1 DEL (1 Delphini). One might think that Number 1 in Delphinus, the Dolphin, would be right at the
forefront of study. Number 1 and all that. But no, it's merely the
westernmost naked eye (or close) star within the swimming constellation, Flamsteed numbers always going
from west to east within a figure's rather flexible eighteenth
century boundaries. Yet the star has an importance belied by its
dim sixth magnitude (6.08) status. 1 Del is a class A (A1)
"emission-line shell star," one with a surrounding equatorial disk
that radiates the hydrogen
spectrum. Most such stars are in class B, and constitute the
populous set of "Be" stars epitomized by Gamma Cassiopeiae and Zeta Tauri. "Ae" stars on the other hand
are rare. In a sense, they are the spillover into class A from hotter
class B. But the bizarre world of astronomical nomenclature strikes
again. Do not confuse them with Herbig Ae/Be stars like AB Aurigae, the class named after George
Herbig, (1920-1913). While they too have equatorial disks, they are
young protostars caught in the act of formation. Just under a second
of arc away from 1 Del (last measured at 0.9 seconds) is an eighth
magnitude companion. The
magnitude difference and the original combined value yield
respective magnitudes of 6.27 and 8.09 for A and B. In spite of their
position in the Milky Way and distance of 743
light years (give or take a large uncertainty of 117), it's surprising
that there is no significant dimming by interstellar dust. Respective but
uncertain (again) temperatures of 9160 (measured?) and 9100
(estimated) Kelvin for 1 Del A and B, needed to account for ultraviolet light, plus
distance give luminosities of 139 and 26 Suns
and radii of 4.7 and 2.1 times solar. Be (and as here, Ae) stars
are characterized by very rapid rotations, and 1 Del A is no
exception, the equatorial spin speed given from 217 to 320 kilometers
per second. That 1 Del is a shell star implies that the disk is more
or less on edge (forming something of a shell around the star), so
that the rotation axis should be fairly perpendicular to the line
of sight, making the observed average rotation speed of 270 km/s
probably close to the mark. 1 Del A then rotates in under 0.9 days.
How rotation is linked to the Be (or Ae) phenomenon is not clear.
Magnetic fields may be involved. Theory applied to luminosity and
temperature gives a mass of two (2.1) Suns to 1 Del B, and show it
to be a hydrogen-fusing dwarf, while 1 Del A, at three
(3.0) solar masses, is apparently a subgiant that, with a dead
helium core, is just beginning its journey into helium-fusing gianthood, after which it will
toss away its outer layers to become a white dwarf of some 0.7
solar masses ("B" someday turning into a lighter one of 0.55 Suns).
At a physical separation of at least 200 Astronomical Units, 1 Del
B takes more than 1300 years to make a full circuit around 1 Del A.
Much father off is 1 Delphini C. Over the past 120 years it has
maintained a separation of just under 17 seconds of arc, and therefore
seems to be keeping a nice pace with the inner AB pair. More than
3900 AU away from the close duo, it takes over 100,000 years to orbit.
Maybe "number 1" is apt after all.
Written byJim Kaler 9/26/14. Return to STARS.