DELTA GRU (Delta Gruis). Double stars abound in the sky, but how easy it is to be fooled. A fine case in point lies in Grus, the Crane, (indeed, two cases in point) with a pair of fourth magnitude stars both called "Delta," the northern (and slightly eastern) one labelled Delta-1, the southern Delta-2. Separated by a little over a quarter of a degree, they make a fine naked-eye pair, one that is entirely a line-of-sight coincidence. Oddly, the two are at almost the same distance. Delta-1 is a rather ordinary class G (G7) giant (magnitude 3.97) 295 light years away, while Delta-2 is a red class M (M4.5) giant (magnitude 4.11) at 325 light years. The errors on the measured distances are large enough that the two could in fact be at the same distance. They are, however, going through space in quite different directions, and have nothing whatever to do with each other. Though of nearly the same visual brightness, Delta-2 is by far the more luminous. Cooler (estimated at 3400 Kelvin), it radiates most of its 2200 solar luminosities in the infrared (Delta-1, at 4900 Kelvin, nearly 10 times dimmer). A truly great star, Delta-2 is 135 Suns across, 0.65 or so Astronomical Units, almost as big as the orbit of Venus. Its mass probably hovers around thrice solar, and within the ageing process, it is most likely a "second ascent giant," one with a dead carbon- oxygen core, its fate to lose a great deal of its mass and turn into a white dwarf. (Delta-1, on the other hand, is a helium-fusing giant that will someday become like Delta-2.) Consistently, Delta-2 is a slight "irregular" variable star with a range of about 0.11 magnitudes. Upon careful examination, such irregular giants are commonly found to be "semi-regular." Unfortunately, Delta-2 has been rather well ignored by the observers. A look through the telescope shows it to be double, with a ninth magnitude class M companion (probably a hydrogen fusing dwarf) a minute of arc away. But here we are fooled again, as over the past 175 years, the two are shifting away from each other much too fast, showing the line-up again to be coincidental. However, Delta-1 (perhaps as much as four solar masses) may have a real twelfth magnitude class K companion over 500 Astronomical Units of arc away from it, the two taking over 5000 years to make a full circuit of each other. Taking the distances of Delta-1 and Delta-2 as correct, each would be wonderfully prominent in the others' sky, shining at magnitude - 1.1, almost as bright as Sirius seems to us on Earth.
Written by Jim Kaler. Return to STARS.