We’re so used to the sun’s constancy. It’s doesn’t suddenly brighten and then fade. It’s the same size day to day, even millenium to millenium. The sun is reliable, which is probably why life has had the opportunity to flourish in so much variety on our planet. Stars like the sun remain well-behaved over much of their lives because they’ve struck a delicate balance between the powerful force of gravity, which wants to crush them into tiny balls, and their tremendous energy output, which would otherwise blow them to bits.

To find the variable star Eta in Aquila the Eagle, first locate the Summer Triangle above bright Jupiter. The star to look for is Altair, the bottom apex of the triangle. Use the map below to zero in on Eta. — created with Stellarium

There are other stars however whose light is not constant. Stars that puff themselves up, flare and otherwise can’t seem to settle down. These are the variable stars and there are lots of them. Most of 9000 or so stars visible across the sky with the naked eye are steady or nearly so, but a few undergo obvious changes in brightness. One of them, which goes by the Greek alphabet letter of Eta (AY-tuh) in Aquila the Eagle, shines over us every clear night of the summer and fall.

Eta is located about one outstretched fist below Altair, on the lower side of the Eagle’s left wing. Eta, and its neighbors Delta and Iota, are dimmer but not faint, and easily visible in suburban and rural skies. — created with Stellarium

Eta is a Cepheid (SEF-ee-id) variable. Cepheids are giant and supergiant stars much larger and brighter than the sun. Changes deep within their nuclear-burning cores cause them to lose their balance, so to speak. Instead of holding steady like the sun, they pulsate or puff in and out with periods of one to 70 days. As they do, their size and surface temperature changes.  Eta, located over 1100 light years away and some 60 times the size of our sun, is one of the few Cepheids whose brightness changes are easy to follow. The time it takes to go from bright to dim and bright again — its period – is just a bit more than seven days. 

Eta is brightest tonight (August 29-30), shining nearly as bright as its neighbor Delta. By next Wednesday the 3rd, it will have shrunk and faded to near minimum brightness, and more closely resemble the fainter Iota. Check for yourself. It’s easy to compare. Once you see the changes, you’ll wonder what else’s been going on up there all these clear August nights.

The animation (at right), created by Professor Robert Buchler at University of Florida, beautifully illustrates the pulsations of stars like Eta. Notice that the star is brightest when it’s expanding most rapidly, and faintest when it’s fading fastest.  After slowly fading in the coming days, Eta will quickly brighten to maximum again on the nights of September 5 and 6. (If you can’t see this animation, please click here for a direct link.)

Cepheids, whose namesake is Delta Cephei in Cepheus the King, may sound like an obscure subject but they allow us the privilege of seeing ongoing physical changes in evolving stars right from our own backyards.

There are many types of variables but stars like Eta might be the most fascinating of all. Back in the early 20th century, scientists learned to use them to probe the vast distances between galaxies. Cepheids were responsible for awakening Earthlings to the fact that we live in a universe much larger than we ever supposed. Later this weekend, we’ll look at these curious stars in more depth when we revisit Edwin Hubble’s discovery of the first Cepheid beyond the Milky Way.