One of the brighter stars in Cepheus, the king, is between 850 and 900 light-years from Earth. Knowing that range is especially important because the star is like a mile-marker. Its distance helps astronomers measure the scale of the entire universe.

Delta Cephei is high in the northern sky as night falls. It’s above the figure of Cepheus, which looks like a child’s drawing of a house.

Delta Cephei is at the end of its “normal” lifetime, so it’s going through some changes. They make the star unstable, so it pulses in and out like a beating heart. Each “beat” takes more than five days. And during that time, Delta Cephei gets noticeably brighter and fainter.

So do many other stars — Delta Ceph is the prototype of an entire class of stars, called Cepheid variables. And in 1908, Henrietta Swan Leavitt found that the longer it took for such a star to “pulse,” the brighter the star was.

By measuring the distances to a few of these stars, astronomers could then determine the true brightness of any of them. So by measuring how bright a Cepheid looks, they can calculate its distance. And Cepheids are bright, so they’re visible even in nearby galaxies. That allows astronomers to measure how far away those galaxies are.

And that acts as a calibration for ways to measure the distances to galaxies that are even farther — all the way to the edge of the visible universe. That’s all made possible by a few stellar mile markers — like Delta Cephei.

Script by Damond Benningfield