The crab constellation as represented in 19th-century Urania's Mirror

Cancer the crab scuttles across the late winter sky, well away from its nemesis Hercules. Cancer (Latin for crab) is a zodiac constellation, the Tropic of Cancer is named for it, and it has existed for over three thousand years. Yet it seems to be a dim and unremarkable constellation. How does it rate so much attention?

Ancient astronomers
Cancer has only two stars brighter than fourth magnitude. (The higher the magnitude number, the dimmer the star.) The constellation is washed out by city lights, but you can see it at a dark sky site. It's an upside down Y tucked between the bright stars of Gemini and Leo. To people for whom the night sky was always dark, even Cancer's comparatively dim stars would have been easily visible. Its significance to ancient astronomers is that the ecliptic passes through Cancer. The ecliptic is the Sun's apparent annual path through the sky as the Earth orbits it. The constellations the Sun seems to transit form the zodiac.

Cancer's second claim to fame is the Tropic of Cancer, which is the northernmost latitude at which the Sun may be directly overhead at noon. This occurs at the June solstice, and in the same way, the Tropic of Capricorn marks the December solstice. A few thousand years ago the Sun was in Cancer at the summer solstice, a time of great importance in most cultures.

But there is a little wobble in the Earth's axis. Over time the celestial location of the solstices and equinoxes seems to move from constellation to constellation. We call this precession. In the case of the Tropic of Cancer, it moved into Gemini long ago, and into Taurus in the late twentieth century.

Mythology
The constellation may not have started off as a crab. It's not certain whether the ancient Babylonians represented it as a crab or as a turtle. But centuries passed and the Greeks took up the Babylonian constellations. Cancer was definitely a crustacean to them, but over time it's been represented as a crab, a lobster or a crayfish.

The ancient Egyptians had their own interpretation. To them the constellation was a scarab (a dung beetle), a creature they associated with Ra the rising Sun. As such it was part of the symbolism of regeneration and renewal.

The main story that comes to us from classical times forms part of the Labors of Hercules. Hercules the hero was the half-mortal son of Zeus from one of his liaisons. Zeus's wife Hera was unforgiving about the amour and vindictive towards the son who was a reminder of it. One of her efforts to kill him was during the battle between Hercules and the many-headed monster Hydra. Hera sent a giant crab to distract Hercules. It bit his foot, but the hero crushed it and killed it. As thanks for its attempt to help, Hera put the crab into the sky. (This reward for failure always seemed to me to be an improbably generous act for the vindictive Hera.)

Another story focuses on just two stars with a fuzzy patch between them. The stars are Asellus Borealis (Gamma Cancri) and Asellus Australis (Delta Cancri), the northern and southern donkeys. Between them is the large open star cluster Praesepe (the manger). During the war between the Titans and the Olympian gods, on behalf of the gods, Dionysos and Silenus rode donkeys to battle. Their braying so terrified the Titans that they fled without even waiting to see them. In gratitude, the gods put the two animals in the sky – apparently, along with a food supply.

Stars and planets
Galileo was the first person to observe Praesepe with a telescope. He could see forty stars, but modern telescopes show over a thousand. Now commonly called the Beehive cluster, there is evidence that it and the Hyades in Taurus were born 600 million years ago from the same giant cloud.

Cancer's brightest star is Beta Cancri, whose common name is Altarf. At 190 light years away, its apparent brightness is of magnitude 3.55. But in fact, it's quite luminous, an orange giant some fifty times the Sun's diameter and nearly seven hundred times brighter. Altarf has a red dwarf companion star in a 76,000-year orbit, and also one known planet. The planet, Beta Cancri b, is some eight times the mass of Jupiter. You'd be hard pressed to see the companion from a planet orbiting Altarf, but if the dwarf has a planet, Altarf would appear thirty times as bright as the full Moon.

Sunlike 55 Cancri is noted for being the first star discovered to have a planetary system with more than four planets. The star has a distant red dwarf companion and at least five planets. Two of these planets are large super-earths (planets whose mass is between that of Earth and Neptune). One is closer to 55 Cancri than Mercury is to the Sun, but the other is in the habitable zone. It's probably a gas planet, but if it has moons, the conditions for life might exist there.

Delta Cancri, the southern donkey, is an orange giant. But perhaps its most memorable feature is that, according to R.H. Allen, it has a name derived from ancient Babylonian, which means “southeast star in the Crab”. You won't be surprised to know that Arkushanangarushashutu isn't in common use.

References:
Jim Kaler, https://stars.astro.illinois.edu/sow/altarf.html
Richard Hinckley Allen, Star Names Their Lore and Meaning, Dover edition 1963