EZ CMA (EZ Canis Majoris = WR6). Among the strangest of stars are those discovered in the nineteenth century by the French astronomers Charles Wolf and Georges Rayet. Though roughly in the O-star temperature range (and higher), Wolf-Rayet (WR) stars fall outside of the normal classification scheme because of their unusual broad spectrum lines. They are characterized by wide emission lines that imply powerful high-speed winds (the Doppler effect widening the lines). There are two major kinds: those with strong emissions from helium and highly ionized nitrogen (the WN stars) and those whose spectra show helium but strong and wide lines of various ionization levels of carbon (the WC stars). Hydrogen is absent. (There is also a rarer variety characterized by oxygen.) Somewhat over 400 Wolf-Rayet stars are known, most of them residing toward the galactic center in the Milky Way and in our satellite galaxies, the Magellanic Clouds. They are truly rare. Their number seems high only because they are so incredibly luminous that we can see them over great distances. Only three are visible to the naked eye, and the light of the brightest, Gamma-2 Velorum (in Vela, the Sails), is entangled with that of a companion O star. Among the dozen or so brighter than magnitude 7.0, EZ Canis Majoris is the only one that carries a classical variable-star name. (It's also catalogued as WR 6.) Though at magnitude 6.95, EZ CMa is still among the "EZ-est" to find, falling just a few minutes of arc north of fourth magnitude Omicron-1 CMa, near the middle of Canis Major. WR stars are born with masses above 40, maybe 60, Suns. As they evolve, they lose most or all of their their hydrogen envelopes through winds. When the hydrogen layers are gone, we begin to see the nitrogen that was been created through nuclear fusion reactions, and a WN star is born. Further mass loss eliminates the N-rich layer to expose the carbon-rich depths, and we see a WC star. By then the masses have been whittled down to 20 solar masses or less. The distance to EZ CMa is highly uncertain. The Hipparcos satellite gives 4500 light years, but the uncertainty is almost as big as the value itself. An earlier distance of 6900 light years is generally adopted from the distances of the stars surrounding it. Because of the strong winds, temperatures are also uncertain. EZ CMa is classed as a mid-temperature (relative to the set) WN4 star of 89,000 Kelvin with a radius of just 2.65 times that of the Sun, a luminosity of 400,000 Suns, and a current mass of 19 Suns. There is no trace of hydrogen. The mighty wind is still blowing at a rate of five hundred-thousandths of a solar mass per year, a billion times the flow rate of the solar wind. Wind speeds approach 2000 kilometers per second. The wind has created a surrounding "ring nebula" (a high-mass version of a planetary nebula) within which shock waves create X-rays. Eventually EZ should turn into a WC star and then blow up as a supernova. The variation in brightness is small, ranging from near zero to a couple tenths of a magnitude over a steady 3.61-day period. The existence of a companion to EX CMa is vigorously argued, however. But not the star's fate, as it will someday collapse and then tear itself apart. (Summary of properties from D.P.Huenemoerder, Astrophysical Journal 815:29 2015 December 10, 2015 and A. Flores, Rev. Mex A&A, 47, 261, 2011. Thanks to Margarita McElroy, who suggested this star.)
Written byJim Kaler 3/10/17. Return to STARS.