UpDate - Vol. 11, No. 2, Page 7
September 12, 1991
Sky high; Sumer triangle of stars lights up evening sky

     September brings the summer triangle high overhead in the
evening hours. This is a configuration of stars for city dwellers.
     Of course, any three stars may be considered a triangle. There
is nothing distinctive about that; but these are the three
brightest stars that are now high in the sky.
     As such, they are best known to those of us for whom a sky
without light pollution means that we have traveled a considerable
distance from home.
     Indeed, those who are fortunate enough to live far away from
city lights may have a harder time finding the summer triangle
because the Milky Way runs down through its middle and dominates
the celestial scene.
     On the map, the course of the Milky Way is from the upper left
corner to the lower right corner. In urban sites where the sky is
never really dark, the countenance of the heavens is reversed: all
we see are the brighter stars and we must strain to make out the
Milky Way.
     Brightest of the trio is Vega in the small constellation of
Lyra (the Lyre). To the east from Vega, and almost directly
overhead at 10 p.m. on Sept. 15 (9 p.m. on Sept. 30) is the star
Deneb. The name Deneb comes from the Arabic and means "tail."
Considering the number of tailed animals among the constellations,
we need not be surprised that there are several "Denebs." But, THE
Deneb, the one now confronting us, is the tail of the swan
constellation, Cygnus. Completing the triangle is Altair, about
halfway toward the southern horizon from Deneb and Vega. Altair is
the bright star of the constellation Aquila (the eagle).
Incidentally, the eagle's tail has TWO stars that have been called
Deneb, sometimes with the spelling Dheneb.
     Today, only the brightest stars are generally designated by
proper names, the rest are better known by their Bayer designation
(the combination of a Greek letter with the possessive form of the
constellation's Latin name) or by some catalogue number. I have
never heard any one actually call either of the eagle's tail
feathers by the name Deneb; they are Zeta Aquilae and Epsilon
Aquilae.
     Lyra, as may be seen on the map, attracts our attention by its
distinctive parallelogram, a skewed rectangle of stars of similar
brightness. Cygnus is equally well known today as the Northern
Cross.
     Indeed, this is a cross much closer to traditional proportions
than the famous Southern Cross, which really doesn't look like a
cross at all.
     Although Deneb appears to us as the faintest of the three
summer triangle stars, it is intrinsically the brightest of all, a
thousand times brighter than Vega.
     Since its surface has a temperature similar to that of Vega,
this greater luminosity is a result not of Deneb's emitting more
light per square inch but rather of its having more square inches
of surface area emitting light. We conclude that Deneb is 20 times
the diameter of Vega.
     Why, then, does Deneb appear fainter than Vega? Because it is
60 times farther away. Traveling at 186,000 miles per second, the
light that we see from Deneb left that star 1600 years ago. We say
that the distance to Deneb is 1600 light years. Vega is only 26.5
light years distant, and Altair is the closest at 16.5 light years.
                                               - Richard B. Herr,
                                           Associate Professor of
                                            Physics and Astronomy