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DELTA VEL (Delta Velorum). Delta Velorum is the second brighteststar ofVela (the Sails, the northeasternportion of ancientArgo, the Ship), and istopped in its constellation only byRegor,Gamma-2 Vel.Canopus andMiaplacidus (the Alpha and Beta starsof Argo) lie inCarina, the Keel, thestars divided prior to the shipwreck. The star is also well-known the brightest member of the "False Cross," which is sometimes confusedwithCrux, the Southern Cross, and which alsoincludesAspidiske (Iota Carinae),Avior (Epsilon Carinae), andMarkeb (Kappa Velorum). Delta Vel has the curiousdistinction of being the brightest star (mid-second magnitude,1.96) in the sky with no proper name. While many other bright deepsouthern 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 ofprecession (the 26,000-year wobble of the Earth's axis), it will be a fineSouth Pole star around 9000 AD and because of its delightful multiplicity. At adistance of 80 light years, this seeming class A1 dwarf is at leasttriple, and may be quintuple. Through the telescope we see asunlike (1 solar mass) class G companion (Delta Vel B) that orbitsthe bright class A star (Delta Vel A, the letter not to be confusedwith the class) every 142 years at an average distance of 49Astronomical Units, a high eccentricity causing a range of between72 and 26 AU, the pair closest in September of 2000.

Delta Velorum B traces a clockwise apparent relative orbit aroundmore massive Delta Vel A (the bright naked-eye star), whoseposition is indicated by the cross. The axes are in seconds ofarc. The orbit is tilted to the line of sight, that is, is notseen face on. In reality, both stars move in mutual orbits arounda common center of mass, which is not located for this system. Delta Vel A is also double (an eclipsing binary), while far outsidethe image lurks another distant double that circulates around thepair shown here, making Delta Velorum a quintuple star. (From anarticle by R. W. Argyle, A. Alzner, and E. P. Horch, inAstronomy and Astrophysics, vol. 384, p. 171, 2002.)

Delta Vel A is itself aneclipsing binary whosesecondary was first detected through interferometry. It seems toconsist of a brighter a class A1 star with a temperature of 9250Kelvin in mutual orbit with a cooler and 6-times-fainter A5 starwith respective masses of 2.7 and 2.0 solar. They orbit in 45.2days at an average separation of half an AU, producing a primary eclipse with a drop of about 0.4 magnitudes. The orbit of DeltaVel B around the bright inner pair gives the total mass for thethree of 5.7 solar, from which we apportion the individual masses. The total luminosity of 83Suns for theinner pair expected from stellar evolutionary theory is in theright range. No dust disk is detected, which given themultiplicity, is not surprising. A bit over a minute of arc awayis another double, Delta Vel C and D, two probable class M reddwarfs of magnitudes 11 and 13.5 at least 150 AU apart that orbitover a period of at least 2000 years. If they are really coupledto the inner triple (which seems likely), they are at least 1700 AUaway from it and take 28,000 years to orbit. From the triple, theymight appear some 5 degrees apart and shine with about thebrightness of Venus in our sky. From the CD pair, the AB pairwould be easily separable, but a telescope would be needed to splitthe eclipsing inner double. Too bad there is no planet (so far aswe know) to watch it all.

Written byJim Kaler. Return toSTARS.

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