The Summer Triangle
Published on Jun 13, 2006 at 4:55 pm.
1 Comment.
Filed under constellations.
Some time back, I had a post about asterisms and constellations. Just to remind you, an asterism is an unofficial grouping of stars. Here in North America, the Big Dipper is the most famous asterism. It is really part of Ursa Major, the Big Bear. Another famous asterism is beginning to make its appearance here in the Northern Hemisphere, the Summer Triangle.Â
As the name suggests, the Summer Triangle is a triangle formed from three stars. Well, big deal, you might say, any three stars make a triangle. Well, yeah, that’s true. What makes this special is that it happens to be three quite bright, easy to see stars.  Even more special is that this particular set of stars was made quite well known by the great astronomy popularizer Sir Patrick Moore about half a century ago. It’s kind of a natural group of stars. At star parties, or any event where I am outside in the summer and people know that I am an astronomer, then I am asked “What are those three stars?” Actually, I often mention it at star parties in my public talk, but some people miss that. It is called the Summer Triangle because these stars are up in the evenings all summer (and into the Fall). The three stars making up the Summer Triangle are Vega (in the constellation Lyra), Altair (in the constellation Aquila), and Deneb (in the constellation Cygnus). That’s right. These three stars are in three different constellations. That’s why it’s an asterism! Patrick Moore made the Summer Triangle famous, and to my knowledge he was the first to use that exact term for it, but others had commented on the asterism before. I have found a reference to Sommerliches Dreieck (meaning the Triangle of Summer) made by Oswald Thomas, an Austrian astronomer, about two decades before Patrick Moore made the asterism famous. Perhaps this is where Patrick Moore got idea. However, even as far back as 1816, Johann Bode marked the triangle on star charts (Though he didn’t actually name it on the charts, he did connect the stars with lines forming a large triangle.). Other astronomers throughout the Nineteenth Century made reference to the conspicuous triangle of stars easily visible in the summer skies.
The first star of the Summer Triangle usually to make an appearance (depending upon your latitude), is Vega. 25 lightyears away, Vega is a bright white star. In fact, Vega is a color standard star. It sets the standard whereby we compare blue magnitudes to visual magnitudes, so it has a color index of zero. It is a type A0 main sequence star of magnitude 0.03. It is the brightest star in the constellation of Lyra, the Harp. Vega is also featured prominently in Carl Sagan’s book Contact, and in the movie by the same name starring Jodie Foster.Â
Deneb is often the next star to appear, if you live far enough north. Deneb, at magnitude 1.25, is the brightest star in Cygnus, the Swan. Deneb means tail. So, Deneb marks the tail of the swan. Many stars have more than one name, and some names are shared with other stars. Since it just means tail, the name Deneb is not really unique, but we all know that this is the star that you mean if you say Deneb. It is also known by the names Arided, Aridif, Gallina, and Deneb el Adige. At spectral type A2, Deneb is also a white star, but it has always looked slightly bluish to me. I don’t really know why. It is not a main sequence star like Vega. Deneb is, instead, a supergiant star. The term invokes the idea of a monstrously huge star, but it really means super brigth more than super big. Deneb is big, but it is by no means the largest star that you see in the sky. Rather, it is one of the brightest. Though it is only first magnitude as seen from Earth, consider that it is nearly 1500 lightyears away! That means that it shines more than 83,000 times brighter than our own Sun!!!! Were it to sit where the Sun is today, we’d be toast. Burnt toast, at that.
The final star in the trio is Altair. If you live far enough to the south, you see Altair rise before Deneb. Altair, at magnitude 0.76 is the brightest star in Aquila, the Eagle. Altair is a spectral type A7 main sequence star located 17 lightyears from us. Altair is interesting because, unlike the average star which is pretty close to being spherical in shape, Altair is an oblate spheroid, with its equatorial diameter some 15% larger than its polar diameter. This strange shape results from its unusually rapid rotation. It takes Altair under 10 hours to rotate, as compared with about three to four weeks for the Sun (depending upon what part you are looking at).  If Altair were to rotate at about the same speed as the Sun, it would be somewhat larger in size, though not excessively, though it still shines about a dozen times more brightly than our own Sun. Altair is the home to the mythical Krel from the movie Forbidden Planet, which was released fifty years ago. See my earlier post about Forbidden Planet if you are interested in knowing more about the movie.
If you want to see the Summer Triangle, you can go out tonight, several hours after sunset, and look to the East. If you wait a couple months, say in August, then it will be high in the sky at sunset. By September and early October, it is nearly directly overhead at sunset for those of us in the Northern Hemisphere. By the end of November or early December, it is setting at sunset. That makes it really sort of a summer and autumn triangle. But, of course, it is still known by the famous designation Summer Triangle.
-Astroprof
michael on December 10, 2008 at 10:26 am: 1
The Big Dipper is not only the most famous asterism in North America but in the world. I guess it is so because it was used for navigation purposes due to its relationship with the small dipper.