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.