The Cancer constellation is one of the 88 officially recognized patterns in the night sky and belongs to the twelve constellations that form the Zodiac. Known historically as the Crab, its name comes from ancient Greek mythology. Despite its status as a zodiacal figure, Cancer is notably inconspicuous when compared to its neighbors. Its faintness makes it a challenge for casual skywatchers to locate without assistance from brighter star groups. This celestial region, while lacking in brilliant individual stars, holds a surprising density of stellar objects.
Defining the Stellar Count
The total number of stars associated with the Cancer constellation depends entirely on the method of counting. For a person observing from a suburban area, the constellation appears to contain a relatively small number of visible stars, perhaps ten to fifteen. This is because only two of its stars are brighter than the fourth magnitude, making Cancer the dimmest of all the zodiacal constellations.
The International Astronomical Union (IAU) offers a more precise, scientific definition, establishing a constellation as a specific, defined area of the celestial sphere. These boundaries were precisely delineated by the Belgian astronomer Eugène Delporte in the 1930s. Every celestial object is officially considered “in” Cancer if it falls within these established coordinates.
Under this official definition, the star count increases dramatically, including many stars too faint to be seen without optical aid. Within the IAU boundaries, the Cancer constellation contains 104 stars that are brighter than or equal to an apparent magnitude of 6.5. This magnitude limit represents the approximate boundary of what the human eye can perceive under perfect viewing conditions. When considering all stellar objects, the number of stars within the constellation’s official boundary reaches into the thousands.
The Most Prominent Stars
The few brighter stars in Cancer form the traditional, albeit faint, outline of the Crab figure. The brightest among these is Beta Cancri, also known as Altarf, which shines at an apparent magnitude of 3.5. Altarf is an evolved orange giant star, significantly larger and more luminous than the Sun, and it represents a part of the Crab’s leg or body.
Another pair of stars, Gamma Cancri (Asellus Borealis) and Delta Cancri (Asellus Australis), form the body of the Crab and are important for locating the constellation’s center. Asellus Australis is the second brightest star, an orange giant with a magnitude around 3.9. Asellus Borealis is fainter, a white-hued subgiant star with a magnitude of 4.67.
The star Alpha Cancri, or Acubens, marks the southern claw of the Crab and has an apparent magnitude of 4.27. Although it bears the “Alpha” designation, it is actually fainter than both Altarf and Asellus Australis. These relatively dim stars highlight the challenge Cancer presents for observers, who must rely on dark skies to discern the constellation’s shape.
Deep-Sky Objects and Star Clusters
While the individual stars are faint, the constellation is famous for containing massive groupings of stars known as open clusters. These clusters dramatically increase the stellar count within the constellation’s boundaries.
The most prominent of these is Messier 44 (M44), universally known as the Beehive Cluster or Praesepe, which is situated near the center of the constellation. The Beehive Cluster is one of the nearest open clusters to Earth, located about 600 light-years away. It is bright enough to be visible as a fuzzy patch to the naked eye under dark conditions. This dense stellar nursery contains approximately 1,000 stars, though only a fraction are bright enough to be clearly resolved with binoculars or small telescopes.
Another noteworthy object is Messier 67 (M67), located in the western part of Cancer, near the star Acubens. M67 is an open cluster that is both farther away and much older than the Beehive Cluster, with an estimated age between 3.2 and 5 billion years. This makes it one of the most ancient open clusters known, containing about 200 stars that are similar in age to the Sun. M67 is much fainter, with an integrated magnitude of 6.1, and requires binoculars or a telescope for clear viewing.
Locating the Crab in the Night Sky
Finding the faint constellation of Cancer requires looking for two brighter, more recognizable constellations that flank it. The Crab is located directly between Gemini to the west and Leo to the east. An imaginary line drawn between the bright stars Castor and Pollux in Gemini and the star Regulus in Leo will pass right through the heart of Cancer.
The best time for observers in the Northern Hemisphere to locate Cancer is during the late winter and early spring months. The constellation reaches its highest point in the sky around 9 PM local time during March. To confirm the location, a dark sky will reveal the Beehive Cluster (M44) as a distinct, hazy glow within the inverted “Y” shape of the faint stars.