SIGMA CRB (Sigma Coronae Borealis). We have a fascination for
solar type stars, perhaps wondering if there is anyone out there on
an orbiting planet looking back at us. Science fiction can have
great fun with double solar type stars. Here is the acme of the
class, a TRIPLE-solar star -- or at least the closest we are
probably ever going to see -- and one just 71 light years away, the
Sigma star in Corona Borealis, the
Northern Crown. Through the telescope, we observe two very solar-
looking hydrogen-fusing dwarf stars just 7 seconds of arc apart,
from west to east seventh magnitude (just barely, 6.66) class G0
Sigma-1 and sixth magnitude (again barely, 5.64) class G1 Sigma-2,
which together make the apparent fifth magnitude (5.3) Sigma CrB
that is unresolved with the naked eye. (Sigma-1 has been called as
cool as class G4.) The spectrograph then shows
something yet more remarkable, that Sigma-2 is ITSELF double, consisting of F9 and G0
dwarfs (Sigma-2 A and Sigma-2 B) that take a mere 1.14 days (1 day
3 hours 21 minutes) to orbit each other. Their respective
characteristics are temperatures of 6000 and 5900 Kelvin,
luminosities 1.17 and 1.02 times that of the Sun, radii 1.14 and 1.10 solar, and masses
1.108 and 1.08 solar. The period combined with the total mass of
the system shows that they are only 0.028 Astronomical Units (AU) -
- just 6 solar radii -- apart. The closeness causes tides that
have forced each of the stars to spin very rapidly, with periods of
1.157 days, nearly synchronized with the orbital period. Solar
rotation and up and down convection of the solar gases cause active
solar magnetism, sunspots, magnetic flares, and so on. The rapid
spins of the Sigma-2 pair, just over 20 times greater than that of
the Sun, cause much greater activity, making the system one of a
class of "RS Canum Venaticorum stars" (after the prototype) like Epsilon Ursae Minoris and Lambda Andromedae.
Sophisticated analysis
with the spectrograph reveals huge starspots that march across the
line of sight as the stars orbit and rotate. There is so much
activity that the system is even observed in the radio spectrum.
Now out to Sigma-1. With a temperature if around 5800 Kelvin and
a luminosity of around 85 percent that of the Sun, the star carries
just about a solar mass. Orbital motion observed over the past two
centuries gives orbital characteristics: a period around the inner
pair of 890 years, an average separation of 128 AU, and a high
eccentricity that carries it as close as 31 AU and as far as 225 AU
from the inner binary. The luminosity, temperature, and lithium
content (an element slowly destroyed in stars by nuclear reactions)
show the system to be very young. A hypothetical planet could
orbit the inner pair, giving a double sun with another off in the
far distance. The activity from the double would make life
hazardous, however. Or, a planet could orbit outer Sigma-1 with
the binary off in the distance. From Sigma-1, Sigma-2 would appear
as double whose maximum separation varied from 25 seconds of arc to
3 minutes of arc, the latter visible to the naked eye, each of the
inner pair shining with (on average) 70 times the light of the full
Moon. No planet, however, is in evidence. But what a scenario for
a story!