The “Summer” Triangle

Summer skies are both great and terrible for stargazing. It’s the time of year when you can go out to feast your eyes upon the night sky without having to bundle yourself up so you feel like the Stay-Puft Marshmallow Man, but it’s also the time of year when the late sunsets mean you’ll have to wait to get your gazing going and also the hot, humid (at least here in Massachusetts) air makes the skies dance and makes haze an issue.

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So I figured a good topic for this post is a pattern that, despite its name, is visible in both the warm summer and cool autumn nights, the Summer Triangle. This trio of stars are not a constellation, but are the brightest stars of three different constellations: Vega in Lyra, Altair in Aquila, and Deneb in Cygnus. 

These stars are so bright that even if you’re in the middle of the city and can’t see the rest of their constellations, you can still see the Triangle itself! So let’s get to know this area of the sky which should definitely be on your search list for your next summer—or fall—sky tour. We’ll start with the stars themselves.

Vega

Vega is the brightest star in the Triangle, the brightest star in the Constellation Lyra the Harp, and one of the brightest stars in Earth’s sky period. It’s fifth overall for brightness among the visible stars, and second if you look only at Northern Hemisphere stars (Arcturus is first, if you’re curious).

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That’s because it is both close and bright. It’s a mere 25 light years from the Sun and is an A-type star. This is not the brightest classes of stars (that would be the O and B-types and if you’re noticing this classification system doesn’t make a lot of intrinsic sense you’re not wrong) but it’s a big, healthy blue star over twice the mass and radius of the Sun. 

Vega also holds the interesting distinction of being on Earth’s circle of axial precession. Basically, Earth wobbles like a top over thousands of years, meaning that its poles point towards different parts of space at different times. At the moment the north pole happens to be pointing almost directly at the star Polaris, so we have a North Star (though no visible southern one). At other points in the circle it’s pointed at nothing in particular. About 13,000 years ago the north pole was pointed at Vega, and Vega was the North Star. It will be so again in another 13,000 years.

As the easternmost of the three Triangle stars Vega is the first to rise and set. It is part of the relatively compact constellation Lyra, a shape which is supposed to represent a lyre, and which, if you ask me, does not look particularly harp-like (but then I’m also not especially musical).

Altair

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Altair is the second brightest of the Triangle stars and is the brightest star in the constellation Aquila the Eagle. It is also the only member of the Triangle to share a name with a video game character, as I discovered when pointing it out to a friend one summer evening only to have them say “Oh, Altaïr! Like in ‘Assassin’s Creed’!” Turns out they both have the same origin, the Arabic word for eagle.

It is also among the brightest stars in the sky, ranked at number 12. That’s not terribly surprising because Altair is actually closer to us than Vega, less than 17 light years away. That makes it one of the closest stars that you can see with your naked eye. It’s not as bright to our eyes as Vega because, although it is also an A-type star, each star classification type actually covers a range of stars.

Vega, for instance, is an A0, the hottest and brightest an A star can be without moving up a type. Altair is an A7, at the fainter and cooler end of things that can still be called A-types (I’ve definitely convinced myself to do a post on stellar classification, it’s much more bizarre than you’d think and it has a fascinating history).

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Altair has a little less than twice the Sun’s mass and its radius…well that’s actually a more complicated story. Altair spins really, really fast, which makes it a squished sphere. At its equator the radius is twice that of the Sun, but at its poles it’s only 1.5 that of the Sun. Do you have any concept of how fast something that big has to spin to squish itself that much? I didn’t, not until I ran some numbers!

Altair is calculated to have an equatorial rotational speed of 286 km/s (639,764 mph). By my calculations, that means it takes less than ten hours to spin once! That is insane. We don’t even have a planet in our solar system that spins that fast! Just as a point of comparison the Sun takes nearly a month to rotate once and has an equatorial rotational speed of 1.9 km/s (4,400 mph)! Altair is spinning so quickly that if it spun much faster it would actually spin itself apart! 

While the implications of that sink in, let’s talk a little bit about the constellation it’s in. Altair is the brightest star in Aquila the Eagle, which looks something like a kite whose tail comes off its short axis rather than its long axis. As constellation shapes go, seeing this one as a large bird with its wings outstretched doesn’t seem that far-fetched. It certainly looks more like an eagle than Lyra looks like a harp.

Deneb

Honestly I started this post mostly because I wanted to talk about Deneb, and then got distracted by Altair’s weird spinning. It’s the brightest star in the constellation Cygnus the Swan, and the name Deneb translates as “tail”. Appropriate as it is the swan’s tail.

Deneb is also a marvel of a star, though it looks very average to the eye. But the next time you go stargazing, I want you to look at all the stars you can see. In fact, look at every naked eye star you can see anywhere at anytime. None of them are as bright as Deneb.

Deneb is so bright that we don’t know how far away from us it is! You will frequently see two distances listed, either roughly 1,500 light years or roughly 2,600 light years. Two different studies using data from the Hipparcos star-mapping satellite produced these two different numbers. Theoretically the departed Gaia telescope could have clarified things—except Gaia was never intended to observe something as bright as Deneb, and images of it require a lot of special processing! The later figure of 1,500 light years is the more widely accepted value today. 

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One way to measure how bright a star is uses luminosity. This is a measurement of how much energy an object radiates in a given time measured in watts. The brightest LED light bulbs have a luminosity of about 20 watts. The Sun has a luminosity of 3.8×10²⁶ watts. Altair’s luminosity is over ten times the Sun’s, and Vega’s is nearly 50 times the Sun’s. Deneb’s luminosity (if it is, in fact, about 1,500 light years away) is at least 55,000 times the Sun’s! That is, to put it extremely mildly, really honkin’ bright!

Deneb is the farthest west of the three Triangle stars, ever so slightly more westward than Altair, so it’s the last of the three to rise. Its constellation, the Swan, looks more like what it is supposed to be than most other constellations do, with a long central line to represent the swan’s body and a longer crooked line crossing it about 75% of the way towards the tail to be the outstretched wings. The central part of this shape is sometimes called the Northern Cross.

The Triangle

While these three stars and their constellations make up the Summer Triangle, it does have other interesting features. The Milky Way runs directly through it, with Vega on one side of it, Altair on the other, and Deneb smack dab in the middle. The faint constellation of Sagitta and Vulpecula, the Arrow and the Little Fox, are also in the middle of these three stars. The slightly easier to spot Delphinus, the Dolphin, is just outside of the line between Deneb and Altair.

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For deep sky objects that you need a telescope to spot, there are a pair of great planetary nebulae, the leftovers of Sun-like stars, in the Triangle’s midst. M27, the Dumbbell Nebula, is part of Vulpecula and is right in the middle of the Triangle. And M57, just called the Ring Nebula for its extremely well-defined shape, is in Lyra, right between the Harp’s bottommost stars. 

I also have to give a shoutout to Albireo, my favorite binary star, which is the Swan’s eye. You can see Albireo as a decently bright star with just your eye, but look through a telescope and you’ll see that not only is it two stars, but that they’re two stars of very different colors! There’s a notably yellow and a notably blue star.

And you can see all of this from the northern hemisphere for a huge chunk of the year, despite its seasonal name. By early June Deneb is clearing the eastern horizon at sunset, and Vega isn’t lost to the western horizon before sunset until December. So don’t worry…even once the hot, hazy days of summer have passed, you’ll still have plenty of opportunities to observe and appreciate this shining trio and everything around them!