The Summer Triangle. Header Image Credit: Jim Thomas, GPL, via Wikimedia Commons
I was surprised to find a clear night When I went outside to find my cat. I took the chance to gawp at myself with a few summer favourites – Cygnus, Lyre, Hercules and a try at spotting Aquila (Altair and Tarazad are always clear but I rarely see anything else). I collided with my cat when I shuffled to the back part of the garden to see what else was visible (to the west), so I trundled back inside with her and set to reading.
Vega
I thought I’d look up Vega not just because it’s so prominent, but from memory it was one of the first stars to have a planet discovered by it. (Now I come to think of it, doesn’t Vega feature in Carl Sagan’s Contact?). When I was younger I mistakenly took it for the wonkey “foot of the cross” of Cygnus (“the Northern Cross”), especially as Lyra is pretty faint (albeit home of the spectacular ring planetary nebula!).
My main take home point from wikipedia is that Vega rotates so rapidly (about once every 16.3 hours), that like Jupiter and Saturn, Vega takes on an oblate spheroidal shape. This leads to large temperature and gravity variations over its surface. The pole of rotation is roughly aligned with the Earth’s point of view to about 5 degrees.
Vega is the brightest star which lies close to the Earth’s circle of precession; it will be the new pole star in years to come. It’s twice as massive as the sun so burns more quickly; at ‘only’ 25 light years away it’s apparent luminosity is 57 times that of the sun (40 x luminous bolometrically).
Vega is one of the 3 stars in the “Summer Triangle” I’ve never heard of that!! The other stars in it are Deneb in Cygnus and Altair in Aquila. I thought I’d look them up too…
Deneb
According to wikipedia, Deneb is a blue supergiant star. (Rigel in Orion is another example of a blue supergiant). It’s distance to Earth is not well known, but estimates place it around 2500 light years away. The distance uncertainty makes Deneb’s luminosity difficult to determine, but estimates place it around 200,000 times that of the Sun!
Altair
Another spinner! Altair rotates in under 8 hours (wikipedia) and like Vega for the same reason, has an oblate shape. The speed at the equator (nearly 300km/s) is close to the star’s breakup speed of 400km/s). (cf the Sun’s rotational speed is 25 days) .
Altair is about twice the mass of the sun, and 11 times as luminous.
When I came inside I felt guilty for not going back outside and taking advantage of the clear skies. But the guilt didn’t last long. It’s inspiring to look up and take it all in – and it’s also good to know what I’m looking at!
