DELTA VEL (Delta Velorum). Delta Velorum is the second brightest star of Vela (the Sails, the northeastern portion of ancient Argo, the Ship), and is topped in its constellation only by Regor, Gamma-2 Vel. Canopus and Miaplacidus (the Alpha and Beta stars of Argo) lie in Carina, the Keel, the stars divided prior to the shipwreck. The star is also well-known the brightest member of the "False Cross," which is sometimes confused with Crux, the Southern Cross, and which also includes Aspidiske (Iota Carinae), Avior (Epsilon Carinae), and Markeb (Kappa Velorum). Delta Vel has the curious distinction of being the brightest star (mid-second magnitude, 1.96) in the sky with no proper name. While many other bright deep southern stars acquired modern, even ultramodern, names (like "Regor"), poor Delta received none at all. It surely deserves one, if for no other reasons than, because of precession (the 26,000-year wobble of the Earth's axis), it will be a fine South Pole star around 9000 AD and because of its delightful multiplicity. At a distance of 80 light years, this seeming class A1 dwarf is at least triple, and may be quintuple. Through the telescope we see a sunlike (1 solar mass) class G companion (Delta Vel B) that orbits the bright class A star (Delta Vel A, the letter not to be confused with the class) every 142 years at an average distance of 49 Astronomical Units, a high eccentricity causing a range of between 72 and 26 AU, the pair closest in September of 2000.
deltavel Delta Velorum B traces a clockwise apparent relative orbit around more massive Delta Vel A (the bright naked-eye star), whose position is indicated by the cross. The axes are in seconds of arc. The orbit is tilted to the line of sight, that is, is not seen face on. In reality, both stars move in mutual orbits around a common center of mass, which is not located for this system. Delta Vel A is also double (an eclipsing binary), while far outside the image lurks another distant double that circulates around the pair shown here, making Delta Velorum a quintuple star. (From an article by R. W. Argyle, A. Alzner, and E. P. Horch, in Astronomy and Astrophysics, vol. 384, p. 171, 2002.)
Delta Vel A is itself an eclipsing binary whose secondary was first detected through interferometry. It seems to consist of a brighter a class A1 star with a temperature of 9250 Kelvin in mutual orbit with a cooler and 6-times-fainter A5 star with respective masses of 2.7 and 2.0 solar. They orbit in 45.2 days at an average separation of half an AU, producing a primary eclipse with a drop of about 0.4 magnitudes. The orbit of Delta Vel B around the bright inner pair gives the total mass for the three of 5.7 solar, from which we apportion the individual masses. The total luminosity of 83 Suns for the inner pair expected from stellar evolutionary theory is in the right range. No dust disk is detected, which given the multiplicity, is not surprising. A bit over a minute of arc away is another double, Delta Vel C and D, two probable class M red dwarfs of magnitudes 11 and 13.5 at least 150 AU apart that orbit over a period of at least 2000 years. If they are really coupled to the inner triple (which seems likely), they are at least 1700 AU away from it and take 28,000 years to orbit. From the triple, they might appear some 5 degrees apart and shine with about the brightness of Venus in our sky. From the CD pair, the AB pair would be easily separable, but a telescope would be needed to split the eclipsing inner double. Too bad there is no planet (so far as we know) to watch it all.
Written by Jim Kaler. Return to STARS.