The stars of the Summer Triangle look a lot alike. Vega, Deneb, and Altair are among the 20 brightest stars in the night sky, and they all shine almost pure white. Vega and Altair really are alike. But except for the way it looks to our eyes, Deneb is nothing like the other two.

The triangle is high in the eastern sky at nightfall. It’s easy to see even through the glare of the almost-full Moon. Vega is the highest and brightest of the three stars. Deneb stands to the lower left of Vega, with Altair farther to the lower right.

Vega and Altair are both about twice the size and mass of the Sun, and a good bit brighter. And both of them spin rapidly – so fast that they bulge outward at the equator. Vega is farther along in its evolution than Altair is. Because of their mass, both stars will spend about a billion years in the “prime” phase of life. That’s compared to about 10 billion years for the Sun.

Deneb is a supergiant – one of the bigger and heavier stars in the galaxy. Some of its details are unclear because its distance is uncertain. It could be about 1500 light-years away, or about 2600. Either way, we know that it’s about 20 times the mass of the Sun, and up to 200 thousand times the Sun’s brightness. It’s no more than 10 million years old, with almost no time left. Before long, it’ll explode as a supernova, then fade away – and the Summer Triangle will disappear.

Script by Damond Benningfield