There is a clear protagonist in the summer night sky, but it’s neither a constellation, nor a planet, nor even another kind of astronomical object. It’s just a simple triangle formed by three stars. Bearing in mind that any group of three stars forms a triangle and that with the naked eye we can observe some 3,000 stars, in theory we should be able to identify billions of triangles in the sky… So why is this one, in particular, so famous?
To discover the importance of this triangle, we embark on an exciting journey through the summer sky, which will reveal its most spectacular constellations. This is an accessible path for beginners—which will take them far beyond identifying the Big Dipper and locating the North Star—and which will end up opening a window to the deep sky, without the need to learn how to handle a telescope: all that is required is a pair of simple binoculars.
But let us return to the great triangle that marks the beginning of this astronomical journey. The first thing that strikes the eye is the brightness of the stars that form its vertices, three of the brightest stars that can be seen in the sky: Vega, Deneb and Altair, each one the brightest in its constellation (Lyra (the Lyre), Cygnus (the Swan) and Aquila (the Eagle), respectively).
These are not just pretty stars. Vega is a reference star in astrophysics. Its brightness is considered the zero point, the starting point from which to measure the brightness of all other stars. And it has also been a movie star, as it is from Vega where the alien message in the film Contact originates.
It is also the shape of the triangle that makes it easily recognisable: it is almost equilateral, (with equal sides and angles), perhaps somewhat isosceles—like a slice of pizza. If it were a right-angle triangle or scalene (with all the sides having different lengths), it would be less easy to identify and probably less famous. Its size is little less than the field of view, which makes it even more spectacular in the sky. And finally, this shape appears during the Northern Hemisphere’s summer from early evening over the eastern horizon, until dawn over the western horizon, giving it the opportunistic name of the “Summer Triangle”.
The three vertices of this colossal triangle are clearly identified against a background of fainter stars. Between Deneb and Altair are two of the most elusive constellations, but also the most beautiful: Cetus (the Dolphin) and Sagitta (the Arrow), which are small but with such curious shapes that it is worth the effort to try to identify them.
For observers with binoculars or a small telescope, between Vega and Altair we find the curious grouping of stars called “the coat hanger”. Although it is neither a constellation nor a celestial object, it is well known by amateur astronomers all over the world.
Spectacular constellations, but a little hard to spot
More difficult to see, but perhaps more spectacular, are the constellations of Sagittarius and Scorpius just above the southern horizon. They are so low in the sky that a tree or a building would cover them up, obscuring their view; this is why we need a sky free of objects on the horizon.
The shape of Sagittarius (mythological figure of a half man, half horse) also draws attention, although it is not an equine silhouette, but something more like a teapot—with its handle and spout, through which even steam seems to emerge (which would be the milky band of the Milky Way). If the constellations were named today, we would no doubt be talking about the teapot.
Next to it is the constellation Scorpius, whose brightest star, Antares, is notable for its glow. It is a red giant, one of the largest types of stars (its mass is about 12 times that of the Sun). Situated at the heart of the scorpion, its Mars-like reddish hue can be seen with the naked eye (and indeed, that is where its name comes from). What cannot be seen without a telescope is that Antares is actually not one star, but two: the red giant forms a binary system with a much smaller and much less bright bluish-white star.
For those with binoculars at hand, this area is the ideal place to take a stroll through the deepest part of the sky. At first glance, no guide is needed—simply by looking around the area it is easy to find nebulae and star clusters. These include the Trifid and Lagoon nebulae, or M11 (also called the Wild Duck Cluster) near the constellation Sagittarius. In addition, with these simple binoculars, next to the planet Jupiter you can see its four largest moons: Io, Calisto, Ganymede and Europa.
Meanwhile, in the Southern Hemisphere…
Observers in equatorial zones will enjoy the spectacle of these two constellations much better than those in the northern hemisphere, as this area of the sky is located near the centre of the celestial vault at nightfall. This provides an ideal excuse to lie down to enjoy them with a beach lounge chair or blanket.
And at latitudes below the equator, stargazers will also be able to see the most famous triangle in the sky, but looking towards the opposite zone (north), and will have to call it the Winter Triangle, since between hemispheres the seasons are reversed. In the Southern Hemisphere, the only noticeable difference compared to northern observers is that the triangle is right-side up, instead of upside down. And thus, during these weeks, the great triangle can inspire us to enjoy the summer or shelter from the winter… but, in any case, it provides us with a magnificent excuse to go outside for a while and enjoy the night sky.