ZETA CRB (Zeta Coronae Borealis). Actually "stars," as it's double. Or is it triple, or maybe quadruple, even
quintuple. To partially quote Laurel and Hardy, "Well here's
another nice mess... " Zeta Coronae Borealis, rather obviously in
Corona Borealis, the Northern Crown, is a
barely-fifth magnitude (4.58) classic double made of a mid-fifth
magnitude (4.96) class B7 dwarf called Zeta-2 CrB coupled
to a sixth magnitude (5.91) class B9 dwarf tagged Zeta-1 (because
it's the more westerly of the two) that lies 6.1 seconds of are away.
They've changed their distance from each other by only 0.9 seconds
of arc since 1779, and pretty clearly belong together. From Smythe
and Chambers in the nineteenth century: "A fine double star, in the
middle of the space over the wreath, and 10 degrees N. a little
easterly, from Alpha. A 5, bluish white; B 6, smalt blue. This is
a beautiful object." Juxtaposition effects do odd things to
apparent star colors, since Zeta-2 should be the more blueish one,
while Zeta-1 is white. ("Smalt blue" refers to deep blue cobalt
glass.) At a distance of 473 light years (give or take 36) for the
pair, Zeta-1 is by far the simpler. Assuming a temperature of 11,000
Kelvin from the class (needed to allow for ultraviolet radiation), it
radiates at a rate of 112 solar luminosities, which gives the star
a radius of three times solar and, with a minimum equatorial rotation
velocity of 20 kilometers per second, a rotation period of under 7.5
Now abandon hope etc. Almost everything about Zeta-2 is
contradictory. Since 1925, it has been known to be a spectroscopic double (with two sets
of spectra shifting back and forth in wavelength) that seems to be
made of similar B7 (or perhaps B6) stars in a tight 12.824-day orbit.
But in 1997, analysis of new data found it to be triple, made of an
inner pair (Zeta-2 A and B)in a 1.72357-day orbit around which goes
another star (Zeta-1 C) with a period of 251 days, the old 12.8-day
orbit dropped as erroneous. But then a further study in 2005
reconstituted it, so now we have a quadruple star with Zeta-1 D (?)
going around the 1.7-day AB pair every 12.5842 days, then with Zeta-1
C orbiting the inner triple in 251 days. Confused yet? Further
analysis leads to even more frustration. Assume that the B7 (B6?)
pair dominates and we split the resulting luminosity in two.
Temperature estimates start at 16,580 Kelvin, much too high for the
B6 class, which should be around 13,000 K. Best we can guess is that
the bright pair shines at around 95 Suns each
with masses of maybe 3.2 times solar. But don't trust it as theory
can't properly fit the parameters. Given what we have, the AB pair
is but 0.05 AU, just 11 solar radii, apart (plenty enough for
clearance). Raising the temperature increases both the luminosity
(because of more UV light) and mass. But we don't know the
contribution of Zeta-2 C or, if really present, Zeta-2 D. Assuming
C and D have inconsequential masses, they orbit the AB pair at mean
separations of 0.20 AU and 1.4 AU. Further speculation is futile.
Looking further afield Zeta 1, separated by at least 930 Astronomical
Units from Zeta-2, must take at least 920 years to make a circuit.
The whole affair shows that while the characters of some stars are
known with exquisite accuracy, when we get into others, especially
close multiples, all bets are off and confusion reigns.
Written byJim Kaler 7/17/15. Return to STARS.