Astronomers describe the brightness of objects in the sky in terms of magnitude. 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. Hipparchus called the 20 or so brightest stars in the sky magnitude 1, and stars just visible to the unaided eye are magnitude 6. 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, because Hipparchus followed the natural tendency to rank the most prominent objects as first magnitude, very bright objects can have zero or even negative magnitudes, and scales with zero and negative numbers are often very confusing to beginners. A zero magnitude star doesn't mean no brightness, it means a very bright star. We can also define fractional magnitudes.
The table below shows the number of stars of each magnitude and the total number of stars of equal or greater brightness. At each step the number of stars visible roughly triples.
|Magnitude||Number of Stars||Number of stars of equal or greater brightness|
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 to fill in unmapped areas (and make a name for the inventor), and map the southern skies. 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.
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.
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. In the case of the Big Dipper, he simply started with alpha at the northern pointer star. 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 and more recently, the Hipparcos Catalog (Sirius is HIP 32349). 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. Astronomers are under no obligation to use purchased star names.
Note: the following is written for Northern Hemisphere viewers.
In order to find your way in the sky, there are two things to bear in mind:
Below are illustrations showing how the Big Dipper changes its position and orientation in the early evening hours of each season.
Distances on the sky are measured in terms of angles. Lines from opposite sides of the Moon to your eye intersect at an angle of about 1/2 degree, so we say the Moon spans, or subtends, an angle of about 1/2 degree. Your fist at arm's length spans about 10 degrees.
The heavens appear to rotate around two points called the celestial poles. The elevation of the pole above your horizon is equal to your latitude. Stars close to the north celestial pole circle the pole without ever hitting the horizon. Such stars are called circumpolar. Around the South Celestial Pole is a similar zone of stars that never rise. In between, is a band of stars that rise and set.
Since the celestial pole is at an elevation equal to your latitude, at the North Pole the elevation of the celestial pole is 90 degrees - straight overhead. All the stars circle the pole endlessly and never rise or set. All the stars are circumpolar, and the opposite half of the sky can never be seen. At the equator, the poles are at an elevation of zero degrees. All stars rise and set. The equator is the only latitude where all stars in the sky are visible at some time or another.
On the star maps below, major constellations are joined by red lines, and less conspicuous constellations are in gray. Not all constellations are shown. Constellations are in bold type, individual stars and asterisms, or informally named groups of stars, are in italics.
Above are the north circumpolar constellations. Best known is Ursa Major, the Big Dipper. The two stars at the end of the bowl are about 5 degrees apart and a line through them points to Polaris, the North Star. It is purely coincidence that we live at a time when there is a bright star close to the North Celestial Pole. There is no South Star.
Apart from Polaris and one star in the bowl, Ursa Minor (the Little Dipper) is inconspicuous. The stars in the bowl include one from each magnitude, second through fifth, so it's a good guide to estimating star brightness.
The star at the bend of the Big Dipper's handle, Mizar, has a faint companion called Alcor that is a good test of your eyesight. A line from Mizar through Polaris and continued beyond takes you to Cassiopeia, easily recognized by its W shape.
Two fainter constellations are Cepheus, shaped like a house, and Draco, which winds between the Dippers. In the movie Dragonheart, the dragon hero Draco sacrifices his life and takes his place among the stars, and the neat thing is that the scene actually shows the constellation accurately.
In the late spring, you can use the Big Dipper as a guide to other constellations. Follow the arc of the Big Dipper's handle to the bright orange star Arcturus, then continue the arc to the bright star Spica. If you extend a line south from the Pointer stars, they take you to the constellation Leo, dominated by the bright star Regulus. A diagonal through the bowl will point to the winter constellation Gemini. South of Leo is the constellation Hydra, not very conspicuous except for one moderately bright star, but notable as the largest constellation in the sky.
Later in the spring and early summer, to the east of Arcturus, you can see the pretty little circle of Corona Borealis, the Northern Crown. To its east is Hercules, whose most obvious feature is the Keystone of four moderately bright stars. North of Hercules is a small trapezoid, the head of Draco, the only conspicuous part of the constellation.
When the stars are first becoming visible, Arcturus and three other stars in Bootes and Corona Borealis form a conspicuous Y shape, shown in green.
The spectacular summer sky has few really bright stars but picturesque constellations galore and a glorious view of the Milky Way.
The Summer Triangle consists of Deneb and Vega, almost circumpolar for the northern U.S., and Altair. Light from Altair left 16 years ago. Light from Vega left 27 years ago. Light from Deneb left before the Roman Empire fell. Deneb is the brightest star of Cygnus, the Swan, and forms its tail (Deneb means "tail" in Arabic). The Swan is flying with its outstretched neck, and the shape really reflects the constellation name. The head of the Swan, Albireo, is one of the most beautiful double stars in the sky as seen through a small telescope. If you omit the wingtips, the rest of Cygnus forms a cross called the Northern Cross.
Vega is the brightest star in Lyra, a small parallelogram capped by an equilateral triangle. Altair is the brightest star in Aquila, the Eagle, another constellation that's not a bad match for its name, though not terribly bright. Between Aquila and Cygnus is tiny Delphinus, the Dolphin, not very brilliant but very attractive and really fitting its name.
Skimming the southern horizon is Sagittarius, whose stars outline a teapot, and Scorpius the Scorpion, yet another constellation that looks like its namesake. Scorpius is dominated by reddish Antares, whose name means "anti-Mars" because it looks similar to that planet. Between Scorpius and Hercules is faint, sprawling Ophiuchus and the two halves of Serpens. Because of how constellation boundaries are drawn, the Sun actually spends more time in Ophiuchus than it does in Scorpius, and every so often the astrology community gets excited at the "discovery" of a thirteenth constellation in the Zodiac.
Two important points are not marked on the chart. Between Vega and Hercules is the direction the Sun is moving relative to nearby stars. Just off the spout of the teapot of Sagittarius is the center of the Milky Way Galaxy, invisible to the eyes but detectable in radio waves.
The Summer Triangle is high in the western sky in the fall but most of the constellations are associated with the myth of vain Cassiopeia, whose vanity offended the gods, who in turn sent a sea monster (Cetus) to ravage the coasts. She and her husband Cepheus consulted an oracle, who decreed that their daughter Andromeda had to be left as a sacrifice to the monster. (Notice how Cassiopeia offends the gods, but Andromeda has to be sacrificed? That's how it went in myths. The fact that Cassiopeia and Cepheus were backed by a bunch of surly guys with swords and spears had only a little to do with the oracle's judgment.) Anyway, just as Cetus and Andromeda are about to do lunch, along comes the hero Perseus, riding the winged horse Pegasus, and saves the day. And they lived happily ever after, or at least until Perseus left his socks on the floor.
The Great Square of Pegasus is the flagship of the autumn sky. Andromeda consists of two arcs of stars pointing between Cassiopeia and Perseus. Not far from the midpoint of the arcs is M31, the Andromeda Galaxy, at 2.3 million light years, the most distant object visible to the unaided eye.
The zodiac constellation Aries is fairly easy to see. Pisces, Aquarius and Capricornus are rather faint. Aquarius consists of a tight little tripod of stars called the Water Jar and to other moderately bright stars. Far to the south is lonely Fomalhaut, a first magnitude star.
Three variable stars of very different types are in this part of the sky. Mira in Cetus is a huge pulsating red giant star that attains second magnitude and then fades to invisibility with a period of about a year. Algol in Perseus is an eclipsing variable in which a faint star orbits a brighter one, eclipsing it every few days. Delta Cephei is the namesake of a class of variable stars that vary regularly in brightness. The period of variation is directly related to the true brightness of the star, making it possible to use these stars as distance indicators in the Universe.
For Northern Hemisphere viewers, the winter sky is the richest in first magnitude stars. At twilight, the first stars to appear form a giant hexagon with reddish Betelgeuse near the center.
Auriga, the charioteer, is dominated by yellow Capella, a nearly circumpolar star in the northern U.S. Taurus, the Bull, is dominated by orange Aldebaran and two star clusters, the V-shaped Hyades and the cute Pleiades. When you first see Capella in the northeast and the Pleiades rising, you know winter is on the way. Gemini is marked by the twins Castor and Pollux. Canis Major is a bright constellation crowned by dazzling Sirius, the brightest star in the sky. Canis Minor consists of yellow Procyon and one other moderately bright star.
Orion dominates the winter sky, with blue white Rigel and reddish Betelgeuse at opposite corners and the three stars of his Belt. Hanging from the belt is his Sword, containing the Great Orion Nebula. Some of the stars in Orion are among the most distant stars easily visible to the eye, over 2000 light years away. According to legend, Orion was stung to death by a scorpion (Scorpius), which is why the two constellations are on opposite sides of the sky.
In winter, we're looking toward the sparse outer edge of the galaxy and the Milky Way is narrow and faint. It is not shown on the chart above but runs across Auriga, Orion and Canis Major.
South of Nashville or Santa Fe, brilliant yellow Canopus clears the horizon. It's the second brightest star in the sky and important in spacecraft navigation. It's so bright it's easy to detect, and nearly at right angles to the plane of the earth's orbit, so if a spacecraft can lock on Canopus and the Sun, it's orientation in space is fixed. First magnitude Achernar appears at Gulf Coast latitudes. Its constellation, Eridanus, represents a river, though which river varies considerably depending on the myth.
Canopus belongs to a constellation called Carina, which means "keel" (of a ship) in Latin. The stars in the lower left corner of the chart once belonged to a huge constellation called Argo, the ship sailed by Jason and the Argonauts in Greek mythology. But Ago was such a big and unwieldy constellation - bigger than the largest constellation today - that it was subdivided into three constellations: Carina, the keel, Vela, the sails, and Puppis, the stern (from which we get the expression "poop deck.")
No region in the sky has four first magnitude stars so close together as the Southern Cross and nearby Centaurus. In the U.S., only Hawaii, the Florida Keys and extreme southernmost Texas can see these constellations. Follow the arc from Arcturus to Spica and keep on going. This part of the sky is just breathtaking, peppered with moderately bright stars and the Milky Way. Crux is not so much a cross as a kite with a little off-center star within it. Nearby are blue white Beta Centauri and brilliant golden Alpha Centauri, third brightest star in the sky and a near twin of our Sun. It's also a spectacular double star and the nearest star beyond the Sun.
Below and right of Spica is Corvus the Crow, a neat little trapezoid. If you can spot Corvus, just drop straight down from it to the Southern Cross. Libra has only two fairly bright stars. At one time they were considered to be the claws of Scorpius.
The upright of Crux points nearly north-south and is used for direction finding in the Southern Hemisphere, since there is no South Pole star. Note that below the word "cross" in the chart there is another similar sized diamond of fairly bright stars. This grouping is called the "False Cross" because it is sometimes confused with the real Southern Cross.
There's a small star within the Southern Cross that spoils the pattern. In Latin America, this star is sometimes called "Intrometida" - "the intruder." In reality, the star has every right to be where it is - it was there long before we named the constellation. It's only an "intruder" because it breaks up a pattern that we think should be there.
Just like on the earth, we have latitude and longitude on the sky. The sky equivalent of longitude is called Right Ascension. It is measured in hours, minutes and seconds of time, that is, how long it takes a given interval to move across the sky. One hour of right ascension equals 15 degrees, one minute equals 1/4 degree. The equivalent of latitude is called declination and is measured in degrees. The Celestial Equator is a line around the sky halfway between the celestial poles and is directly above the earth's equator.
On the diagram above, we are seeing the celestial sphere as if we are outside it. Since we are inside it, the hours actually increase from east to west (toward the right as you face south). The numbers ascend to the right.
Since the earth's axis is tilted with respect to the plane of the earth's orbit, the Sun and planets appear to travel a path at an angle to our celestial coordinates.
We call this path the ecliptic because eclipses occur when the Moon is exactly on it. It is tilted 23.5 degrees with respect to the celestial poles. We use the ecliptic to define zero right ascension. Where the ecliptic crosses the celestial equator going up is zero right ascension. The sun is at this point on March 21 so this point is often called the vernal equinox. The ecliptic reaches a maximum declination of 23.5 degrees at 6 hours right ascension (summer solstice), crosses the equator headed south at 12 hours (fall equinox), and reaches a minimum of -23.5 degrees at 18 hours (winter solstice).
We can define latitude and longitude based on the ecliptic, with the ecliptic as the equator of the sphere and two ecliptic poles 90 degrees away. This system is useful for describing the motions of the planets. Ecliptic latitude and longitude are described in terms of degrees, like terrestrial latitude and longitude, with zero longitude at the vernal equinox, 90 at the summer solstice, and so on.
The diagram above shows how the circumpolar stars relate to celestial coordinates for 45 degrees North.
Created 07 July 2008, Last Update 14 December 2009
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