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07 August 2019

Triangles in the Sky

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In the summer night sky we have a clear protagonist, but it’s neither a constellation, nor a planet, nor even a celestial object; it’s a triangle formed by three stars. Bearing in mind that any group of three stars form a triangle and with the naked eye we can contemplate 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?

The first thing that calls our attention is the brightness of the stars that make up 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—never better said— in astrophysics. Its brightness is considered 0, 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 pulling a little isosceles –like a pizza slice. 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 bit less than that which can encompass one’s field of vision, which ends up making it more spectacular in the sky.  And finally, this shape appears during the Northern Hemisphere’s summer from the beginning of the night on the eastern horizon, until dawn on the western horizon, which gives it the opportunistic name of the “Great Summer Triangle“.

The Great Triangle, over the horizon at night. Credit: Óscar Blanco

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, you can see the constellations of Sagittarius and Scorpio just above the southern horizon. They are so low that a tree or a building would cover them up, preventing their view; this is why we need a sky clear of objects on its 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 Scorpio. Its brightest star, Antares, stands out for its red brightness, not surprisingly since red giants are one of the largest types of stars. These weeks Antares competes in brightness with nearby Jupiter. They seem to be sizing each other up to determine the prize for the brightest star… and Jupiter wins by a landslide, because although the planet is much smaller than Antares, it is much closer to us than the giant star.

Sagittarius and Scorpio, in the center of the sky seen from Chile. Credit: Óscar Blanco

For those with binoculars on hand, this area is the ideal place to have a look around in the deepest sky. At first glance, no guide is needed—simply by searching the area it is easy to find nebulae and star clusters. Among them are the nebulae of Trifid and the Lagoon, or the cluster M11 (also called the Wild Duck Cluster) near the constellation of 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 from equatorial zones will enjoy the spectacle of these two constellations much better than the inhabitants of the northern hemisphere, since this area of the sky is located at the beginning of the high night, near the centre of the celestial vault. This provides an ideal excuse to lie down to enjoy them with a lounge chair or a blanket.

And at latitudes below the equator, star aficionados will also be able to see the most famous triangle in the sky, but looking towards the opposite (north) zone and they will have to call it the Great Winter Triangle, since between hemispheres the seasons are reversed. In the southern hemisphere, the only difference that is noticeable compared to observers from the north is that the triangle is from the right, instead of turned. And so it is that during these weeks the great triangle can inspire us to enjoy the summer or protect us from the winter… but in any case, it provides us with a magnificent excuse to go outside for a while to enjoy the night sky.

Borja Tosar


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