asterisms, clusters of stars, star names
Credit: Mount Wilson Observatory StarMap program by Bob Donahue. StarMap is fortran program, but it's been broke since 2000jan03. Download site: Univ. of Tennesse, Knoxville Astro course; more precisely here.
Since prehistory humans have noticed the stars, of course. Over the course of a human life-time and even many generations don't seem to move much, except for the daily westward overall circling and the much slower overall motion due to the precession of the equinoxes.
Both daily circling and precessions are physically caused by the motions of the Earth, of course---but this was not understood until after Copernicanism was accept in the course the 17th century.
These Earth-originated motions do not change the relative angular positions of the stars.
There is a small relative motion among the stars due the Earth's revolution around the Sun: the famous stellar parallax not discovered until 1838 (No-419).
Actually, the stars do have significant relative motion if you wait long enough or measure carefully enough. This motion is due to both the motion of the stars through space and the motion of the Sun through space.
Now in fact the stars close enough to the Earth to be easily seen are to a large degree actually spread randomly in space. There are some physically groupings of stars (i.e., star clusters), but they are relatively few in our naked-eye range. The Milky Way has, of course, a varying distribution, but over relatively small part where we see bright stars, the distribution is relatively constant---allowing for random spacing. We do see the Milky Way's variation, of course, but only through the Mikly Way band (which is what the ancients thought of as the Milky Way itself) which was not resolved into stars until the advent of the telescope (No-335).
From the above two points, we can say that the naked-eye stars are relatively fixed and relatively randomly placed in space. This means that there will random spatial groupings of stars in space. Furthermore there will be random groupings of stars in angle as seen from Earth.
These random groupings in angle do not correspond to spatial groupings in general. Some stars close together in angle may be hundreds of light-years apart. Since stars vary in intrinsic luminosity by more than 10**9 (Se-605, small star catalogue ), the apparent brightness of any individual star is not at all a reliable measure of that star's distance. A bright star may have low luminosity, but be relatively close or it could have a high luminosity and be relatively distant. Without modern astronomical techniques, one simply cannot tell.
The upshot of the proceeding points is that groupings of stars in angle usually have no fundamental physical significance. They are usually just a consequence of our vantage point on Earth. But in pre-modern times before the size and nature of space were known, it was plausible to believe that the groupings were of fundamental importance: this belief is a basic ingredient of astrology, of course. The fading of the belief dealt yet another death blow to astrology---but it never died.
The groupings are NOT CLEAR-CUT. It is possible to divide the stars into many different sets of groups. And historically this is what happened. Different cultures selected different sets of groupings to recognize as fixed groups which in English, of course, are called CONSTELLATIONS.
We can't know for sure how ANCIENT CONSTELLATIONS were settled on or why. Probably the process was somewhat random and the name assigned to a constellation in many cases may have been just MNEMONIC without implying anything intrinsic about the nature of the constellation. The BIG DIPPER was certainly so called because it looks like a set of dots outlining a dipper. But even with connecting lines (which, of course, arn't on the sky) most constellations look like the object they are named for ONLY in an abstract-in-eye-of-the-beholder way. Without connecting lines, except for Big and Little Dippers, the constellation shapes have almost no relation to the names assigned to them.
The constellation names were no doubt often assigned to honor a god or a legend. For example, TAURUS (the Bull) which goes back at least to the Babylonians of the 5th century BC and, perhaps, much earlier, may honor a bull god or a sacred bull. The Golden Calf of the Bible is not forgotten. When the GREEKS acquired Taurus from the Babylonians, they assigned their own bull myth to Taurus: it is ZEUS in the shape of a bull that he used to carry off Europa: but enough of that.
It is not clear to the author of these notes (that's me you know) how SACRED constellations ever were in the tradition that Europe inherited. They don't seem to have ever been directly worshipped or ritually honored so far as I can tell. In astrology, the SIGNS of the Zodiac are technically not the constellations of the same name, but 30 degree segments of the Ecliptic that constellation of that name occupied about 500 BC. However, a popular identification of the sign with the constellation of the same name seems perfectly fair to me---there's no reason to be consistent about astrology.
As mentioned above there have been many assigments of constellations. For example, Chhien Lu-Chih (5th century) working in the CHINESE TRADITION of astronomy grouped 1464 stars into 284 CONSTELLATIONS (No-139--140). The Yin-Yang symbol (shown to the right) doesn't represent a constellation, but astronomical interpretations of it are possible: e.g., night and Moon (Yin) and day and Sun (Yang).
(Actually the web offers a plethora of sites on the Yin-Yang that mostly don't look very trustworthy, but the Yin-Yang page of Dorothy Wilkins has references. The quote on the figure comes from a herbal medicine page, but it agrees sort of with the better documented Tao Te Ching translation of Stan Rosenthal. I've also figured out that the Tao Te Ching is not the same as the I Ching.)
At least some of ANCIENT CONSTELLATIONS that have found there way into the modern constellation canon may go back to the ANCIENT MESOPOTAMIANS of 2000 BC. The uncharted southern part of the sky known to the Greek poet Aratus (circa 270 BC) would have been below the southern horizon for observers of latitude about 36 degrees north circa 2000 BC allowing for the precession of the equinoxes. Babylon is about 33 degrees north.
The GREEKS had close contacts with BABYLONIAN ASTRONOMY after Alexander's conquest of the Persian empire (circa 330 BC) (No-17,35,39,93) and probably acquired the Babylonian constellations sometime after 330. Aratus describes them in his poem Phaenomena. PTOLEMY in his catalogue groups his 1022 fixed stars in 48 constellations many (most???) following the Babylonian constellations. Ptolemy's constellations are the basic set of classical constellations from which modern constellations are derived.
In the 16TH AND 17TH CENTURIES new constellations were added by celestial cartigraphers to group southern stars that were below the horizon of the Mediterranean area. The first new southern constellations seem to have been introduced by JOHANN BAYER in his Uranometria (Augsburg, 1603): he made up 12 new ones including Tucana (Toucan), Grus (Crane), and Phoenix.
In the 17th and 18th centuries there was a lot of making
up of new constellations to fill in gaps between the ancient ones.
Many of these didn't survive at all.
For example, JULIUS SCHILLER in his
Coelum Stellatum Christianum
(Augsburg, 1627) replaced all the traditional constellations
with Christian versions: the
Zodiac got changed into the
TWELVE APOSTLES and the Argo Navis (an ancient constellation
that didn't survive) became
Noah's Ark.
JOHANNES HEVELIUS introduced 11 new constellations in his Firmamentum Sobiescianum sive Uranographia (Gdansk (Danzig), 1690) of which 7 survived to the modern list: SCUTUM SOBIESCANUM (the modern name is just Scutum: see figure to the right), Canes Venatici, Leo minor, Lynx, Sextans, Lacerta (the lizard), and Vulpecula (the fox with the goose). (Hevelius was one of the great observational astronomers of the 17th century.) Some of these constellation names are merely following traditional nomenclature: animals and simple devices (e.g., Sextans means sextant). But Scutum Sobiescanum means Shield of Sobiesky named in honor of the Polish King John III Sobiesky who repulsed a Turkish invasion 1673.
A later attempt to insert new constellations occurs in the 1795 PARIS REVISION of John Flamstead's (1646--1719) star atlas. In proximity to ORION AND TAURUS are the TELESCOPE OF HERSCHEL in honor of William Herschel the discoverer of Uranus and the HARP OF GEORGE in honor of maybe George III---actually it doesn't seem likely that a king would find honor in Paris in 1795---maybe it's George Washington's harp: I never knew he played.
In 1922 the INTERNATIONAL ASTRONOMICAL UNION (IAU) at it's first meeting decided---perhaps arrogating to itself the right to decide---on a fixed list of 88 constellations. These include many of the traditional constellations of Ptolemy and some of the modern inventions particularly for southern hemisphere sky. The Telescope of Herschel didn't make the cut though there is a Telesopium in the sourthern sky. Some ancient constellations failed to get in too like the southern Argo Navis---it was divided into Carina (the Keel), Puppis (the Poop [i.e. stern]), and Vela (the Sails)---and Antinoos: see Ian Ridpath's Star Tales. In 1930 the IAU assigned to each constellation a fixed area on the Celestial Sphere : these areas cover the entire Celestial Sphere and effectively become the constellations for modern astronomical purposes.
A good list of the modern 88 constellations is at the Munich Astro Archive . The Archive gives the astronomical details and the mythical background if there is one. The constellations include 14 humans, 19 land animals, 10 water creatures, 9 birds, 2 insects, 2 centaurs, a head of hair (Coma Berenices [Berenice's Hair]), a serpent, a dragon, a flying hourse (Pegasus), a river, and 29 inanimate objects including a telescope and a ship's sail. There are more objects in the above enumeration because some constellations include multiple objects.
In addition to the IAU OFFICIAL CONSTELLATIONS, there are OBSOLETE and UNOFFICIAL CONSTELLATIONS and other recognized groups of stars. Any of these groups can be called an ASTERISM. (Asterism is from the Greek asterismos derived from asterizein meaning to mark with stars [Ba-76].) The most famous asterism is the Big Dipper which is still often called a constellation in it's own right, but it is not in the official IAU 88 constellation list: it is part of Ursa Major (the Great Bear). A good photographic image of Ursa Major is at the Stellar Scenes site of Naoyuki Kurita: Naoyuki Kurita's Ursa Major (the Great Bear). Similarly the Little Dipper (part of Ursa Minor [the Small Bear]) is an asterism. Polaris (the North Star or the Pole Star) is at the end of the handle of the Little Dipper: in year 2000 epoch coordinates it is only 44 arcminutes, 9 arcseconds from the North Celestial Pole (NCP). Other well well known asterisms are presented by the Munich Astro Archive.
(To find Polaris easily locate the Big Dipper (which overall is a much brighter, more obvious asterism than the Little Dipper) and the two stars farthest from the handle: a line from these stars (called the pointer stars) up from the Big Dipper runs pretty nearly into Polaris: see the northern winter-night sky map.)
As mentioned above constellations are generally NOT physical groups of stars: in fact none of the IAU 88 constellations are physical groups???. But some of the asterisms are physical groups. The most famous are probably the PLEIADES (M45) (see below right and SEDS Pleiades or Steven Gibson's Pleiades ) and HYADES (see SEDS Hyades) which are both OPEN STAR CLUSTERS in Taurus. (A good photographic image of Taurus showing the Pleiades and Hyades is at the Stellar Scenes site of Naoyuki Kurita: Naoyuki Kurita's Taurus.)
The Pleiades, an open star cluster, in Taurus at about 410 lyr and spanning about 40 lyr. The glow around the star is reflected starlight from interstellar dust.
Credit: NOAO/AURA/NSF.
An open cluster is group of stars formed in a common star forming region. Because of spatial compactness and because the stars formed nearly at the same time (by astronomical standards), open clusters are of great astrophysical importance in learning star properties. As groups of stars the Pleiades and Hyades have been known in many cultures since prehistoric times. Hesiod (circa 8th century BC) mentions them in his Works and Days:
All good rural advice I assume. Chaucer cleverly alludes to them in the Canturbury Tales: see Chaucer.
The name Pleiades may be derived from Pleione (a mythical mother) which is also the name of one of the brighter stars. In Europe they have also been called the Seven Sisters and their Japanese name is Subaru like the car and the telescope. Usually 6 Pleiades at least can be seen with the naked eye; 9 can be seen under good conditions; 14 were claimed visible by Kepler. Telescopically, the cluster has over 500 stars.
To find the Pleiades, one can use the constellations as landmarks---well SKYMARKS First, locate Orion and Sirius (the brightest star in the sky) off to the lower left of Orion (south-east on the sky). A line from Sirius though the belt of Orion and then through the bright orangy Aldebaran (the eye of Taurus) leads pretty much to the Pleiades---a distinct close little group of six or more stars. The northern constellations map below illustrates the method.
The northern constllations: a mid-winter night-time view judging from the position of old man Orion.
Credit: Mount Wilson Observatory StarMap program by Bob Donahue. StarMap is fortran program, but it's been broke since 2000jan03. Download site: Univ. of Tennesse, Knoxville Astro course; more precisely here.
(Yours truly admits to never having done this, but in principle it should work.) The Hyades are the stars about Aldebaran. But Aldebaran is not part of the physical Hyades cluster: it is a foreground star at about 60 lyr; the Hyades are at about 150 lyr---they are closest star cluster to Earth, except for the very spread out Ursa Major cluster.
Part of the sky lore that goes with constellations are the TRADITIONAL STAR NAMES given to the brightest stars. These names have, of course, derived from a hodge-podge of sources over the millennia.
Possibly Al Sufi (903--986), the Persian astronomer who fixed many of the names of the fixed stars in his Book of the Fixed Stars.
Credit: Medieval Islamic artist; modern credit ??? (but believed to be public domain); download site Wolfgang Steinicke's list of NGC/IC observers.
Johann Bayer in his Uranometria made the innovation of naming stars
according to their relative brightness in the constellation in which they were found (Se-12). To indicate their brightness he used GREEK LETTERS in order of decreasing brightness: alpha for brightest, beta for 2nd brightness, gamma for third brightest, etc. To indicate the constellation he appended the Latin possessive form of the constellation name: hence Aldebaran, the brightest star, in Taurus becomes ALPHA TAURII or alpha Tau for short.
Bayer's assignment of brightnesses doesn't always accord with the modern assignment---in fact the agreement is pretty poor---but his nomenclature is retained. The Bayer names complement and supplement the traditional names: they don't replace them. The invention of the telescope in 1608 (No-328, Lecture 4.8), of course, revealed myriads of new faint stars and Bayer's system became inadequate eventually. Other star name systems became necessary: the most basic of these is simply to name the star by its declination and right ascension coordinates. A longer discussion of modern star naming systems is given by Jim Kaler of the University of Illinois.
Why do we astronomers still want constellations? As we argued above we now understand that they have no fundamental physical significance? Partially, it is just that astronomers and folks in general are FOND of their constellations---they're traditional and part of the romance of astronomy---so we should keep them in an orderly fashion. There is also a practical use for both professional and amateur astronomers. The modern constellations (i.e., patches of the sky) provide a useful rough and easily memorized location system--- the constellations act as SKYMARKS. One can always locate an object precisely using declination and right ascension, but for just a ROUGH LOCATION one can use constellation mnemonics: e.g., for Polaris and the Pleiades as described above. Also for a rough position one can say the object is in such or such a constellation. For example, one can say there is a bright supernova in Virgo: this is a relatively frequent occurrence since there is a large nearby cluster of galaxies in Virgo (the Virgo cluster), and so bright supernovae are relatively frequently found in Virgo. (Supernovae occur only a few times per century in large galaxies, but if you are looking at many galaxies in a cluster you'll see them much more often.)
The locution object x is in constellation y, although perfectly natural given the modern definition of constellation, does have astrological suggestiveness as if there was a magic sympathy between object and constellation---Venus is Virgo or Venus is in Taurus---but this is just a vestige of where we've come from.
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