GRUMIUM (Xi Draconis). With a name like that, were you a star,
you'd probably prefer "Xi." Allen calls this faintest (fourth
magnitude, 3.75) star in the head of Draco, the Dragon, a "barbarism." Kunitzsch and Smart,
on the other hand, have it derived from a Latin word meaning a
pig's snout, sort of appropriate for the "serpent's jawbone." In
the celestial art found on old star atlases (Bayer's Uranometria), the Dragon's body twists four
times starting at the head, these "nodes" marked by four stars.
The best known of them is the second, "Nodus II," also called Altais, more commonly Delta Draconis. Our
Grumium, Xi Dra, marks the first twist, and is hence Nodus I.
Because of the easy confusion between the lower-case Greek "Xi" and
"Zeta," Zeta is occasionally erroneously tagged as Nodus I, whereas
it is actually Nodus III (a term used beyond rarity). To round us
out is Theta (or possibly Iota) Dra as Nodus IV. Aside from the
complexity of the names, as an orange class K2 giant, Xi Dra at first seems
pretty ordinary. It does, however, have its own evolutionary twist
in addition to its cultural one. From a distance of 111 light
years, Nodus I shines with the light of 53 Suns, not all that much for a giant star, which
leads to a radius of just 12 times solar and a mass of 1.5 times
that of the Sun. Born three billion years ago, instead of being a
quiet helium (to carbon) burner, it seems to have recently (160
million years ago) left its hydrogen-fusing dwarfhood behind and is
now brightening as a giant with a dead helium core. (Its physical
state is uncertain, however. It could also already have reached
its peak luminosity and be dimming after initiating helium fusion.)
A 16th magnitude star 316 seconds of arc away is listed as a binary companion, but since
there is only one measure, there is no way to tell if the two are
tracking each other through space. Most likely, the pairing is
just a line-of-sight coincidence. If they are indeed connected,
then Xi Dra B would be an M6 dwarf at least 11,000 Astronomical
Units away, leading to an orbital period of 800,000 years. But the
companionship seems unlikely at best.
Written by Jim Kaler 8/29/08. Return to STARS.