The beautiful stars of the Summer Triangle set against the Milky Way (Image credit: NASA, ESA, A. Fujii)
Around this time of year, go outside after 9pm on a clear night and look at the sky. Even if it is not quite dark a bright star will be visible in the south. This is Vega, the third brightest star in our sky (only Sirius and Arcturus are brighter). If you look a little more you will see two more bright stars, these are Deneb and Altair. Together with Vega they form a large triangle with the point downwards. This has been known as the Summer Triangle for a century or more. The three stars are members of three different constellations and as it gets darker you will be able to see more of their fellow stars. Deneb is the tail of Cygnus (the Swan), Altair is in Aquila (the Eagle) and Vega is in Lyra (the Lyre). The Summer Triangle is not actually a constellation itself, just a nice handy grouping of stars, astronomers call these asterisms.
What about the stars themselves? Well, all three are brilliant white A class stars, to our eyes Vega is the brightest, next brightest is Altair and Deneb is the faintest. However, although it looks the dimmest, Deneb is actually by far and away the biggest and brightest of the three stars. In fact, it is one of the brightest stars in the night sky. So why does it look relatively dim compared to Altair and Vega? Deneb is thousands of light years from us, much, much further away than the other two stars which are very close to us in astronomical terms. Altair is located just 17 light years away from Earth, so it is one of the closest stars visible to the naked eye. Vega is another near neighbour of ours, a mere 25.4 light years from the Sun.
Altair is just a little bigger than the Sun but is spinning very, very fast. The star rotates once around its axis every 6.5 hours (our Sun takes more than 25 days). As a result Altair’s shape is extremely flattened, it you were close enough you would see that is not a sphere like the Sun but shaped more like a thick discus. If its rotation rate was much faster it would have been pulled apart.
Vega is twice as big as our Sun and about fifty times as bright, it spins very quickly too, but not as fast as Altair. Vega is very interesting to astronomers, as in 1983 scientists using a satellite called IRAS unexpectedly discovered Vega is surrounded by a shell of dust and ice bigger than our Solar System. Nobody is sure why this material is there. Some scientists suggest that it is evidence for huge comets orbiting the star, others say it is material forming into new planets or even debris left over from colliding planets!
The stars of the Summer Triangle are easy to see and individually fascinating. Why not point them out to your friends before exploring the other wonders of the summer sky?
