2 LAC (2 Lacertae). The only star that makes any kind of visual impact on obscure Lacerta, of all things a celestial Lizard, is the Alpha star, and it is of just magnitude four (3.77). Beta Lac then lies at the other end of fourth magnitude (4.43), and that is it for the Greek letters. The rest of the "names" are but Flamsteed numbers, the best known star of which is the blue, class O dwarf 10 Lacertae. Just over the fifth- magnitude line (4.57), though, Flamsteed's 2 Lac holds its own by giving us a bit of uncertainty and a look back into the astronomical past. The blue-white class B (B6) hydrogen-fusing dwarf has a tight spectroscopic companion in a very short 2.6164-day period. The two are quite inseparable. Estimates from the spectrum suggest a full magnitude difference, rendering 2 Lac A (the B6 star) magnitude 4.94, 2 Lac B then 5.94, which would make it a class B8 dwarf. (Off in the distance, 48 seconds of arc away, lies 12th magnitude 2 Lac C, which from its large motion is no more than a line-of-sight coincidence.) A thirteen percent correction for dimming by interstellar dust, a distance of 554 light years (give or take 9), and respective temperatures of 15,100 and 12,000 Kelvin (the latter an estimate), yield luminosities of 863 and 203 times that of the Sun and radii of 4.3 and 3.3 solar. Theory then leads to masses of 5 and 3.5 solar masses, and for the system an age of around 70 million years, the more massive of the pair about three-fourths of the way through its hydrogen fusing lifetime. Given the masses and period of 2.62 days, Kepler's Laws reveal a mean separation of but 0.076 Astronomical Units, just 16 solar radii and only four times the radius of the system's brighter member. Measures show the orbit to be closely circular. A projected rotational velocity of 44 kilometers per second for 2 Lac A gives a rotation period under 4.9 days. Given their closeness, the two are most likely in synchronous rotation, the rotation periods the same as the orbital, which makes the axial tilt 32 degrees to the line of sight. Their proximity must also produce significant tidal distortion. Though they do not eclipse each other, as they orbit, they present different apparent cross- sections to us, and thus the system varies some as an ill-studied "ellipsoidal variable." The real fun for the two begins when 2 Lac A's core hydrogen runs out and the star begins to expand as a giant, whence it will impinge on 2 Lac B, causing mass to flow from the evolving star to the denser dwarf, as is now the case for Algol. As to the reference to the past? The orbital period, given repeatedly throughout the literature on the star, remarkably harkens back to a detailed study in 1904 by then-young R. H. Baker, who eventually authored a wildly popular astronomy textbook and various popular writings, and in the latter part of the first half of the 20th century, became head of the Department of Astronomy at the Star of the Week's home institution, the University of Illinois.
Written by Jim Kaler 10/08/10. Return to STARS.