NODUS SECUNDUS (Delta Draconis). With a rather unromantic name,
Nodus Secundus, Draco the Dragon's Delta
star, is less-well-known from Arabic as Al Tais, the Goat. Better
to use the Latin "Nodus," as it more refers to its host
constellation, where it marks the second (Secundus) of four loops
in the Dragon's long winding mythological body. Nicely visible at
mid-third magnitude (3.07), and one of the brighter stars of the
far north, it lies only a bit more than 20 degrees from the sky's
north pole (marked by Polaris), and is
circumpolar (always visible) from everywhere north of the Tropic of
Cancer. Within its daily circle about the North Celestial Pole, it
is exceeded in brightness only by Polaris, Kochab, and Pherkad, the Alpha, Beta, and Gamma stars
of Ursa Minor. Physically, Nodus number
two is a class G (G9, almost class K) giant star, though since it
is a bit warm (4830 Kelvin) for such a giant, not quite so large as
many. At a distance of almost exactly a light-century (100 light
years), the star radiates at a rate of 63 solar luminosities,
giving it a radius 11 times that of the Sun.
Its luminosity and temperature combine to yield a mass almost
exactly 2.5 times solar and an age of 700 million years. Nodus
Secundus is a fine example of a helium-fusing giant, one that is
now converting its core helium into carbon and oxygen. Sometime in
the astronomically near future, the helium will run out and the
star will brighten as it prepares to slough its outer envelope and
become a mid-mass white dwarf. It is not clear whether Nodus-2 is
a single or double star. Lying 88 seconds of arc away is a faint
twelfth magnitude "companion" about which nearly nothing is known.
The seeming proximity may be just a line of sight coincidence. Yet
the two seem to be regarded as a real double. If so, "Delta
Draconis B" is at least 2700 Astronomical Units from the giant.
Nearly 70 times Pluto's distance from the Sun, the orbital period
would be at least 87,000 years. From a hypothetical planet
orbiting the giant, the companion (which from its brightness would
be a class M1 dwarf) would shine redly at roughly the brightness of
Venus, while from the dwarf the giant would shed the light of a
half-dozen full Moons.