PSI AND (Psi Andromedae). As a rare class G (G5) supergiant 1000 light years away
(give or take 150), fifth magnitude (4.95) Psi Andromedae (in the northwest corner of Andromeda near the border with Cassiopeia, 1.4 degrees east of brighter Lambda and just barely the most northerly
Greek-lettered star) excites one's
curiosity. A closer look intrigues even more, as the mid-temperature
(5050 Kelvin) star seems to be the Everest of a veritable cluster of
six fainter stars that have been related to it. Psi And B, C, D, and
E (magnitudes 14.6, 13.1, 9.5, and 10.4) scatter outward from Psi A
at separations of 24, 63, 186, and 241 seconds of arc. However, small
shifts relative to Psi A can be explained by its motion against the
background relative to the Sun, so at least for now (and maybe the next
century) we have to put them on hold. Odds are they are just in the
line of sight. However, the remaining two stars (Ab and Ac) are so
close to Psi proper, just a couple tenths of a second of arc, that they
are surely real companions. Ac
is clearly the brighter as its spectrum has
long been known to mingle with that of the supergiant (Aa), revealing
it to be an eighth magnitude (8.1) class A (A0) ordinary dwarf. Except for its presence
nothing is known about Ab. After an uncertain correction of a quarter
magnitude for dimming by interstellar dust, a minor fix
for infrared radiation, and
subtraction of the light from Ac (assuming Ab is not significant), Psi
And Aa is seen to shine with the light of 1225 Suns, which shows the star falling between the
lesser supergiants and the "bright giants" (a term not a
description). Combination with temperature gives a radius 46 times
that of the Sun. Theory is a bit ambiguous. The star could either
be heating its dead helium core or it could be a full-fledged helium
burner. It doesn't matter much, as the derived mass is still somewhere
between 5 and 5.5 Suns. There is some agreement that the star is
metal-rich, various studies averaging an iron content relative to
hydrogen of 25 percent greater than solar.
Though bright in absolute terms, with a luminosity of 65 Suns, a radius
2.6 times solar, carrying a heft of 2.75 times the solar mass, the
A5 dwarf companion (Ac) pales beside the class G5 supergiant. While the
ages of dwarfs are hard to determine, those of evolved stars often are
not. The age of Psi And Aa must be around 60 million years, a figure that
would of course apply to both Ac and Ab (which for the sake of further
argument we assume to have insignificant mass). The angular separation
between Aa and Ac gives us an orbital size of at least 125 Astronomical
Units. Kepler's laws then
yield an orbital period of more than 475 years. Ab must be at least
90 AU from Aa and orbit in over 375 years. The problem is that we have
no idea of the foreshortening. Ac could be well inside the orbit of
Ab or well outside of it. For that matter, Ab could be in orbit around
Ac. Before long, Psi And Aa, the luminous supergiant, will shed its
outer envelope, produce a planetary nebula, and
die as a white dwarf about
the size of Earth with a mass of around 85 percent that of the Sun.
The star our descendants see as Psi And will then be classed as an A5
dwarf. It will take some probing to find the small faint companion
that once dominated the system, as Sirius B once did for Sirius.
Future astronomers will have what is probably a faint unevolved red
dwarf (Ab) to contend with as well.
Written byJim Kaler 12/18/15. Return to STARS.