56 PEG (56 Pegasi). Just barely fifth magnitude (4.74, almost
fourth), 56 Peg (commonly designated by its Flamsteed number), lies
practically on the outline of the Great Square of Pegasus about 2.5 degrees due south of
Beta Peg (Scheat). At first it looks like
just another class G or K helium-fusing giant that so abound (and make
much of our constellations), though one a
bit brighter than most. Originally classed as G8 lesser supergiant, it's also been
called a K1 subgiant. We'll
go here with the last one, though it is of little consequence.
Common as it appears, the star harbors a secret, though one not all
that rare. At a healthy distance of 592 light years (give or take
25), assuming there is no interstellar dimming (of which
there is some evidence), and adopting a temperature of 4509 Kelvin
(the average of several measures) from which we can assess the amount
of infrared radiation), we
come up with a luminosity of 570 times that of the Sun (less than we'd expect for a supergiant),
a radius of 39 times solar, and a mass of 4 Suns, putting it smack
into the so-called "clump" of helium-fusing giants with similar
properties though of differing masses. A maximum interstellar
dimming of 1.25 magnitudes raises the luminosity to 1800 Suns, the
radius to 70 times solar, and the mass to 6 Suns. It's really just
a good old K1 or so giant with a rotational period of a year of less.
An interferometric measure of angular diameter gives 46 solar radii,
not that far from the mean of the dimmed-undimmed values and about
half the size of Mercury's orbit. At least we are more or less on
the right track. 56 Peg is special because it's a "barium star" with
an elevated abundance of barium as well high levels of other elements
produced by the slow capture of neutrons. Such abundances cannot be
innate to a simple giant, so it must have been contaminated by
enriched gas flowing to it from a companion that was once a highly
advanced giant with a dead
helium core that is now lost on the glare of 56. Sure enough, 56 Peg
is orbited by a white
dwarf with a temperature of 32,000 Kelvin that makes the star
shine brilliantly in the ultraviolet where it is
easily detected. Doppler shifts in the spectrum of 56 proper reveal
an orbital period of 111 days, which is next-to-shortest for
barium-star white dwarfs after 80-day HD 77247. To have already
evolved to a white dwarf, the smaller star must once have been the
more massive of the two. Not too long ago, the current giant must
have been surrounded by a planetary nebula,
the older star's ejecta illuminated by what is now the white dwarf.
The lost mass, rich in new chemical elements will eventually find
its way into another generation of stars. (Thanks to R. F. Griffin
in "The Observatory," February 2006.)
Written byJim Kaler 12/30/16. Return to STARS.