PHI AQR (Phi Aquarii). Want to see what will someday happen to the
Sun? Here is a fine chance, from a star
that lies within the constellation of Aquarius, the "Water Bearer." Phi Aquarii
(with no proper name) is a fourth magnitude (4.22) class M (M1.5,
sometimes called M2) red giant
that lies a dozen degrees southeast of Aquarius's famed "Water Jar" and a bit less to the south of
the western end of Pisces' "Circlet." Just a degree off (to the south
of) the ecliptic, a line
drawn from it to Lambda Aquarii more or
less parallels the apparent solar path. Rather well-observed, the
temperature from various studies is pretty consistent around the
average of 3760 Kelvin, the star producing much of its radiation in
the invisible infrared. When
that is accounted for, and Phi's distance of 202 (give or take 9)
light years is factored in, we find a luminosity 265 times that of
the Sun and a radius of 38.5 solar, which is about half the size of
Mercury's orbit. Since Phi Aqr is close to the ecliptic, it's
regularly occulted (covered over) by the Moon. From the time it takes the star to
disappear behind the lunar disk, and knowing the rate at which the
Moon moves across the sky, the star's angular size is readily
found. Distance then leads to a smaller physical size of 32.7
solar radii. (The errors of measurement, however, are such that
the two values are consistent with each other.) The projected
rotation is so slow that it has never been measured, but it could
be as long as 1.3 years. The real interest, though, is that when
compared with calculations from the theory of stellar structure and
evolution, Phi Aquarii has almost exactly the luminosity and
temperature expected for the Sun after a life of 12.2 billion years
since birth (7.6 billion years from now). Having given up core
hydrogen fusion 2.4 billion years ago (at an age just shy of 10
billion years), Phi Aqr is now most likely brightening as a red
giant with a dead helium core, which will shortly begin to fuse
into carbon and oxygen, the star then shrinking and dimming a bit
to become a quiet class K giant helium burner of the sort that
flocks the sky. At its coming peak, Phi Aquarii will reach a
luminosity of nearly 1300 solar and a radius half the size of
Earth's orbit. Alternatively, it could now already be in that
dimming phase. In any case, any inner planets like Mercury the
star might have (or had) are gone. And that is minor compared to
what will happen when the star's core helium is used up (having
fused to carbon and oxygen) and it becomes thousands of times
brighter than the current Sun, as big as the orbit of Earth, loses
its outer envelope, lights up a surrounding planetary nebula, and dies as a white dwarf. Phi Aquarii is
sometimes listed as having a spectroscopic companion, though
that is not confirmed. Like the Sun, the star will probably have
to go through its long evolution by itself.
Written by Jim Kaler 11/05/10. Return to STARS.