1. Introduction

Cancer, a zodiac constellation located in the northern celestial hemisphere, holds both historical significance and astronomical interest. Known by its Latin name for "crab," Cancer is one of the oldest recognized constellations, dating back to ancient Mesopotamian and Greek civilizations. Its association with the crab stems from Greek mythology, where it represents the crab sent by Hera to hinder Hercules during his battle with the Hydra.

Positioned between the constellations of Gemini and Leo, Cancer occupies a relatively small area of the sky. Its celestial coordinates lie approximately between right ascension 6 hours and 9 hours and declination +20 degrees and +36 degrees (Figure 1). Due to its location along the ecliptic, Cancer is part of the zodiacal band through which the Sun, Moon, and planets appear to move throughout the year.

Figure 1. IAU chart of Cancer. Source: https://www.iau.org/static/archives/images/screen/aqr.jpg. Credit: IAU and Sky & Telescope. Reproduced under CC BY 4.0 license.

Cancer is characterized by its lack of bright stars, with no star brighter than fourth magnitude. Its most notable feature is the open star cluster known as the Beehive Cluster or Messier 44 (M44), located near its center. M44 is a prominent object visible to the naked eye under dark skies and appears as a fuzzy patch of light containing hundreds of stars.

In addition to M44, Cancer also contains several other deep-sky objects, including galaxies and star clusters, albeit less prominent than those found in some other constellations. Despite its modest stellar population, Cancer offers astronomers and stargazers opportunities for observation and exploration, particularly for those interested in open clusters and fainter celestial objects.

2. Historical Background and Mythology

Cancer, one of the twelve zodiac constellations, has a rich historical background rooted in ancient mythology and cultural significance. Dating back to ancient Mesopotamian and Greek civilizations, Cancer's association with the crab and its position along the ecliptic has made it a prominent fixture in celestial lore for millennia.

In ancient Mesopotamia, Cancer was known as the "Crayfish" and was associated with the summer solstice. The constellation's appearance in the sky coincided with the Sun's northernmost point on the celestial sphere, marking the longest day of the year. This astronomical event held significant cultural and religious importance, as it signaled the beginning of the summer season and the ripening of crops.

The Greek mythological origins of Cancer are intertwined with the Twelve Labors of Hercules. According to legend, Hercules, the mighty hero of Greek mythology, was tasked with slaying the Hydra, a monstrous serpent with multiple heads. During the battle, Hera, the queen of the gods and a sworn enemy of Hercules, sent a giant crab to distract him and aid the Hydra. Despite its formidable appearance, Hercules crushed the crab underfoot, earning the ire of Hera and solidifying Cancer's place in the heavens as a constellation. In another version of the myth, the crab was said to have been sent by the water nymph, Eurystheus, to assist the Hydra in its battle against Hercules. However, Hercules dispatched the crab with a swift kick, and the creature was placed among the stars by the gods as a reward for its bravery.

The association of Cancer with the summer solstice and its depiction as a crab are also reflected in other ancient cultures. In Egyptian mythology, Cancer was associated with the scarab beetle, a symbol of resurrection and transformation. The scarab's ability to emerge from the earth and roll its dung ball, reminiscent of the crab's movement, was seen as a metaphor for the Sun's journey across the sky during the summer months.

In Hindu astrology, Cancer is known as Karka and is associated with the deity Chandra, the god of the Moon. According to Hindu mythology, Chandra was cursed by Daksha, the father of his wife, to suffer from a disease. Seeking a cure, Chandra prayed to Lord Shiva, who granted him the boon of residing in the constellation Cancer, thus granting relief from his affliction.

Throughout history, Cancer has remained a symbol of resilience, transformation, and rebirth, reflecting the cyclical nature of life and the changing seasons. 

3. Notable Stars

Altarf (Beta Cancri): Altarf, also known as Beta Cancri, is the brightest star in the constellation Cancer. It is a binary star system consisting of two main-sequence stars orbiting each other. Altarf has a combined apparent magnitude of about 3.5 and is located approximately 290 light-years away from Earth. The primary star is a yellow-white subgiant, while the secondary star is a red dwarf.

Acubens (Alpha Cancri): Acubens, or Alpha Cancri, is another notable star in Cancer. It is a binary star system composed of two main-sequence stars. Acubens has a combined apparent magnitude of about 4.2 and is located approximately 174 light-years away from Earth. The primary star is a white A-type star, while the secondary star is a fainter companion.

Asellus Australis (Delta Cancri): Asellus Australis, or Delta Cancri, is a binary star system located in Cancer. It has a combined apparent magnitude of about 3.9 and is approximately 182 light-years away from Earth. The primary star is a yellow-white dwarf, while the secondary star is a fainter companion. Asellus Australis is one of the brighter stars in Cancer and is sometimes referred to by its traditional name, which means "southern donkey colt."

Asellus Borealis (Gamma Cancri): Asellus Borealis, or Gamma Cancri, is another binary star system in the constellation Cancer. It has a combined apparent magnitude of about 4.7 and is located approximately 158 light-years away from Earth. The primary star is a yellow-white dwarf, while the secondary star is a fainter companion. Asellus Borealis is sometimes referred to by its traditional name, which means "northern donkey colt."

4. Deep-Sky Objects

Messier 44 (M44) - The Beehive Cluster: Messier 44, also known as the Beehive Cluster or Praesepe, is one of the most prominent open clusters in the night sky. Located near the center of the constellation Cancer, M44 is visible to the naked eye under dark skies and appears as a fuzzy patch of light. Through binoculars or a small telescope, observers can see dozens of stars packed closely together, resembling a swarm of bees, hence its name. M44 is relatively young, with an estimated age of about 600 million years, and contains hundreds of stars, including several binary systems. It is situated approximately 577 light-years away from Earth.

Messier 67 (M67): Messier 67 is another open cluster located in the constellation Cancer. It is less prominent than the Beehive Cluster but is still an interesting target for observers. M67 is older than M44, with an estimated age of about 4 billion years, making it one of the oldest known open clusters. It contains hundreds of stars, including several red giants and white dwarfs. M67 is located approximately 2,700 light-years away from Earth.

NGC 2775: NGC 2775 is a spiral galaxy located in the constellation Cancer. It is characterized by its bright central bulge and tightly wound spiral arms. NGC 2775 has a relatively low surface brightness, making it challenging to observe visually without the aid of a telescope. It is classified as a Seyfert galaxy, indicating that it has an active galactic nucleus that emits intense radiation. NGC 2775 is situated approximately 55 million light-years away from Earth.