A pair of stellar “frogs” hops low across the south on November evenings. The brighter one is Fomalhaut, the “mouth” of Piscis Austrinus, the southern fish. It’s due south in mid-evening. It’s the brightest star in that part of the sky, so it’s hard to miss. In ancient Arabia, Fomalhaut was known as the “second frog.” The first frog stands well to the upper left of Fomalhaut. It’s known as Diphda, from an Arabic term for frog. But it’s also known as Deneb Kaitos — the tail of the whale — because it’s the brightest star of Cetus, the whale. It’s only about half as bright as Fomalhaut, but still easy to spot. Diphda is about 96 light-years away. That means the light we see from it tonight actually left the star around 1919. And that applies to all forms of light — not just what we see with our eyes. Diphda is much bigger than the Sun, and its surface is much cooler, so it emits much of its energy in the infrared. Much of it is absorbed by water vapor in Earth’s atmosphere. So to detect i
