Stargazing: The Summer Triangle

As darkness slowly returns to our evenings, with the summer solstice now more than three weeks behind us, we are approaching a time of the year that can produce the most memorable of night skies.  The Milky Way — that dim gray lane that runs from horizon to horizon, created by the billions of stars that lie in the plane of our vast galaxy — is rising higher and higher in the eastern sky after each sunset.  Although the Milky Way itself, here in Connecticut, is at best a faintly gray appearance against a darker surrounding background, there are a myriad of nearby stars in the general direction of the Milky Way — anywhere from a few dozen to several thousand lights years distant — that are visible to the unaided eye, and pointing binoculars or a small telescope in the direction of the Milky Way will bring a never ending feast for the eyes.

But for this week, I want to focus on only three stars that bracket the river of gray — the stars that form the “Summer Triangle."  As you face east on these July evenings, find the brightest three stars in the sky, using the included map to guide you.  The triangle is longer than it is wide, and points toward the southeast horizon on mid-July evenings. 

The highest star of the three is Vega, the brightest star in the sky this time of year, the fifth brightest of all stars visible from Earth, in the small but distinctive constellation Lyra.  The rest of Lyra is formed as a parallelogram to the right and below Vega as you face east.  Vega lies just above the Milky Way as we face east.  The ancient Greeks and Persians identified the harp of the gods in Lyra — it is the harp of Orpheus, who charmed the guards of the underworld with it to allow his wife Eurydice to escape the land of the dead (and who then lost her again by disobeying the gods and looking back at her before they had reached the land of the living).  It is also the harp of Mercury (Hermes), Apollo, and even King Arthur, depending on local cultures.

Vega itself is a massive, young and hot star, about twice the mass of our Sun, generating 58 times as much energy, with a diameter roughly 2 and a half times the Sun, and an age of only 450 million years.  Vega is located 25 light years away, making it one of the closest stars to Earth.  A telescope reveals a faint companion to Vega located sufficiently close to suggest that Vega is a double star;  however, careful measurements reveal that the companion is not gravitationally attached to Vega, and is in fact hundreds of light years more distant than Vega.

Because of its brightness, proximity, and excellent positioning for Northern Hemisphere viewers, Vega has many astronomical “firsts” associated with it.  It was originally used to set “0” on the astronomical brightness (magnitude) scale, though modern redefinition of that scale results in Vega having a magnitude of 0.03.  It was the first star to be photographed, in 1850 using a daguerreotype (an earliest form of camera using a silvered copper plate as film).  It was also the first star to have its spectrum photographed, revealing lines of absorption caused by the presence of chemical elements in the star’s atmosphere.

Vega is one of the few stars whose diameter has been accurately measured.  It is found that the star is far from round due to its incredible rotational velocity, completing a rotation once every 12 hours (our Sun, in comparison, completes a rotation once every 24 days).  This results in a great distortion of Vega – its diameter around the equator is 2.8 times that of our Sun, while around its poles it is about 2.4 times as large as the Sun.

The second star in the Summer Triangle is Deneb in the constellation Cygnus, lying in the center lane of the Milky Way.  Unlike diminutive Lyra, Cygnus is a huge constellation forming a cross pattern, representing a swan with the body running roughly north to south along the Milky Way and its vast wings spreading out to the east and west.  Most ancient cultures associated this massive cross of the northern sky with a bird.  The very bright star Deneb is at the tail of the swan – the name derives from the Arabic al-Dhanab al-Dajajah, or “the Tail of the Hen”. 

While Vega is a large star, Deneb is almost unfathomably colossal.  Look back at Vega, and recall that it is 25 light years distant — a near neighbor of the Sun.  Now realize that Deneb, only slightly less brilliant than Vega, lies approximately 1500 light years distant.  At that distance, our Sun would be visible in only the largest of telescopes as an insignificant point of light among millions.  Deneb produces about 200,000 times as much energy as the Sun!  To get some concept of what this means, if our Sun were replaced by Deneb, the temperature on Pluto would be about 7 times hotter than the temperature we currently have on the surface of Mercury. 

At a mass of roughly 20 Suns, and a diameter just smaller than the diameter of the orbit of Venus,  Deneb, a young blue-white supergiant,  is destined to live a short and ultimately violent life.  Star lifetimes decrease as their masses increase — while the Sun will remain a stable yellow star for about 10 billion years, Deneb will rapidly progress to becoming an even larger red supergiant before destroying itself in a stupendous supernova, whose remains will form a black hole.  Its total lifetime may last only a few million years.

The final member of the Summer Triangle is Altair, in the constellation Aquila, the Eagle.  Altair lies below the Milky Way facing east, and is the nearest of the three stars of the triangle, at a mere 16 light years from Earth.  Altair is more sun-like than its brethren in the triangle, with an energy output only 10 times that of the Sun, and a mass and diameter around twice that of our star.  Similar to Vega,  Altair spins at an extreme speed, completing a revolution in only 9 hours.

Altair has the scientific distinction of being the first star other than the Sun to have its surface directly imaged.  In the attached false-color image, we see that its high rotational speed has flattened the poles of this star in a manner identical to what we observe on Vega.  The star is also significantly hotter at its poles than at its equator due to this change in diameter.

During the first few nights of my course in astronomy, we learn to identify the constellations Lyra, Cygnus and Aquila, using the Summer Triangle to guide us on our quest.  Once we have learned the general map of the heavens from these starting points, we can proceed to discover the astronomical treasures that lie closely guarded by the Harp, the Swan, and the Eagle.