A constellation is formally defined as a region of the celestial sphere, with boundaries laid down by the International Astronomical Union (IAU). Constellations are usually based on asterisms, which are chance groupings of stars in the sky that resemble familiar patterns. Some constellations contain asterisms within them. For example, the Big Dipper is an asterism located within the constellation Ursa Major. The constellations are actually much larger than the asterisms they contain. Today there are a total of 88 constellations that cover the entire night sky. 36 of these are located in the northen hemisphere of the sky white the remaining 52 are in the southern hemisphere. The official borders of these constellations were defined by the IAU in the 1920s. Astronomers use these constellations as a location marker for identifying the part of the sky where a particular star or other object can be found.
The asterisms that make up the constellations were seen by ancient people as patterns in the stars. Their origins date back hundreds or even thousands of years. Ancient cultures in different parts of the world have assigned different patterns to these star groupings. There are 50 constellations that date back to ancient Greece, Rome, and the Middle East. Many of these ancient constellations are associated with the rich mythology of the time. The other 38 are much more recent. The Greek astronomer Ptolemy cataloged 48 constellations in the second century. They were based on the work of the Greek astronomer Eudoxus of Cnidus. Some of the southern constellations are more recent because they could not be seen by any of the ancient civilizations north of the equator. They were charted by Dutch navigators in the late 16th century and included in Johann Bayer’s star atlas Uranometra in 1603. Bayer added a total of 11 new constellations to the sky. Several more were added by the French astronomer Nicolas Louis de Lacaille in his catalogue published in 1756. Lacaille created 14 new constellations while mapping the southern skies from an observatory in South Africa. The International Astronomical Union defined the final boundaries of the 88 constellations in the 1922.
Classifying the Stars
Stars in the night sky are classified by their brightness, which is referred to as their magnitude. The magnitude of a star is measured on a logarithmic scale, similar to that used for measuring earthquakes. Each order of magnitude equals a variation in brightness of approximately 2.5 times. Therefore, a magnitude 1 star is 2.5 times brighter than a magnitude 2 star, and is 2.5 x 2.5 times brighter than a magnitude 3 star. Astronomers use two different types of magnitude. Absolute magnitude measure the brightness of a star as it would appear if it were placed at a fixed distance from the Earth. Apparent magnitude is the brightness of the star as it appears in the night sky from Earth. The stars in the constellations are classified using their apparent magnitude, also called visual magnitude.
Classification by Magnitude
Most of the brighter stars in the constellations were assigned names by the German astronomer Johann Bayer in 1603, when he published his star atlas known as Uranometra. Bayer assigned a lower-case Greek letter such as alpha, beta, gamma, etc. to each star he cataloged. Each star’s name consists of a Greek latter plus the Latin name of the constellation in genitive, or possessive, form. In most cases, Bayer assigned the letters in alphabetical order based on the star’s brightness. For example, the brightest star in the constellation Taurus is Alpha Tauri. The second brightest star is Beta Tauri. The next brightest is Gamma Tauri, and so forth. These classifications are known as Bayer designations and are still used today. However, in Bayer’s day the brightness of a star could not be measured precisely. Stars were usually assigned to one of six magnitude classes. As a result, the brightest star in each constellation did not always get assigned as Alpha. This means than in some of the constellations the Bayer designations do not follow the star’s apparent magnitudes in order.
In addition to the Bayer designation, many of the brighter stars have also received traditional names over the years. Most of the traditional names used today are based on Arabic star names. A few are based on Greek or Chinese names. The Greek astronomer Claudius Ptolemy calculated the position and brightness of 1,025 stars in the second century. His book, known as Almagest, was translated into Arabic in the 8th and 9th centuries. It was later translated into Latin where it became popular in Europe. Many of the Arabic names became widely used as names for the stars.
Many people wonder why so many of the constellations do not look like their names. This is because in most areas of the world today, light pollution from city lights hides many of the dim stars. For many of the constellations, these dim stars fill in the fine details of the patterns, making them more recognizable. It still takes some imagination to see some of these shapes, but it is much easier when all of the stars can be seen.
Constellations by Month
As the Earth revolves around the Sun, the stars that are visible in the night sky change from month to month. Because of this, the constellations are often grouped according to the month in which they are best visible. These monthly listings assume that you are looking at the sky at 9:00 PM. For every hour later than 9:00, add half of a month. For every hour before 9:00, subtract half a month. At the bottom of the page is an alphabetic listing for those looking for a particular constellation. Please note that the illustrations of each constellation in this section will show only the brightest stars for purposes of simplicity. If you need a more accurate rendering of any of these star groupings, please consult a star chart. Click here for a list of credits and sources.