What Are Constellations?

1. What Are Constellations? The first thing you need to know is that constellations are not real! Constellations help by breaking up the sky into more manageable bits The constellations are totally imaginary things that poets, farmers and astronomers have made up over the past 6,000 years (and probably even more!). The real purpose for the constellations is to help us tell which stars are which, nothing more They are used as mnemonics, or memory aids

2. On a really dark night, you can see about 1000 to 1500 stars. • Stars are identified by their Brightness or Magnitude 0 magnitude • Being the brightest then becoming fainter as you count 1, 2, 3… • There are about 21 zero and first magnitude stars. • There are about 50 2nd magnitude stars including Polaris. • The 3rd magnitude stars total about 150 • There are some 600 4th magnitude stars • 5th magnitude stars are about the faintest you can see on a good • night. There are about 1500 of these stars, but less than 100 • of them appear on the charts. • f. Some 6th magnitude stars can be seen by the “keen of sight” in • constellations such as the Dolphin, Cup, and the Fishes. • g. For anything fainter binoculars or a telescope is needed. The magnitude scale was invented by an ancient Greek astronomer named Hipparchus in about 150 B.C

3. As it turns out, the eye senses brightness logarithmically, so each increase in 5 magnitudes corresponds to a decrease in brightness by a factor 100. The absolute magnitude is the magnitude the stars would have if viewed from a distance of 10 parsecs or some 32.6 light years. Obviously, Deneb is intrinsically very bright to make this list from its greater distance. Rigel, of nearly the same absolute magnitude, but closer, stands even higher in the list. Note that most of these distances are really nearby, on a cosmic scale, and that they are generally uncertain by at least 20%. All stars are variable to some extent; those which are visibly variable are marked with a "v".

4. What are apparent and absolute magnitudes? a. Apparent is how bright the appear to us in the sky b. Absolute magnitudes are how bright a star would appear from some standard distance, arbitrarily set as 10 parsecs or about 32.6 light years. Betelgeuse (alpha Orionis) is the second-brightest star in the constellation Orion and one of the brightest stars in the sky.

5. The Brightest Stars, as Seen from the Earth

8. There are 88 recognized constellations, with their names tracing as far back as Mesopotamia, 5000 years ago. Currently, 14 men and women, 9 birds, two insects, 19 land animals, 10 water creatures, two centaurs, one head of hair, a serpent, a dragon, a flying horse, a river and 29 inanimate objects are represented in the night sky (the total comes to more than 88 because some constellations include more than one creature.

9. Star Groupings and Asterisms Some of the more familiar "constellations" are technically not constellations at all. For example, the grouping of stars known as the Big Dipper is probably familiar to most, but it is not actually a constellation. The Big Dipper is part of a larger grouping of stars called the Big Bear (Ursa Major) that is a constellation

10. Constellations Are Not Physical Groupings The apparent groupings of stars into constellations that we see on the celestial sphere are not physical groupings. In most cases the stars in constellations and asterisms are each very different distances from us, and only appear to be grouped because they lie in approximately the same direction.

11. The Constellations of the Zodiac The zodiac is an imaginary band 18 degrees wide and centered on the ecliptic. The constellations that fall in the zodiac are called the 12 constellations of the zodiac. The constellations of the zodiac are still of importance because the planets, as well as the Sun and Moon, are all near or on the ecliptic at any given time; thus, they are always found within one of the zodiac constellations.

12. The 12 Constellations of the Zodiac Aquarius, the water bearerAries, the ramCancer, the crabCapricorn, the goatGemini, the twinsLeo, the lionLibra, the scalesPisces, the fishSagittarius, the archerScorpius, the scorpionTaurus, the bullVirgo, the virgin

13. When you look in a sky atlas, you might see diagrams like this: This type of schematic draws the stars as different sizes to represent different brightness You might also notice that every star on the chart is labeled In addition, other things besides stars are also labeled on the chart

14. Common Names Most of the brighter stars in the sky have common names that are of historical and mythological significance. For example, the bright red star in the shoulder region of the constellation Orion (the Hunter) is called Betelgeuse, which comes from Arabic and means (roughly) "the armpit of the mighty one" (see adjacent figure). The brightest star in Orion is a blue-white star called Rigel that is situated at the opposite corner of the constellation from Betelgeuse (adjacent figure).

15. The Bayer Naming System One more systematic method is the Bayer system, which names the brighter stars by assigning a constellation (using the Latin possessive of the name) and a greek letter (Alpha, Beta, Gamma, Delta, Epsilon, . . .) in an approximate order of decreasing brightness for stars in the constellation. The adjacent figure illustrates for Orion. Betelgeuse is also called Alpha-Orionis and Rigel is called Beta Orionis in the Bayer system.

16. The ordering of stars by brightness in the classical Bayer system is only approximate. For example, Rigel (Beta Orionis) is actually slightly brighter than Betelgeuse (Alpha Orionis), and Kappa Orionis is considerably brighter than the position of Kappa in the Greek alphabet would suggest. The brightest star in the nighttime sky is Sirius, which is in the constellation Canis Major and is termed Alpha Canis Majoris in the Bayer naming system.

17. The Flamsteed Naming System The Flamsteed naming system can in principle be used to name any number of stars. In this system one uses the same Latin possessive of the constellation name as in the Bayer system, but the stars are distinguished, not by their brightness, but by their nearness to the western edge of the constellation by assigning an arabic numeral. Thus, the closest star to the western edge of the constellation Cygnus is called 1-Cygni in the Flamsteed system and 61-Cygni denotes the star that is the 61st closest to the western edge.

18. Star Maps To use star maps effectively, you need to know your latitude and longitude on the surface of the Earth, and the offset of your timezone from the Greenwich meridian.

19. The celestial sphere is an imaginary sphere of infinite radius centred on the Earth, on which all celestial bodies are assumed to be projected. This Earth-centred Universe is, of course, not an accurate model of the real Universe, so why introduce it? First, it forms a convenient pictorial representation of the different directions of astronomical objects, and second, calculations involving these directions can be performed using some formulae of spherical trigonometry.

20. The celestial sphere is assumed to be fixed, so as the Earth rotates the celestial sphere appears to rotate in the opposite direction once per day. This apparent rotation of the celestial sphere presents us with an obvious means of defining a coordinate system for the surface of the celestial sphere - the extensions of the north pole (NP) and south pole (SP) of the Earth intersect with the north celestial pole (NCP) and the south celestial pole (SCP), respectively, and the projection of the Earth's equator on the celestial sphere defines the celestial equator (CE). The celestial sphere can then be divided up into a grid in a similar manner to the way in which the Earth is divided up into a grid of latitude and longitude.

21. Celestial CoordinateSystem Declination The celestial equivalent of latitude is called declination and is measured in degrees North (positive numbers) or South (negative numbers) of the Celestial Equator.

22. Right Ascension The celestial equivalent of longitude is called right ascension. Right ascension can be measured in degrees, but for historical reasons it is more common to measure it in time (hours, minutes, seconds): the sky turns 360 degrees in 24 hours and therefore it must turn 15 degrees every hour; thus, 1 hour of right ascension is equivalent to 15 degrees of (apparent) sky rotation.

23. Right ascension (RA; symbol α: Greek letter alpha) is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination. RA is comparable to longitude, measured from a zero point known as the vernal equinox point. RA is measured in hours, minutes, and seconds. Being closely tied with sidereal time, it is both a unit of time and of angle. An hour of right ascension is equal to 15 degrees of arc, a minute of right ascension equal to 15 minutes of arc, and a second of right ascension equal to 15 seconds of arc.

24. The apparent path of the Sun in the sky is known as the ecliptic and is actually the intersection of the plane of the Earth's orbit with the celestial sphere. Because the rotation axis of the Earth (which defines the celestial sphere) is tilted at an angle (23.5°) with respect to the plane of the Earth's orbit, the ecliptic is inclined at an angle to the celestial equator.

25. The ecliptic and the equator intercept at two points, associated with the zodiacal constellations of Aries and Libra. spring equinox (or vernal equinox), on March 21 autumnal equinox, on September 21

26. The maximum altitude of the Sun in the sky, as viewed from the northern hemisphere, gradually increases from the spring equinox until it reaches a maximum on June 21 - the summer solstice (when the Sun appears to `stand still' in the sky before starting to move back towards the celestial equator)

27. The Sun reaches its minimum altitude in the sky when viewed from the northern hemisphere on December 21 - the winter solstice - which marks the beginning of northern hemisphere winter.

28. The Earth rotates from west to east and hence the stars appear to revolve from east to west about the celestial poles on circular paths parallel to the celestial equator once per day. Some stars never set and remain visible at night all year. These are called circumpolar stars upper culmination. Which stars are circumpolar depends on the latitude of the observer lower culmination

29. REVIEW

30. Coordinates on the Celestial Sphere The right ascension (R.A.) and declination (dec) of an object on the celestial sphere specify its position uniquely, just as the latitude and longitude of an object on the Earth's surface define a unique location. Thus, for example, the star Sirius has celestial coordinates 6 hr 45 min R.A. and -16 degrees 43 minutes declination, as illustrated in the following figure.

31. The Constellations of the Southern Hemisphere (some are seasonally visible in the Northern Hemisphere): Apus, the bird of paradiseAra, the altarCarina, the ship's keelCentauras, the centaurChamaeleon, the chameleonCircinus, the compassCrux, the southern crossDorado, the swordfishEridanus, the riverGrus, the craneHydrus, the water snakeIndus, the IndianLepus, the rabbit Mensa, the tableMusca, the flyNorma, the surveyor's levelOctans, the octantPavo, the peacockPhoenix, the phoenixPictor, the easelReticulum, the netTriangulum Australe, the southern triangleTucana, the toucanVela, the ship's sailsVolans, the flying fish

32. The Constellations of the Northern Hemisphere (some are seasonally visible in the Southern Hemisphere): Andromeda, the princessAntlia, the pumpAquila, the eagleAuriga, the chariot driverBootes, the herdsmanCaelum, the chiselCamelopardalis, the giraffeCanes Venatici, the hunting dogsCanis Major, the big dogCanis Minor, the little dogCassiopeia, the queenCepheus, the kingCetus, the whaleColumba, the doveComa Berenices, Berenice's hairCorona Australis, the southern crownCorona Borealis, the northern crownCorvus, the crowCrater, the cup

33. Cygnus, the swanDelphinus, the dolphinDraco, the dragonEquuleus, the little horseFornax, the furnaceHercules, the heroHorologium, the clockHydra, the water snakeLacerta, the lizardLeo Minor, the little lionLupus, the wolfLynx, the lynxLyra, the harpMicroscopium, the microscopeMonoceros, the unicornOphiuchus, the sepent holderOrion, the hunter Pegasus, the flying horsePerseus, the Medusa killerPisces Austrinus, the southern fishPuppis, the ship's sternPyxis, the ship's compassSagitta, the arrowSculptor, the sculptorScutum, the shieldSerpens, the snakeSextans, the sextant Telescopium, the telescopeTriangulum, the triangleUrsa Major, the big bearUrsa Minor, the little bearVulpecula, the little fox

34. Constellations Sorted by Month January Caelum Dorado Mensa Orion Reticulum Taurus

35. Bibliography http://www.enchantedlearning.com/subjects/astronomy/ http://csep10.phys.utk.edu/astr161/lect/time/naming.html http://www.astro.wisc.edu/~dolan/constellations/constellations.html http://www.astro.columbia.edu/~archung/labs/fall2001/lec02_fall01.html