PHI PEG (Phi Pegasi). Several are the classic stellar tests for minimal
as well as excellent vision. The best known is certainly Mizar and Alcor in
the Big Dipper, the Arabs' "Horse and
Rider?" Most people can rather easily make them out. How many stars
can you see in the Pleiades, the "Seven
Sisters?" Most people see six (hence the "Lost Pleiad), but keen-eyed
observers can make out seven, even eight or nine or more. A much tougher
test is to try to split the "Double-Double," Epsilon Lyrae, whose stars (each of which is
double yet) are about three and a half minutes of arc apart. Among
the lesser-practiced tests is to see how many stars are visible inside
the nearly blank Great Square of Pegasus. There is no agreed-upon or
absolute value of course, but good eyes should at least see the four
that carry Bayer Greek letters, as
these came out of Tycho's visual observations. Here we find fourth
magnitude Upsilon Peg (at 4.40 the brightest), fifth magnitude (4.60)
Tau Peg, fifth magnitude Psi (4.66) and
fifth magnitude (5.08) Phi. To these we might add 71 Peg (mag 5.32)
and 56 Peg(5.38). Of great significance is HR
8799, which has an orbiting planets that can be seen with direct
imaging, though at sixth magnitude (5.99) the star's a tough naked-eye
find about halfway between Beta and Alpha Peg. Near it is fifth magnitude (5.49)
51 Peg with the first-known exoplanet, but
it's just outside the square and doesn't count. As a separate but
curious issue, Phi Peg is just east of the equinoctial colure, the great
circle that connects the equinoxes
and the celestial poles. Because of the 26,000-year precession of the
Earth's axis, the equinoxes and solstices are moving east against the
stars. Phi Peg will cross over around the year 3030. (It's never too
soon to plan a party). Phi Peg is actually a reddish class M (M2.5)
giant. (The square seems to
attract them: so are Psi and 71 Peg.) Very little is known about the
star, which lies at a distance of 463 light years (give or take 18).
A temperature of 3400 Kelvin adopted from the spectral class allows an estimate
of the cool star's copious infrared
radiation, yielding a total luminosity of 1860 Suns, which in turn gives a radius of 125 times
that of the Sun, 0.58 Astronomical Units, 1.5 times the radius of Mercury's orbit.
Given all the uncertainties, the star probably carries
a mass of around twice that of the Sun. Some 1.5 billion years old,
the best guess is that it has run out of helium fuel and is brightening
for the second time with a dead carbon core and will before long shed
its outer layers, allowing the core to turn into a white dwarf. There seems to
be no evidence for any sort of companion
to watch the action, the star
circling the Galaxy on its own.
Written byJim Kaler 12/02/16. Return to STARS.