -12, the Sun about -27.
The Sun is about 25 magnitudes brighter than Sirius, and since each step of fivemagnitudes corresponds to a brightness step of 100 times, the Sun appears 100 x 100 x 100x 100 x 100 or 10 billion times brighter than Sirius. If Sirius were a star like the Sun,it would have to be 100,000 times farther away to appear as faint as it does (100,000 x100,000 = 10 billion). That distance is about 1.5 light years. Sirius is actually quite abit more luminous than the Sun and is really 8.6 light years away.
The brightness of stars as we see them from Earth is called Apparent Magnitude.If we can determine the distances to the stars we can determinetheir true brightness compared to each other. Astronomers define the AbsoluteMagnitude of a star as the brightness it would have at a distance of 10 parsecsor 32.6 light years. The absolute magnitude of the Sun is 4.8; it would be barely visibleto the unaided eye 32.6 light years away and entirely invisible 50 light years away, astone's throw in interstellar terms. This humbling fact suggests that interstellarnavigation might just be a bit more challenging than many science-fiction epics lead us tobelieve. Astronomers use the term Luminosity to refer to the total energyoutput of stars.
Most stars are constant in brightness, but there are many Variable Stars whosebrightness varies. There are many types of variable stars. Eclipsing Binariesconsist of a faint star orbiting a bright one. Normally we see the combined brightness ofboth stars, but when the faint star hides the bright one, the brightness dropsdramatically. By timing the disappearances of eclipsing binaries, astronomers obtainvaluable information about the diameters of the stars. There are many types of stars thatpulsate and vary in brightness, some regularly and rhythmically, others spasmodically andunpredictably. Other stars, called Novae, experience explosive outburststhat may increase the brightness of the star by hundreds of times. Finally there are starsthat experience catastrophic explosions that may destroy the star. These stars, called Supernovae,flare briefly to billions of times their normal brightness and are perhaps the mostviolent events ever witnessed.
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Created 26 March 1998, Last Update 11 April 2000