The Stars as Seen from Earth

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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The Stars as seen from Earth

Constellations

Every culture has told stories about the stars, and grouped the stars into Constellations. These differ radically from one society to the next; traditional Chinese constellations bear almost no resemblance to Western constellations. Most of the constellations now accepted in astronomy are based on traditional Western and Near Eastern star lore, with a few 17th and 18th-century additions. At one time, a few stars were parts of two constellations, and about a quarter of the stars in the sky were "unformed"; they belonged to no constellation. Now the constellations have fixed boundaries 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 only one, constellation. The constellation names are Latin.

The constellations we see from Earth are patterns of stars more or less in the same direction as we see them from Earth. Most of the objects in a constellation are at widely different 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 Cluster of galaxies (40 million light years). If we were to travel to a distant star, none of the constellations would be recognizable any more.

Names of the Stars

Only a few of the brightest stars have proper names. Each culture has its own star traditions, and most of our star names were invented by the Arabs. They came into English when Arabic learning came to Europe in the Middle Ages, often with interesting misspellings. For example, the Arabs called a bright red star in Orion Yad-al-Jawza, the Shoulder of Orion, but the Arabic characters for Y and B look much alike. A medieval scholar mistook the Y for a B when transcribing the name into Roman letters, and Betelgeuse it has been to this day. A few star names originated in other ways. Regulus means "little king" in Latin. Cor Caroli, Latin for "heart of Charles" was named in honor of the deposed and executed Charles I of England.

Star Catalogs

Most stars are designated in accordance with some star catalog. A common system is that of 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 roughly in order of decreasing brightness. Since Bayer estimated brightness by eye, his system is only approximate. When he ran out of Greek letters, he used lower-case Roman letters and then capitals. In Bayer's system, Sirius, the brightest star besides the Sun, is Alpha Canis 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 crossed the meridian. In Flamsteed's system Sirius is 9 Canis Majoris, the ninth star in Canis Major. Fainter stars are listed in great catalogues like the Bonn Durchmusterung (Sirius is BD -16 1591 in this catalog) or the Palomar Sky Survey. Usually each star designation has 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 happen to be of unusual interest. At times various organizations have offered to name stars after people, in return for a fee. Some such enterprises have been run in jest, to raise money for charity; others are merely aimed at the gullible, but none have any official standing.

Colors of the Stars

When we look at the night sky, the stars vary in color and brightness. They vary in color from blue-white through white, yellow, and orange to red. The colors are actually very pale; what we call a red star is actually a pale pinkish-orange. Color in stars is due mostly to differences in temperature: blue-white stars are hottest and red stars are coolest.

Brightness of the Stars

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 amounts of light. Two bright stars in the summer sky, Altair (16 light years) and Deneb (about 1,600 light years) appear about equally bright, even though Deneb is 100 times farther away. Light intensity drops off in proportion to the square of the distance, so Deneb must be about 100 x 100 = 10,000 times as luminous as Altair. Deneb is a stupendous star; its light left before the fall of Rome, yet it is still a bright star in our sky. It is about 70 million kilometers in diameter (half as big as the orbit of Mercury) and about 50,000 times 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 few million years longer.

Magnitude

Astronomers describe the brightness of objects in the sky in terms of Magnitude. The 20 or so brightest stars in the sky (like Deneb and Altair) are Magnitude 1. Stars just visible to the unaided eye are Magnitude 6. This scale was originally devised by the Greek astronomer Hipparchus over 2,000 years ago, and the modern version is largely an extended and more accurate version of the ancient scale. Faint objects have large magnitude numbers, bright objects have small numbers. Each step of one magnitude corresponds to an increase or decrease of about 2.5 times in brightness; a five-magnitude step means a 100-fold increase or decrease. Unfortunately, very bright objects can have zero or even negative magnitudes, and scales with zero and negative numbers are often very confusing to beginners. 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 its brightest has a magnitude of -4, the full Moon about -12, the Sun about -27.

Apparent and Absolute Magnitude

The Sun is about 25 magnitudes brighter than Sirius, and since each step of five magnitudes corresponds to a brightness step of 100 times, the Sun appears 100 x 100 x 100 x 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 x 100,000 = 10 billion). That distance is about 1.5 light years. Sirius is actually quite a bit 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 determine their true brightness compared to each other. Astronomers define the Absolute Magnitude of a star as the brightness it would have at a distance of 10 parsecs or 32.6 light years. The absolute magnitude of the Sun is 4.8; it would be barely visible to the unaided eye 32.6 light years away and entirely invisible 50 light years away, a stone's throw in interstellar terms. This humbling fact suggests that interstellar navigation might just be a bit more challenging than many science-fiction epics lead us to believe. Astronomers use the term Luminosity to refer to the total energy output of stars.

Variable Stars

Most stars are constant in brightness, but there are many Variable Stars whose brightness varies. There are many types of variable stars. Eclipsing Binaries consist of a faint star orbiting a bright one. Normally we see the combined brightness of both stars, but when the faint star hides the bright one, the brightness drops dramatically. By timing the disappearances of eclipsing binaries, astronomers obtain valuable information about the diameters of the stars. There are many types of stars that pulsate and vary in brightness, some regularly and rhythmically, others spasmodically and unpredictably. Other stars, called Novae, experience explosive outbursts that may increase the brightness of the star by hundreds of times. Finally there are stars that 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 most violent events ever witnessed.


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Created 26 March 1998, Last Update 11 April 2000

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