The Summer Triangle
The title of this article reads like the name of a romance novel or even perhaps a Hollywood mystery. Alas, that is not the case; instead this is about the nighttime sky. The Summer Triangle involves three special stars, each located in a separate celestial constellation. Taken together these three stars, Altair, Deneb and Vega, form a very large triangle, easily discernable in the overhead (from Tucson) sky with the naked eye, from July through mid-October around 9pm. A star chart of the northern hemisphere is the best reference for finding the Summer Triangle and its three component stars. You can locate star charts on the Internet, in astronomy magazines and books or purchase the charts. The public library is an excellent source for all of these items and it’s free! Viewed from a dark-sky site, away from urban lights, it is easy to see that the Summer Triangle is positioned in the middle of a luminous glowing band known as the Milky Way.
The Milky Way we see is really one of several spiral arms the make up our galaxy, also known as the Milky Way Galaxy. The Greek word, “gala” refers to milk, and is how they thought of the appearance of the arm; our word “galaxy” is derived from it. Our solar system: Sun, nine planets with their moons, asteroids is positioned between two of the spiral arms, the Orion Arm, that we see, and the Sagittarius Arm. Astronomers estimate our galaxy resembles an immense pinwheel containing a conservative 20 billion stars!
The anchor stars for the Summer Triangle are each associated with a separate constellation and are also the brightest star in their respective constellation. Altair, in Aquila, The Eagle, is a white star about 16.5 light-years distant from our solar system and is almost a first magnitude star. The distance measurement means that it takes light leaving Altair today 16.5 years to reach Earth. Each change in magnitude is a 2.5 times difference in brightness, most people can see stars as dim as positive sixth magnitude. Thus a sixth magnitude star is 100 times dimmer than a first magnitude star. Negative magnitude values are increasingly brighter stars, Sirius at magnitude -1.45 is the brightest naked-eye star visible from Earth. Venus, the Moon and the Sun have magnitudes of -4, -12 and -27 respectively! Altair has one of the fastest rotations known for a star, six-and-a-half hours or about 160 miles per second. The Sun, about two-thirds Altair’s, diameter has a rotation period of over 25 days! As a result of its extremely rapid rotation Altair is presumed to be considerably deformed, being compressed at its poles, with an equatorial diameter almost twice its polar diameter.
Looking towards South, Altair is the apex of the Triangle; its base is an imaginary line that connects the first-magnitude star Deneb to Vega, both north of Altair. So not only are two of the stars of the same approximate first magnitude they are all about the same white color, too. Stellar color is temperature-dependent, blue being the hottest, red the coolest. Blue stars may have surface temperatures of 70 thousand degrees F or higher and red stars are relatively cool at five thousand degrees F or less. White stars have a surface temperature range of 13-17 thousand degrees F. Our nearest star, the Sun, is a yellow star, with a surface temperature of about 10 thousand degrees F. At its core, our Sun is about 27 million degrees F! That temperature makes the hottest Earthly summer temperatures seem extremely cold, if you’re a star.
The star Deneb, dimmest of the Triangle’s three anchor stars, is located in Cygnus, The Swan or The Northern Cross, one of the oldest known constellations related to human history. Deneb, Arabic for “the tail”, is about 1,600 light-years distant and is a super-giant about 60 times the diameter of our Sun or close to 50 million miles in diameter! It is also the “tail”-end of the Swan and it anchors the eastern apex of the base of the Triangle. The Northern Cross seems a more appropriate name, because that is the constellation’s visual appearance. The Cross’s entire length and most of its horizontal cross-member are within the Triangle’s boundary. In fact, Deneb is the top of the upright part of the Cross, the bottom ending at the double-star Albireo. Albireo, also known as “The Blue and The Gold”, because of their distinctive colors, is about 410 light-years away. The two stars are about 400 billion miles apart, that’s about 100 times the distance of Pluto from the Sun! The Veil Nebula is an ancient supernova remnant, a star that literally blew up an estimated 30-40 thousand years ago. It is visible even in binoculars of 7x50 size. It is about 1500 light-years away and has a diameter estimated at 70 light-years. The Veil is a breathtaking object to behold, especially from a dark site. Speaking of distance we now move from near the beginning of the alphabet to near its end, for the Triangle’s last star, Vega.
Vega, Arabic for “Eagle” is a mere 26 light-years distant in the constellation of Lyra, The Lyre or Harp. Vega represents the western apex of the Triangle’s base and it is the brightest of the three stars at almost zero magnitude. Vega has the singular distinction of being the first star to be photographed on the night of July 16-17, 1850 at the Harvard Observatory. Vega was the Pole Star some 12 thousand years ago and will be again in the year 12,000 AD. Lyra is the smallest of the Triangle’s constellations, yet it has several interesting objects contained within its borders. The Messier catalog object, M57, also know as “The Ring”, a planetary nebula, is widely known to both professional and amateur astronomers and it is one of the more frequently photographed celestial objects as well. Binoculars or small telescopes reveal it though it is obviously not as distinct, despite being almost one light-year in diameter, unless one uses larger instruments. That is understandable because it takes light some two thousand years to reach Earth from M57! Lyra has a most interesting star, this is Epsilon Lyrae, also named The Double-Double, visible as a binary in binoculars, In reality each star is a binary and so there are four stars! The distance separating the two pair of binaries is about one-fifth of a light-year, a distance in which can be fit over 160 of our solar systems end-to-end!
You now know a little about The Summer Triangle, but there is a wealth of information yet to learn and ample resources are available to help you find it. There is a surprise inside of the Triangle, another entire constellation, can you find it? Good hunting and Clear Skies!
