By Jim Kaler

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.