HOT CRAB SANDWICH
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
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
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.