Online Astronomy eText: The Sky
Stars and Constellations
(also see Star Names / Sky Atlas)
The Scattered Stars
      (add a discussion of the number of stars in sky of various brightnesses, and the number visible at any given time and place)
Diagram of the stars visible to the naked eye in a reasonably dark northern sky
Stars visible in a relatively dark northern sky, looking toward the Celestial Pole.

Constellations and Asterisms
     Constellations are groups of stars which have been designated as representing a particular figure in the sky. Different cultures have done this in different ways, and many ancient and modern constellations are no longer in use. Eighty-eight of the traditional and modern Western constellations (based on Babylonian and Greek constellations of two to five thousand years ago) have been designated as "official" constellations by the IAU (the International Astronomical Union).
     Constellations can be represented by stick figures, some of which are traditional, and others which are created by the illustrator of one book or another. The latter are often copyrighted (e.g., those used in The Stars by H.A.Rey). In the past, constellations were usually depicted by artistic drawings based on allegorical figures, and different books typically had different drawings, which didn't always include exactly the same stars. In 1930 the IAU defined constellation boundaries which enclose the regions traditionally occupied by allegorical figures, so that every part of the sky is inside one constellation or another. Stars which are inside the boundary of a constellation are usually said to be "in" that constellation, meaning that they are in a direction enclosed by that boundary.
     Asterisms are groups of stars such as the Little Dipper, Big Dipper and the Pleiades, which are not constellations but have well-known names of their own. The Big and Little Dippers are part of the Big and Little Bears (Ursa Major and Ursa Minor). The Pleiades are an example of a cluster of stars which happens to be visible to the eye in the constellation of Taurus (the Hyades is another asterism, also in Taurus, which is also a cluster).

Diagram of constellations and asterisms near the North Celestial Pole
Constellations and Asterisms Near The North Celestial Pole (Courtney Seligman)
Cassiopeia and Cepheus are constellations; but the Big and Little Dippers are asterisms -- groups of stars which are only parts of constellations (in this case, Ursa Major and Ursa Minor).

An image of the Big Dipper, showing how Dubhe and Merak, called the Pointers, 'point out' the location of the Pole Star, Polaris
     Two of the stars in the bowl of the Big Dipper have nearly the same right ascension (an east-west position similar to longitude on the Earth); as a result, a line through them, being nearly north-south, points toward Polaris, the Pole Star. As a result, Dubhe and Merak are often referred to as the Pointers. (Jerry Lodriguess, Catching the Light, apod070108)

Image of the constellations of Orion and Taurus, and the asterism known as the Pleiades
Orion and Taurus (constellations), and the Pleiades (an asterism).

DSS image of the constellation of Orion, the Hunter
The constellation of Orion, based on a mosaic of dark-sky images
Save for the faint glow of the Orion Nebula, the nebulosity shown here is not visible without special filters

Color image of the constellations of Canis Major, the Big Dog, and Orion, the Hunter
Canis Major (left) and Orion (right)
Above: a typical modern color image. Below: a traditional black and white (negative) image
(Images based on apod070203, by Babak Tafreshi)
A traditional black and white negative image of Canis Major, the Big Dog, and Orion, the Hunter

Image of the Pleiades, an asterism or group of stars in Taurus, and also a cluster of stars
The Pleiades -- An Asterism In Taurus
Even though a well-known group of stars, the Pleiades is not a constellation, but simply part of the constellation of Taurus. Many people confuse the Pleiades with one of the Dippers (which are also asterisms). The misty glow around the stars is light reflected from clouds of dust that the cluster happens to be passing through, and has nothing to do with the dust which surrounded them during their formation. (ESA, AURA/Caltech, NASA)

Ways Of Showing Constellations
Allegorical Figures (immediately below) / Stick Figures / IAU Constellation Boundaries

     Until fairly recently maps in celestial atlases showed allegorical figures which more or less encompassed the region and stars traditionally associated with the constellations in that region. Examples of these are shown on most of the constellation pages in the Celestial Atlas that I am working on, and a few samples are shown immediately below:
Portion of Bayer's Uranometria showing the region near Orion
Orion in Bayer's 1603 Uranometria (Image from the USNO copy of the 1661 edition of Bayer's Uranometria)

Portion of Bayer's Uranometria showing the region near Taurus
Taurus in Bayer's 1603 Uranometria (Image Credit & © Tartu Observatory Virtual Museum; used by permission)

Portion of Bode's Uranographia showing Orion and Taurus
Orion and Taurus in Bode's 1801 Uranographia
(Image Credit & © Tartu Observatory Virtual Museum; used by permission) Image of the constellations of Orion and Taurus, and the asterism known as the Pleiades
An example of constellation stick figures shown earlier on this page (Courtney Seligman)

Cover of The Stars, by H. A. Rey
Gemini, as shown on the cover of
The Stars, © H. A. Rey
An excellent introduction to the sky, used as a supplementary text when I taught astronomy lab classes
The illustrations in Rey's book are copyrighted, but I expect no complaint about what amounts to free advertising

IAU Constellation Boundaries
     Since both allegorical and stick figures only give a general idea of where constellations are located and rarely show the "boundaries" of the constellations (and when they do, those boundaries vary from one atlas to another), in 1930 the International Astronomical Union "regularized" the constellations by assigning specific boundaries to each constellation. The boundaries jogged in and out to try to place stars that traditionally belonged to a given constellation inside that constellation's boundary, but were otherwise aligned with the hour circles of right ascension and parallels of declination according to the way the Earth rotated in 1875 (that being the approximate date when the idea was first proposed). Because of precession, the hour circles and parallels are gradually moving and tilting relative to their position more than a century ago, so the boundaries do not align with the current hour circles and parallels, and will become more and more misaligned as time goes on.
The IAU constellations showing their boundaries using the Equinox of 1875
Map of Sculptor by Courtney Seligman
A map of Sculptor showing the IAU boundaries (Image Credit Courtney Seligman)

Wikimedia Commons map of Scutum by Torsten Bronger
Modified version of Wikimedia Commons map of Scutum by Torsten Bronger