GOING TO THE DOGS
As the cold of northern winter descends, Orion's two hunting dogs bound over evening's eastern
horizon, enthusiastically chasing their master across the sky as
he perpetually hunts his prey. On successive nights rising ever
earlier, they are with us until spring returns flowers and green
grass. The Dogs, Canis Major (the
Larger Dog) and Canis Minor (the
Smaller), could not be more different, the Big Dog loaded with
bright blue and white stars and host to numerous star clusters, the little one sparsely
populated. Yet there is a relation between the two that
transcends mythology. Lying symmetrically on opposite sides of
the faint winter Milky Way (as seen
from the northern hemisphere), each contains a brilliant Alpha
star: Major's minus-first magnitude Sirius (almost minus second and the
brightest star in the sky) and Minor's zeroth magnitude Procyon (number 8 in brightness). Far
more curious is that they both harbor famous white dwarf companions,
the dead remains of stars that long ago outshone their hosts.
On a frosty winter night, Sirius, its name from the Greek for
"scorching" or "searing," immediately draws the eye. Flashing
colorfully across the spectrum,
it looks like a celestial traffic light controlling interstellar
missions as they proceed to the ends of their journeys. As
starlight enters the Earth's atmosphere, it refracts downward,
making all stars seem higher in the sky than they really are.
The ride is far from smooth. The turbulent air behaves as if
filled with myriad shifting prisms that erratically disperse the
light and make the star colorfully twinkle. Sirius's brilliance
and low position from mid-northern climes (its light passing
through much more air than does that from the stars overhead)
make it the jewel of the sky.
While fairly luminous, shining with the power of 25 Suns, this
snow-white class A1 (temperature 9900 Kelvin) ordinary hydrogen-
fusing dwarf's brilliance comes mostly from its proximity. Only
8.6 light years away, the seventh closest star and the fifth
closest star system, Sirius is just twice as far as the nearest
star, triple Alpha Centauri. On
the other side of the Milky Way, Canis Minor's Procyon is not
only farther (11.4 light years, the 14th closest system), but is
the lesser of the two: a cooler (6530 K) class F5 star with a
luminosity of but 7 Suns. Unlike Sirius, Procyon -- classed as a
dwarf-subgiant -- is beginning give up hydrogen fusion in its
core; it is beginning to die. Unlike Procyon, Sirius is a
"metallic line star" with odd metal deficiencies and enhancements
caused by gravitational settling and radiative lofting in its
quiet atmosphere. Procyon's name, from Greek meaning "before the
dog," pays homage to Sirius across the way, as from middle
northern latitudes Procyon rises first.
Both stars were long ago seen to wobble along their paths among
the surrounding stars, implying that they were being periodically
shifted by orbiting
companions. Sirius's mate was the first to fall, in 1862, to
Alvan Clark. Devilishly difficult to see, Sirius B is 10 magnitudes fainter than "A"
and huddles only a few seconds of arc away. Every 50.1 years the
two circuit each other in elliptical orbits with an average
separation of 19.8 Astronomical Units, about Uranus's distance
from the Sun. Application of orbit theory
gives masses of 2.1 times the Sun's mass for Sirius A and 1.03
solar for Sirius B. But Sirius B has a huge temperature of
25,000 Kelvin. To be so hot and faint it must be terribly small,
only 92 percent the size of Earth. A solar mass stuffed within
our planet yields an average density of nearly two million grams
per cubic centimeter -- two tons in a sugar cube. Sirius B the
classic "white dwarf," one with a nearly pure hydrogen atmosphere
(heavier elements sinking out of sight). The term was invented
to distinguish such bodies from normal dwarf stars like Sirius A.
The mass and radius of Sirius B have been closely confirmed
(respectively 98 percent solar and 94 percent that of Earth) by
precise observations made with the Hubble Space Telescope, which
involved the measurement of Einstein's gravitational red shift.
A ball thrown upward in the air slows down as it works against
Earth's gravitational field. Light must work to escape a
gravitational field as well. However, instead of slowing down,
it loses energy by increasing its wavelength, by reddening (the
longer the waves, the lower the energy). So one could also take
the Hubble observations (and similar ones that have been made) as
a critical test of relativity. If the gravitational field is
strong enough, light reddens to infinite wavelengths and the body
producing it disappears. Sirius B thus directs us toward the
infamous "black hole."
White dwarfs are the end products of normal solar-type stellar
evolution. The core of the Sun will someday run out of hydrogen
and turn to helium with the temporary creation of a much more
luminous red giant star. The internal helium will then fuse to
carbon (Arcturus, Aldebaran) to create carbon and oxygen. As it
ages, a giant loses so much mass that it strips itself down to
the bare core. Part of the lost mass is seen briefly as a
planetary nebula, and what remains dies as a very dense, cooling
white dwarf that is supported by quantum-mechanical effects
acting on its electrons. Sirius B is such an old core. Theory
tells us that high mass stars live shorter lives. From the
estimated age of the system, at birth Sirius B should have been a
six or seven solar mass hot class B3 star that dwarfed Sirius A.
Yet B is now considerably the less massive. It must have lost
some 85 percent of itself back to interstellar space. The binary
thus provides marvelous proof that stars really do lose large
quantities of matter as they expire and recycle much of
themselves back into interstellar space.
Procyon's white dwarf was sighted in 1896. Eleventh magnitude
(10.82) and closer to its host than Sirius B, Procyon B is even harder to see. The two
orbit each other every 41 years at an average separation of 15
AU, closer than Sirius and its mate. Both components are also
less massive than their Sirian counterparts. Procyon proper
comes in at 1.4 solar. At 7700 Kelvin, just a bit warmer than
Procyon A, Procyon B is much less massive than Sirius B, just
0.60 solar. The radius, however, is larger, 1.35 times that of
Earth. Behaving oppositely from normal dwarfs, the more massive
a white dwarf, the smaller it is, higher gravities causing a
greater squeeze. Procyon B's original mass must have been larger
than Procyon A's, but not by all that much since "B" is already
starting to die (probably near 2.1 solar, about what Sirius's
mass is now). Procyon B is a white dwarf with a helium
atmosphere, its hydrogen envelope stripped off, but one oddly
loaded with heavier elements. White dwarfs, the remains of stars
created with masses under 8 or 10 solar, flock the sky. We were
led to their meanings by the two dog stars -- and by the river
Eridanus from which they drink -- which contains the original
white dwarf, 40 Eridani B. The trio is
critical in testing the theoretical relationship between white
dwarf masses and radii.
With a backward glance at a few long period variables (ageing
giants like Mira), we leave Canis Minor for Major. Johannes
Bayer did not pay much attention to brightness in the
constellation. The second brightest star, actually at the end of
first magnitude, is Adhara, Epsilon CMa (while Gamma CMa, at fourth magnitude, ranks
way down on the list). A hot nearby class B2 bright giant
(perhaps even supergiant), Adhara's simple spectrum is invaluable
in the measure of the local interstellar medium through
superimposed absorption lines. At around 20,000 Kelvin, Adhara
is the brightest star in the sky as seen in ultraviolet light.
Equally hot, Mirzam (Beta CMa, ranking
fourth in the constellation in visual brightness) is a classic
"Beta Cephei" star that subtly varies
by nearly a tenth of a magnitude with several different periods,
the main ones around a quarter of a day. The star is so well
known that the variable class is sometimes called "Beta Canis
Majoris stars." (In contrast, Gomeisa, Beta Canis Minoris, is a class B
rapidly-spinning dwarf surrounded by a rotating circumstellar
disk.)
Just beneath Sirius, Canis Major cradles one of the sky's great
open clusters, Messier 41, a compact group that makes an
impressive sight in a small telescope. At a distance of 2250
light years, this youngish cluster (about a quarter of a billion
years old) is loaded with half a hundred stars more luminous than
the Sun packed into a volume just 25 light years across. Farther
down in the constellation, to the northeast of the triangle of
bright stars that includes Adhara (the others Wezen and Aludra,
Delta and Epsilon CMa) lies NGC 2362. Much farther (4500 light
years) and younger (only eight million years old, and filled with
hot B stars), the cluster is visually dominated by the
spectacular class O9 multiple-star supergiant, Tau CMa (which is assumed to be a cluster
member). Shining with the light of half a million Suns, the Tau
CMa system is one of the most luminous in the Galaxy.
The grandest system, though, fills much of the constellation. As
for many of the brighter constellations, Canis Major's stars are
not entirely random. Within the lower half of the figure is a
large, loose "association" of luminous stars that includes Wezen,
Omicron-2 CMa, and Xi-1 CMa. The
stars of such associations, like those that make much of Orion,
Scorpius, Centaurus, and Perseus, were
born at more or less the same time from the same interstellar
cloud complexes, but unlike clusters are not gravitationally
bound together. Instead, these huge structures expand with time
and gradually dissipate. The association, called "Collinder
121," takes its name from an obscure unrelated cluster that lies
within its boundaries. (Per Collinder was a Swedish astronomer
who created a large cluster catalogue in 1931.) The Col 121
association is nearly 15 degrees in diameter, which from its
distance of 1900 light years gives a physical diameter of some
500 light years. The original faint cluster (which has been
confused with the association) may be twice as far, showing the
difficulty and confusion that can arise from mere
alignment.
Associations like these lead the way to an even greater structure
discovered by John Herschel and B. A. Gould, who called attention
to a ring of bright stars around the sky (now called the Gould
Belt) that does not quite match the ring of the Milky Way: Orion
and Canis Major lie on one side of the Milky Way, while Scorpius
lies on the other. The Gould Belt seems to be made of a tilted
collection of local associations, all of which are related to one
another through a "superassociation" known as "Cassiopeia-Taurus" that
extends across 100 degrees of sky. As the massive stars in an
association explode as supernovae, their shock
waves slam into nearby interstellar clouds. The resulting
compression generates more stars and associations, and so on down
the line, the older associations dissipating, the new ones born
to take their places.
At the end, everything is seemingly connected to everything else,
sometimes directly, as in the associations, sometimes subtly, as
in Sirius and Procyon, and sometimes just in ancient and charming
myth, as witness Orion's two faithful dogs.
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 January/June 2006 Newsletter of the Lowestoft and Great
Yarmouth Regional Astronomers, who are gratefully acknowledged.