By Jim Kaler

Among the constellations of the Zodiac, few get less attention than Cancer, the Crab, sandwiched as it is between two far more prominent figures. To the west stands bright Gemini with Pollux, one of the Zodiac's five first magnitude stars (not to mention Castor, the brightest of second magnitude), while to the east roars Leo with another first magnitude star, Regulus. Pollux, an orange giant, tends to a planet of at least three Jupiter masses, Castor is a classic sextuple star, and Regulus, accompanied by a white dwarf, is quadruple. See what happens? We start with Cancer and wind up with Gemini and Leo. The figure gets so little respect that the astrologers even tried to change its name, people born under its sign for a time known as "Moon Children" to avoid any reference to the disease.

Cancer has, however, a number of things that claim interest. Chief among them is a prominent circle on the Earth at 23.4 degrees north latitude, the "Tropic of Cancer." The line represents the most northerly point at which you can experience a zenith Sun, which occurs around June 21 when the Sun passes the Summer Solstice in Gemini. But wait, that makes no sense. Why is it not the Tropic of Gemini? In addition to daily rotation and annual revolution, the Earth has a prominent third motion, the 26,000-year wobble of its rotation axis around the orbital perpendicular, during which it closely maintains the 23.4 degree tilt. The movement, called "precession," changes the direction of the celestial poles. Polaris as pole star is just temporary; 4700 years ago, during the time of ancient Egypt, Thuban in Draco held that honor. The wobble also forces the four main points of the ecliptic -- the equinoxes and solstices -- to move to the west against the stars at the rate of 50 seconds of arc per year. During classical times, between about 1500 BC and the year zero (which actually does not exist, the calendar going from -1 to 1, but never mind), the Summer Solstice indeed WAS in Cancer. Moreover, since the astrologers pin the signs of the Zodiac to the drifting points, the sign of Cancer now more or less overlays the constellation Gemini. Even though the Twins hold the Solstice, the Sun passes through Cancer between late July and early August, when northern hemisphere temperatures are near their maximum, hence the title of this article. (That said, around 1992 the Summer Solstice moved across the artificial constellation border into Taurus, but let's not start calling the Earthly circle the "Tropic of Taurus" as things are confused enough already.)

Cancer's chief attraction, and its a fine one, is one of the few open clusters that can be seen with the naked eye, Messier 44. Better know as the "Beehive," it's a beautiful sight in large binoculars or a wide-angle telescope. Just a degree or so north of the ecliptic, the Beehive's regular visits by the planets only add to its allure. At a well-determined distance of 595 light years (give or take 20), the cluster's age of three-fourths of a billion years precludes really massive stars (which, unlike those of the much younger Pleiades, died out long ago), but does give it some color with a smattering of orange class K giants. One star, sixth magnitude Epsilon Cnc, a metal-heavy class A dwarf, is bright enough to carry a Greek letter name (which it actually borrowed from the cluster as a whole). While Epsilon just seems particularly metal-rich as a result of separation of elements in its atmosphere, the whole cluster oddly seems to follow suit with an iron abundance (relative to hydrogen) that is some 60 or more percent above the normal solar value.

The Beehive then leads us to a lesser known, much fainter, even obscure, open cluster called Messier 67 that has a more normal solar-type iron abundance. Its dimness comes in part from a large distance of 2500-3000 light years (five times farther than the Beehive). It's prominent, however, in the age department. In the textbook case, a cluster is born from an interstellar cloud with a full range of stellar masses, from dozens of times solar on down. High mass, high luminosity stars use their fuel fast and die first, so all we have to do to get age is to find the lowest mass star that is still a hydrogen-fusing dwarf and apply theory. With an age of between 2.5 and 4 billion years, M 67 is old enough to have burned off all the dwarf stars down nearly to the luminosity of the Sun (another reason for its faintness). M 67 thereby opens the door to a deep study of when various parts of the Galaxy were born.

From the observation of globular clusters (of which Cancer is barren), the age of the Galaxy is around 13 billion years. But globulars are in the extended Galactic halo. What about the Galactic disk, whose manifestation is the Milky Way? Here we call on the open clusters that abound within it. Unlike globulars, which are all about the same age, open clusters have a terrific range of ages, from just born to ancient. We need only to find the oldest one to date the disk.

However, open clusters have a problem. Globulars, at least the ones we see, are sufficiently densely packed to survive disruption. Open clusters, though, have a strong tendency to fall apart. Gravitational interactions among their members cause the more massive stars to sink to the center, while the less massive move to the outside, where they are stripped away by tides raised by the Galaxy and passing giant molecular clouds. Star-loss is made worse by binary-star interactions. Old clusters are therefore quite rare, and survive only in the outer parts of the Galaxy where there is less to disturb them. Among the oldest are NGC 188 in northern Cepheus near the Pole at 6-7 billion years and 8 to 11 billion-year-old NGC 6791 in Lyra. From the whole set of data, the disk appears to have been formed roughly 10 billion years ago, confirming the long-held concept that it is younger than the halo.

M 67 is also relatively rich in "blue stragglers," dwarf stars that are too massive for the cluster's imputed age. More common in globular clusters, blue stragglers are believed to be the result of stellar mergers, either in binary systems or by direct collision, which raises the mass above the birth mass and delays evolution to the giant state.

The Beehive, along with two flanking stars, is also known as the "Praesepe," or "manger," where the pair -- made of fourth and fifth magnitude Delta and Gamma Cancri -- are the "Aselli" or "Donkeys." Gamma (Asellus Borealis) is a rather ordinary 2.3 solar mass class A1 dwarf 180 light years away, while Delta (Asellus Australis) is an equally common two-solar-mass class K0 giant 130 light years distant. Don't expect much from Acubens (the "claw"), the fourth magnitude Alpha star, another metallic line class A dwarf 190 light years off, which actually ranks fourth in the constellation. The constellation's luminary is fourth magnitude Beta Cnc, Al Tarf ("the End"). Replicating the Aselli, it's yet another helium-fusing K giant 300 light years away.

The "star" of Cancer's stars, the one most likely to be picked, may be fifth magnitude Zeta Cancri (Tegmine, referring to the "shell"), which lies 82 light years away. Tegmine is a well-known quadruple star in which not only is the orbit of the inner pair of class F dwarfs known (a period of 60 years averaging 22 AU apart), but so is the orbit of another close pair of class G sunlike dwarfs as they go around the inner pair every 1000 years or so at an average distance of some 200 AU. There is even some sketchy information about the outer pair's mutual orbit, the two seeming to take about 17 years to make a turn.

But Tegmine's "star" may be eclipsed by 55 (Rho-1) Cancri, a star with planets, and not just a few either, making it look something like our own planetary system. So far five have been found to orbit this G8 dwarf 41 light years away. Planets with minimum masses of 0.14, 0.17, 0.82, and 0.034 Jupiter- masses (the latter just 11 Earths) orbit between 0.78 and 0.04 AU with an outer "Jupiter" of at least 3.8 Jovian masses hovering above them all some 6 AU out. The outer one takes 14 years to orbit, the inner a mere 2.8 days.

Variables, which hardly abound, seem to be led by the long-period Mira-type star R Cancri, which varies between 6th and 12th magnitude over a period of 361 days, giving the binocular viewer the chance to see a star appear and then quite disappear. Though it's measured to be 2400 light years away, the error is so high that the distance is effectively unknown. Next up is probably RS Cancri, a semi-regular sixth magnitude variable 465 light years away that changes by only about a magnitude over a period of 120 days and is closer to Lynx than to classical Cancer. Unless it's X Cnc, a deep red seventh magnitude semi-regular carbon star 1100 light years away that is practically on the ecliptic and that has dredged freshly made carbon upward from its nuclear-burning interior. The huge star is measured to be 3 AU across.

Perhaps our Hot Crab Sandwich now seems a bit more appetizing. Certainly it does to Hydra, the Water Serpent, whose head lies just below our Zodiacal constellation, the giant snake seeming to be ready to gobble the whole thing down.
Copyright © James B. Kaler, all rights reserved. These contents are the property of the author and may not be reproduced in whole or in part without the author's consent except in fair use for educational purposes. First published in the August/December 2009 Newsletter of the Lowestoft and Great Yarmouth Regional Astronomers, who are gratefully acknowledged.