Cancer

The quiet Crab

Cancer, the Crab, is a zodiac constellation tucked between the bright stars of Gemini and Leo. It is the faintest of the 12 zodiac constellations: its brightest star only reaches magnitude about 3.5, so from a town you may see nothing there at all.

What it lacks in bright stars it makes up for with one of the finest star clusters in the sky, the Beehive, visible to the naked eye from a dark site. The Sun passes in front of Cancer from about July 21 to August 10, so the constellation is hidden in the solar glare during midsummer.

How to find it

Cancer is a late-winter and spring constellation for the Northern Hemisphere. In early March it stands due south and highest around 10 p.m. local time. Because the stars rise about 4 minutes earlier each day, by early April it is highest around 8 p.m., or 9 p.m. with daylight saving time.

Find Castor and Pollux, the twin stars of Gemini, and Regulus, the brightest star of Leo. Cancer fills the dim space between them. To find the Beehive, picture the midpoint of a line from Regulus to Pollux, then look a little to the south and west.

From the Southern Hemisphere, Cancer is an autumn evening sight in March and April and stays visible into May.

The main stars and their names

Al Tarf (Beta Cancri), at magnitude about 3.5 to 3.8, is the brightest star and lies about 290 light-years away. It is an orange giant star with a faint red dwarf companion.

Acubens (Alpha Cancri), magnitude about 4.2 and about 174 light-years away, takes its name from the Arabic for 'claws'. Despite its Alpha label, it is not the brightest star in the constellation.

The most charming names belong to Asellus Borealis and Asellus Australis (Gamma and Delta Cancri), the northern and southern donkey colts. They sit beside the Beehive, whose Latin name Praesepe means 'manger', so the picture is two young donkeys feeding at a manger. Delta, the southern one, is the brighter of the pair at magnitude about 3.9 to 4.2.

The Beehive Cluster

An open cluster is a loose group of stars that formed together from the same cloud and still travel as a family. The Beehive, also called M44 or Praesepe, contains about 1,000 stars and lies roughly 580 to 600 light-years away, which makes it one of the closest open clusters to us. It is about 600 to 700 million years old, young compared with our galaxy.

It shines at magnitude about 3.7 and spreads across about 1.5 degrees of sky, roughly three times the width of the full Moon. That is too wide for most telescopes at high power, so binoculars give the best view: they show about 20 stars sprinkled like bees around a hive.

To ancient observers it was just a cloudy patch. Galileo, using his early telescope, was the first to show that it is made of stars, counting about 40 of them.

M67, a cluster as old as the Sun

Close to Acubens, about 2 degrees to its west, lies M67, a fainter open cluster of magnitude about 6 to 7. It is about 2,800 to 3,000 light-years away and holds more than 500 stars, including about 100 that resemble the Sun.

M67 is about 4 to 5 billion years old, roughly the age of our Sun, and far older than the Beehive. It lies about 1,500 light-years above the flat central plane of the Milky Way. Its stars include many red giants and white dwarfs, and about 30 'blue stragglers', stars that look younger than their neighbours. Johann Gottfried Koehler first recorded it in 1779, and Charles Messier added it to his list in 1780.

Mythology and the Tropic of Cancer

In Babylonian tradition the four stars around the Beehive formed the Seat of the God of Heaven. The Greeks saw a crab instead. In their story, a crab attacked Heracles during one of his battles and was crushed, and the goddess Hera placed it among the stars. Some early modern drawings even show it as a lobster.

About 2,000 years ago the Sun stood in Cancer at the June solstice, when it reaches its farthest point north. The line of latitude where the Sun is overhead on that day was named the Tropic of Cancer, and the name stayed even though precession, Earth's slow wobble, has since moved the solstice Sun into Taurus.

Practical observing tips

Pick a moonless night between March and May, away from city lights, and wait until full darkness. Find Pollux and Regulus first, then look for the faint glow of the Beehive between them; if you can see it with the naked eye, your sky is fairly dark.

Binoculars are the ideal tool here. Scan slowly across the Beehive, then move south toward Acubens and step about 2 degrees west to find the smaller, fainter haze of M67. Because Cancer lies on the ecliptic, the Moon and planets pass through it; a planet beside the Beehive makes a lovely binocular sight.