Sky & Telescope - October 2022 - 45

Evenings with the Stars by Fred Schaaf
Autumn's
Lonely
Southern
Beacon
Solitary Fomalhaut shares
connections with several
bright stars.
A
utumn is a very lonely season for
bright stars. From mid-northern
latitudes, the constellations of spring
have three stars of first magnitude or
brighter; summer displays four such
stars; and winter boasts seven luminaries.
But the constellations of autumn
have, remarkably, just one: Fomalhaut.
Fomalhaut dominates the otherwise
undistinguished constellation of Piscis
Austrinus and is even more isolated
than the above statistics suggest.
Excluding the far southern sky, Fomalhaut
is the only first-magnitude star
occupying a vast span of about eight
hours of right ascension - one-third of
the way around the entire heavens.
If you're not sure that the glint near
the southern horizon is Fomalhaut, you
can confirm your sighting by first locating
the Great Square of Pegasus. Extend
a line 45° south through the Square's
western stars and you'll land right at
Fomalhaut's position. But since it has
no competition anywhere nearby, unless
something's blocking your view, the star
should be easy to identify.
pALONE AGAIN, NATURALLY Sitting between
the glitter and spectacle of the summer
and winter constellations are the comparatively
faint stars of the autumn sky. The sole exception
is 1st-magnitude Fomalhaut, which is
shown in the photo above near the lower right
of the frame between a pair of trees.
Now, here's something else to
consider: Fomalhaut might not look as
bright as you expect. With a declination
of -29.6°, it never rises higher than 20°
above the horizon for observers at 40°
north latitude. When a star is that low,
its light is dimmed by the thick layer
of atmosphere it has to travel through.
Even on a clear, transparent night,
atmospheric extinction dims Fomalhaut
from magnitude 1.2 down to around
1.6 - slightly below the first-magnitude
cutoff of 1.5. Viewers around 50° north
latitude never see Fomalhaut higher
than 10° and, therefore, never brighter
than about magnitude 2.2.
Fortunately, lonely Fomalhaut has
some statistical connections to other
bright stars, including summer's brilliant
Vega. Lyra's alpha star is 25.0 lightyears
from Earth, while Fomalhaut is
just a tiny bit farther, at 25.1 light-years.
The two have other interesting connections.
About 40 years ago, the Infrared
Astronomical Satellite (also known
as IRAS) discovered what are probably
belts of comets (or Kuiper Belt Objects)
encircling each sun. And about 20 years
ago, researchers found evidence that
Fomalhaut and Vega may also each be
orbited by a planet (or planets). The
similarities don't end there. Studies also
indicate that both stars are members
of the Castor Moving Group, which
includes some 16 stars that are drifting
through space together.
When it comes to brightness, Fomalhaut
is a very close match to Pollux, the
brightest star in Gemini. Measurements
show that Pollux is only 1
/50 magnitude
fainter - a difference that's impossible
to perceive visually.
Finally, there's an interesting
connection between Fomalhaut and
Capella. At mid-northern latitudes,
both stars rise at about the same time.
This is remarkable considering that
Capella is about six hours of right
ascension farther east than Fomalhaut.
But that difference is offset by Auriga's
luminary being at a declination 76°
farther north than Fomalhaut. As
an evening star, Capella is viewable
over three seasons (autumn, winter,
and spring), and for those at around
latitude 50° north, it's circumpolar and
visible even in summer. By contrast,
Fomalhaut is essentially a one-season
star, being well placed almost exclusively
in autumn. However, even from
mid-northern latitudes, you can sight it
in the predawn as early as May, and at
dusk as late as January.
¢ In January 1970, FRED SCHAAF
located his first comet (Tago-SatoKosaka
1969g) low in the southwest,
below Fomalhaut.
sk yand tele scope .o r g * OCTOBER 2022 45
ALAN DYER
http://skyandtelescope.org
Sky & Telescope - October 2022

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