ALTAIS (Delta Draconis). "Altais," while an actual Arabic word, is
here a corruption of a different Arabic word that refers to a
serpent, and consequently, to Draco
itself. Delta Draconis also goes yet another name, Nodus Secundus
(Nodus II), as 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, Altais
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. Altais 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 Altais is a single or double star. Lying 82 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 2500 Astronomical
Units from the giant. Nearly 65 times Pluto's distance from the
Sun, the orbital period would be at least 75,000 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 four or so full Moons.
Written by Jim Kaler 9/14/01; revised
8/22/08. Return to STARS.