The Cancer constellation, known as the Crab, is one of the twelve constellations that make up the Zodiac. It is recognized as the faintest of the zodiacal constellations, lacking any stars of the first or second magnitude. This ancient figure holds a place in astronomical history, but its subdued appearance often leads to confusion about its contents. The simple question of how many stars belong to Cancer is complicated because the answer depends entirely on how the term “star” is defined within the context of the constellation’s boundaries.
Defining the Count: Official Boundaries and Cataloged Stars
The International Astronomical Union (IAU) formally defines the borders of all 88 constellations, dividing the entire night sky into precise zones. Cancer occupies a medium-sized area of 506 square degrees, a region that contains an immense number of distant, faint stars. Within these official boundaries, any celestial body, no matter how dim or distant, is technically considered part of the constellation.
A common metric is the number of stars brighter than magnitude 6.5, which is generally accepted as the limit for human vision under perfectly dark conditions. Based on this criterion, Cancer contains approximately 104 to 108 stars that are visible with the naked eye or with basic optical aid. Only a small handful of these stars are noticeable to the casual observer.
The recognizable pattern, or asterism, of the Crab typically involves only four or five stars. Because of the faintness of these stars, an observer under typical night sky conditions might see only about 10 to 20 individual stars within the figure.
The Brightest Stars of the Crab
Despite its overall dimness, Cancer contains several named stars that anchor its shape, though none are brighter than the third magnitude. The brightest star is Altarf (Beta Cancri), an orange giant shining with an apparent magnitude of 3.53. It marks the southwestern extremity of the Crab’s traditional figure.
The second brightest star is Delta Cancri, named Asellus Australis (magnitude 3.94). Asellus Australis and its neighbor, Gamma Cancri (Asellus Borealis, magnitude 4.66), are often referred to as the “donkey” stars. Acubens (Alpha Cancri) is the fourth brightest star at magnitude 4.25 and is a multiple-star system, historically marking one of the Crab’s claws.
These four stars are the most prominent light sources used to outline the constellation, forming a distinct, albeit faint, inverted “Y” shape. The names Asellus Borealis and Asellus Australis reference their mythological role as the animals feeding at a nearby manger.
Praesepe (M44): The Beehive Cluster
The most famous feature within the Crab’s boundaries is the Praesepe Cluster, also known as Messier 44 (M44). This is a large, nearby open star cluster located near the center of the constellation. Praesepe is one of the closest open clusters to Earth.
The Beehive Cluster appears to the unaided eye as a faint, cloudy smudge of light, which led ancient observers to refer to it as a “nebulous mass.” The cluster contains an estimated 1,000 gravitationally bound stars, though only a fraction are easily visible with a small telescope or binoculars. Galileo resolved approximately 40 individual stars when he first observed the cluster through his telescope in 1609.
With an integrated apparent magnitude of 3.1 to 3.7, M44 is one of the brightest star clusters in the sky and is readily visible under dark conditions. Its location is nestled between the two donkey stars, Asellus Borealis and Asellus Australis, giving rise to its Latin name, Praesepe, meaning “manger.”
Locating and Viewing the Constellation
Finding the Cancer constellation requires navigating between brighter, more recognizable neighbors. Dark skies are necessary for a good view, particularly of the fainter stars that outline the Crab. The constellation is located between Gemini to the west and Leo to the east.
A common technique for locating Cancer is to use the two brightest stars in Gemini, Castor and Pollux, and the bright star Regulus in Leo. Cancer lies approximately midway between these two star pairs. The best time to observe the constellation from the Northern Hemisphere is during the late winter and spring months, when it is high in the evening sky.
Once the general area is found, the Praesepe Cluster (M44) often appears first as a hazy patch of light, marking the heart of the constellation. Using binoculars or a small telescope will resolve the cluster into many individual stars, confirming the location of the Crab.