SADALBARI (Lambda and Mu Pegasi), a two-for-one special! The more or less random distribution of stars leads to some curious coincidences. Yellow class G dwarf stars like the Sun are not all that common, yet there are two side by side, our Sun and the brightest member of our nearest companion, Alpha Centauri, which shares our G2 subclass. Northern Pegasus has another odd pairing. Just to the southwest of Scheat, Beta Pegasi, lies a duo just one and a quarter degree apart that so impressed the Arabic astronomers that they gave them a shared name, Sadalbari, "the Lucky Stars of the Excelling One." Nobody, however, now seems to know what it means (as for Sadalmelik and Sadalsuud in Aquarius). They are discriminated by their Greek letter names, which lie in sequence, the southwestern one fourth magnitude (3.95) Lambda Pegasi, the other barely-third-magnitude (3.48) Mu Peg. More oddly, not only do they hold the same proper name, they -- like the Sun and Alpha Cen -- are both not only of class G, they are of the same subclass, G8. But this time are both giants rather than dwarfs. Like our two dwarfs, though, they are not a real pair, but have very different distances from us, Mu Peg 106 light years away, Lambda 365 light years, 3.4 times farther. Here then we see that they part company. To be roughly the same brightness as seen from Earth, the more distant one, Lambda Peg, must be by far the more luminous. Indeed it's been classed as a G8 supergiant (which physically it is not, as noted below).

With a temperature of 4995 Kelvin, the lesser of the two (Mu) shines with the light of just 45 Suns, not all that much for a giant, from which we derive a radius of 9 times that of the Sun, which agrees very well with a value of 8.75 found through direct interferometry. The mass then comes in at between 2.0 and 2.5 solar depending on the evolutionary condition (probably, as a helium-fusing star, the former case).

Now for Lambda. Somewhat warmer, 4710 Kelvin, Lambda radiates at a rate of 400 times that of the Sun (9 times that of Mu), which gives us a radius of 30 times solar (3 1/3 times that of Mu), half that of the size of Mercury's orbit around the Sun. The slow rotation period could be as long as 190 days (that of Mu not known). Lambda's mass is a hefty four times solar if it is just starting to fuse its helium into carbon and oxygen, 3.7 if the process is already quietly underway. In opposition to the alternative designation of "supergiant," Lambda clearly has the characteristics of a fairly massive giant. It's expected to die as a 0.75 solar mass white dwarf, whereas Mu will pass away at 0.63 solar. Lambda's metal content is 0.7 times that of the Sun, while Mu's is close to the solar value. From Mu, Lambda would appear somewhat brighter, third magnitude (3.2), while from Lambda, Mu would shrink to near sixth (5.4), any residents (and is that ever unlikely!) paying little attention to each of the two to which we actually gave a joint name. (Thanks to Paolo Colona, who suggested these stars.)
Written by Jim Kaler 10/23/09. Return to STARS.