Watching a star physically change in a matter of a week is something you’d think impossible to see. On the contrary. The star Eta Aquilae (AY-tuh AK-will-uh), located just down the block from the bright star Altair in the Summer Triangle, brightens up and then fades into the background every 7.17 days. And it does this with the precision of a fine watch.
I’ve kept my eye on it starting last Wednesday night, when it reached it greatest brightness of magnitude 3.5 (about a level fainter than the stars of the Big Dipper). By the weekend it had faded noticeably and finally bottomed out this past Monday and Tuesday at 4.3 or nearly a magnitude fainter. Last night the star quickly shot back up, and tonight will be shining again at its brightest.
Eta is a yellow-white supergiant that shines 3400 times more brightly than the sun with a diameter 65 times larger. Too bad it wasn’t closer, or it would be one of the brightest stars in the sky. Its distance of 1170 years tames its light, so Eta looks like an ordinary star. Eta is a Cepheid (SEF-ee-id) variable star, named after the prototype Delta Cephei in the constellation Cepheus the King.
Cepheids are unstable stars that pulsate in size, temperature and brightness. They shrink and expand right before our eyes! Naturally, because of the distance, we can’t see the star’s outer surface puffing out and sinking back in, but we know it’s happening by watching the star’s light and color change. Eta is brightest when it’s expanding the fastest and dimmest when it’s contracting the fastest. All this wonderfully visible instability has to do with the extremely hot temperatures in the star’s core. Most stars we see in the sky burn hydrogen as fuel to create energy; a Cepheid takes it to the next level by burning helium after it’s used up its hydrogen. This causes the star’s outer surface to rapidly expand until it overshoots a stable size, then shrink back – but a bit too far – which leads to another phase of expansion. In a sense, the star is trying to achieve a stable balance like the sun, but it can’t because of instabilities in its core due to helium burning.
Eta Aquilae was first recognized by the sharp-eyed English amateur astronomer Edward Pigott in 1784. It’s one of the brightest Cepheids known and a perfect one for beginning sky watchers to get acquainted with the wonders of variable stars. Tonight, watch for Eta to shine its brightest and then very slowly fade over the next several nights. By next Monday and Tuesday, it will be dimmest, then quickly return to maximum light Wednesday and Thursday. Amazing!