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Exploring the Summer Triangle in the Night Sky

Updated on April 11, 2017
The Summer Triangle
The Summer Triangle

Out On The Lawn

In 1933 W H Auden wrote a poem (“A Summer Night”) that begins with the lines:

Out on the lawn I lie in bed

Vega conspicuous overhead

In the windless night of June …

Leaving aside the merits of the poem – and there are many – nobody could fault Auden’s astronomical knowledge. On a clear night in June, if you lie looking up at the stars on an English lawn in June you will hardly fail to be aware of the prominent star Vega, which is the fifth brightest star in the entire sky.

Vega forms one of the extremities of what is known as the Summer Triangle, the other points being Altair and Deneb. The Triangle is what is termed an “asterism”, by which is meant an arrangement of stars other than an officially recognised constellation. Altair forms the lowest point of the triangle, with Vega to the top right and Deneb at the top left.

All three stars are also members of constellations: Vega is in Lyra (The Lyre), Deneb in Cygnus (The Swan) and Altair in Aquila (The Eagle). Each named star is the brightest in its constellation.

An Optical Illusion

It is easy to imagine that the constellations and asterisms seen with the naked eye represent actual associations of stars that are relatively close to each other. However, this is rarely the case, because what we are seeing are stars along certain lines of sight when viewed from Earth. A star can be many times further away from us than its apparent neighbor, and the Summer Triangle provides an excellent example of this fact.

The closest of the three stars to Earth is Altair. It is 16.7 light years away, which means that we are seeing it as it was 16.7 years ago. Vega is 25 light years away but appears to be somewhat brighter than Altair. That is because Vega is considerably more luminous than Altair, which is a main sequence dwarf star – as is our own Sun. Altair is 11 times more luminous than the Sun, but Vega is 52 times more luminous and therefore appears brighter than Altair despite being considerably further away.

Even more striking is the case of Deneb. It appears to be three times fainter than Vega, but that is because it is nowhere near as close. It has been estimated that it could be between 1,550 and 2,600 light years away, and would be invisible to the naked eye were it the same sort of star as Vega or Altair. However, Deneb is a white supergiant star with a diameter 200 times greater than that of our Sun and it is 200,000 times more luminous. Were Deneb to be at the same distance from us as is Vega, it would appear so bright that it would cast shadows at night and be visible in daylight!

The Summer Triangle and the Milky Way
The Summer Triangle and the Milky Way | Source

The Milky Way

If the sky is dark and the Moon not shining, a viewer of the Summer Triangle will not fail to be impressed by the sweep of the Milky Way crossing the Triangle between Vega and Altair. It is even more impressive when viewed through a telescope.

This “river” is the combined light of the many millions of stars that form part of our galaxy. The tilt of Planet Earth means that viewers in the Northern Hemisphere can only look “outwards” whereas in the Southern Hemisphere you would look “inwards” towards the heart of the galaxy. That is not a great limitation as far as northern viewers are concerned, because our Sun lies in one of the outer swirls of the galaxy (the Orion Arm) and there is plenty to see that is in the same swirl as ourselves and in swirls even further out, such as the Perseus and Cygnus Arms.

Alberio

This star is just to the left of a line drawn between Vega and Altair and about halfway along that line. It can be seen with the naked eye, but even a modest telescope will reveal it to be a double star with strongly contrasting components in terms of color – Beta Cygni A is amber and Beta Cygni B is blue-green. It has been suggested that Beta Cygni A is itself a double star, making Alberio a triple-star system.

However, the jury is still out over whether Alberio represents a true double/triple, with the components orbiting a common centre of gravity, or whether this is an optical double with Beta Cygni A and Beta Cygni B merely being seen along very close lines of sight.

Epsilon Lyrae

If there are questions about the composition of Alberio, there are none about Epsilon Lyrae, which lies quite close (visually) to Vega. This is a genuine “double double”, with the two main components also being doubles. You would need very good eyesight – or modest binoculars – to resolve the main division, but something much stronger in terms of telescopic power to see the system in all its glory – around 200x magnification in a four-inch aperture telescope should do the trick!

The impression that these stars are sitting on top of each other is easy to gain but somewhat misleading. The Epsilon Lyrae system (which probably contains more than the four stars indicated above) is 162 light years away, which is 6.5 times further away than Vega. The fact that it is possible to resolve the components at all using standard ground-based equipment must mean that they are at some distance from each other. Each of the two doubles are around 120 AUs (astronomical units) apart. Given that an AU is the distance from Earth to the Sun, that distance takes you considerably beyond the orbit of any known object in the solar system – the most remote object discovered to date is at 103 AUs and Pluto is almost cheek by jowl with the Sun at an average distance of 39.5 AUs!

The distance between the two main pairs in Epsilon Lyrae is on a completely different scale, at 10,500 AUs. When seen in the context of its own star system that sounds like a vast distance; however, when one considers that the distance to the Sun’s nearest star neighbor, Proxima Centauri, is 268,000 AUs, the Epsilon Lyrae stars sound to be almost touching each other!

Nebulas M57 and M27

Two impressive planetary nebulas can be seen in the Summer Triangle. A nebula looks like a fuzzy star when seen through low-resolution equipment, but a better telescope will reveal its true nature, namely the expelled outer layers of an ancient red giant star that reached the end of its life as a giant and continued as a white dwarf.

M57, which is known as the Ring Nebula, is about halfway between Vega and Alberio, slightly to the right of an imaginary line between them. A three-inch aperture or larger telescope will reveal the dramatic colors of what has been described as a “celestial smoke ring”. M57 is 2,300 light years away from us.

Alberio lies halfway between M57 and M27, which is known as the Dumbbell Nebula. It is brighter and larger than M57 and closer to us at 1.360 light years. It is easier to spot than M57 and was in fact the first planetary nebula to be identified. It has been calculated that the original red giant star threw off its outer layers about 14,500 years ago, leaving its core behind as a white dwarf star that is the largest such star discovered to date.

Time spent with a moderately powerful telescope pointed at the Summer Triangle will be well rewarded!

Ring Nebula (M57)
Ring Nebula (M57) | Source
Dumbbell Nebula (M27)
Dumbbell Nebula (M27) | Source

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