Online Astronomy eText: The Sky
Stars and Constellations
(also see Star Names / Sky Atlas)

     I am working on a celestial atlas, which contains constellation maps and lists of stars and deep sky objects in each constellation, and pages devoted to those deep sky objects (NGC and Messier objects, in particular). Although very incomplete at the moment, I have posted a preliminary version of every page, and more objects and descriptions will be added every day, until the atlas is complete.

The Scattered Stars
      (number of stars in sky of various brightnesses, number visible at any given time and place)

Stars visible in a relatively dark northern sky.

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 Internation 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 some 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).

Constellations and Asterisms Near The North Celestial Pole

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).

     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)

Orion and Taurus (constellations), and the Pleiades (an asterism).

Wikisky image of Orion
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

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)

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 which 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)