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DELTA TRI (Delta Trianguli). Tucked as it is just aboveGamma Trianguli (inTriangulum, the Triangle), fainter Delta Tri is notgiven much attention, except that with 7 Tri (just southwest ofGamma) it helps make a line-of-sight triple star, the three havingnothing to do with each other. At fifth magnitude (4.87), Delta isroughly midway in brightness between the other two. It is,however, by far the closest of them, lying a mere 35 light yearsfrom Earth. Its relative faintness is the result of the intrinsiclow luminosity of a solar type star, actually two of them, oneshining visually at magnitude 5.2, the other at 6.5, 3.3 timesfainter (the two inseparable by eye). In close orbit about eachother every 10.0201 days, these two suns, each less luminous thanour ownSun, is a science fiction writer'sdream. The primary star, like the Sun, is class G (G0, at 5900Kelvin somewhat warmer than our G2, 5780 Kelvin Sun), the otheraround G9 (5300 Kelvin). Both are fairly young, hydrogen-fusingdwarfs. Their total luminosities are respectively 0.8 and 0.3times that of the Sun (the allowance for invisible infraredradiation making the secondary star a bit more respectable), theirradii 0.86 and 0.64 solar, their masses (estimated fromluminosities and temperatures) 1.0 and 0.8 solar. Other analysesgive somewhat different results. While the pair cannot be splitvisually, sophisticated interferometer observations haveestablished the orbit. They move about each other in circularpaths at a separation of 0.106 Astronomical units, only 28 percentMercury's distance from the Sun. From the orbit, their combinedmasses are 1.60 times that of the Sun, somewhat less than the 1.8solar found from their evolutionary status. The brighter may be asmuch as 6 times as visually luminous as the fainter, which wouldlessen the mass of the secondary and bring the two estimates oftotal mass into better agreement. The system is rather likeAlpha Centauri, except here a planet(were there one) would have to orbit the pair rather than theindividuals (as the components of the Alpha Cen system are widelyseparated). To achieve the same irradiance we have on Earth, anorbiting planet would have to lie at a distance only four percentfarther than we are from the Sun. From there we would see twoyellow suns looping around each other, separated by as much as 6degrees (just a bit greater angle than separates the front bowlstars of the Big Dipper). Given a 24 hour rotation period of ourhypothetical planet, a person at the equator would see one rise orset at most half an hour after the other. A third star, of 14thmagnitude, lies a minute of arc away from the pair. Alas, at adistance of 360 light years, 10 times farther than Delta Tri, thealignment is a mere coincidence.
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