MU-1 and MU-2 PAV (Mu-1 and Mu-2 Pavonis), a two-for-one special. Odd coincidences abound amongst the stars and here's "one for the books" as they say. At sixth and fifth magnitude (5.76 and 5.31), Mu-1 and Mu-2 Pavonis (in Pavo, the Peacock) appear as a faint naked-eye double just a degree southwest of much brighter Delta Pav. Deep in the southern celestial hemisphere, the two are only 23 degrees from the South Celestial Pole and are circumpolar for points south of the Tropic of Capricorn. You'd likely not notice them. But they are so close together, just 9.1 minutes of arc apart, that they begin to intrigue as a possible, though wide, physical pair. Not only are they close together on the sky, but both are also class K giants! Mu-1, the western of the two, is a K0 giant, while Mu-2, the eastern (the numbers usually based on west-east position, though not always, as seen in Orion's "Pi stars") is formally classed a K2 subgiant . Theory, however (see below), reveals it to be a real red giant (such dichotomies not unusual as the criteria are for the two modes of classification are different). More oddly, they are nearly at the same distance, Mu-1 at 240 light years (give or take 28), Mu-2 at 236 light years (with a formal uncertainty of 6). They must then be partners in loose orbit, an unusual giant pair. But no. Their motions are quite different. Mu-2 is moving roughly southeast at 50 kilometers per second relative to the Sun, while Mu-1 is clipping along to the southwest at a high rate of 74 km/s, six times normal. The two are just "strangers passing in the night," perhaps making them even more intriguing. With distances and respective temperatures of 4660 and 4434 Kelvin (from which we evaluate the amounts of infrared radiation) we find luminosities of 33 and 57 times that of the Sun and radii of 9 and 12 times solar. It's hard to tell exactly what is going on. With masses of say one and a quarter Suns, the stars could be brightening with dead helium cores or fading some to become ordinary K-giant helium burners (the cores turning into carbon and oxygen in the process). But let's be creative and use these two stars, along with nearby Delta Pav, to illustrate what will happen to the Sun. Delta is a hydrogen-fusing dwarf with about a solar mass. It will someday (ignoring mass differences) appear as Mu-1 Pav and then as Mu-2, all three examples of stellar evolution within about a degree of one another. Being so close, the two Mu's must be very bright in the others' skies. At the given distances, they are but 2.6 light years apart, and each would glow for the other at the bright end of magnitude -4, roughly about that of our Venus. If they are at the same distance, allowed by the uncertainties, they would be but half a light year apart and would shine more at -8, corresponding to some 30 Venus's. Unfortunately there is no evidence for any companions, let alone planets, so there is nobody there to see them shining with their orange light as they pass, never to see each other again.
Written byJim Kaler 11/20/15. Return to STARS.