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.