September evening skies find the remnant of summer as it slides towards the Autumnal Equinox- the first day of fall in the Northern Hemisphere, September 22nd. Summer’s amazing splendor includes the most conspicuous view of the Milky Way Band if you can escape bright light pollution. Straddling the “summer Milky Way” is a large triangle of three bright stars, Vega, Deneb and Altair.
This is informally known as the “Summer Triangle,” an asterism, rather than one of the official 88 constellations. There are many asterisms; the Big Dipper is one of them, the most well known part of the large constellation Ursa Major the Big Bear.
As evening twilight deepens, look approximately straight overhead on a September evening (if you are in the mid-northern latitudes) for the brilliant blue-white star Vega. This marks the top of the Summer Triangle. The other two stars are not quite as bright. Deneb marks the corner on the left (east-northeast); Altair is the star on the right (southeast) and it is down lower than Deneb.
Vega is part of the compact constellation, Lyra the Lyre (Harp). Deneb marks the tail of Cygnus the Swan, or the top of its own asterism, the Northern Cross. Altair is the eye of the constellation, Aquilia the Eagle.
Vega is relatively close, a mere 25 light years away. That’s only 145 trillion miles- or so. Vega is the fifth brightest star in the night sky, shining at +0.02 magnitude.
The lower the magnitude number, the brighter the star. One might think 0 magnitude would be the brightest, but it isn’t. Star magnitudes are measured on a logarithmic scale, based on a first magnitude star being 100 times as bright as a sixth magnitude star. Vega is used as a standard reference star of 0.0 magnitude. This is “apparent magnitude,” the brightness it appears from Earth.
Bright orange star Arcturus, glowing in the western September evening sky, is a little brighter, at -0.04. The night sky’s brightest star is Sirius, a winter evening star, at -1.47.
The faintest star typically visible to the unaided eye under very good conditions is around +6.5.
Back to the Summer Triangle!
Altair glows at +0.77 magnitude and is the 12th brightest star in the night sky. Altair is only 16.7 light years from the Sun, and thus closer to us than Vega. It appears white.
Deneb, a blue-white star, shines at magnitude +1.25. It is the 19th brightest star in the night sky. Approximately 2,600 light years away, the star is nearly 104 times as far as Vega and 155 times as distant as Altair. Imagine the output of this far off star, to shine as bright as it does and visible across this great chasm of outer space!
To put stars on an equal footing, astronomers also rate brightness in “absolute magnitude,” how bright the star would appear at a fixed distance compared to others. The distance used is 10 parsecs, one parsec being 32.6 light years. Placing Deneb at this distance, it would shine at - 8.38, which is about as bright as a crescent Moon. Vega would be +0.5 (very similar to its apparent magnitude) and Altair, +2.2 (or about as bright as the brighter stars of the Big Dipper appear to us).
In terms of luminosity compared to the Sun’s output, Vega is 52 times the Sun; Altair is 10 times the Sun and Deneb is an outstanding 70,000 times the Sun.
The shape and size of the “Summer Triangle”, like the constellations, is of course limited to how it appears from our Earthly abode in space. A cosmic explorer would find the three stars form a triangle in an infinite number of ways, depending on the astronaut’s vantage point in the galaxy. Vega and Altair are only 14.8 light years from each other. With Deneb so much further from them, the triangle would be long and steep as seen from the side!
Last quarter Moon is on September 13.
Keep looking up!
—Peter Becker is Managing Editor at The News Eagle in Hawley, PA. Notes are welcome at firstname.lastname@example.org. Please mention in what newspaper or web site you read this column.