Cepheid variables are a type of star whose brightness fluctuates over time. They played a major part in the discovery of the Universe's true scale, since the time it takes a Cepheid to go through its cycle of bright-and-dim is related to its intrinsic luminosity, the average brightness you'd measure if you examined the star from a standard reference distance away. If you know how bright a star would be at, say, ten parsecs away, and you know how bright it appears in your telescope, then you can compare those intensities to figure out how far away the star has to be to appear the strength it does. Thus it was found that Andromeda is a separate galaxy far outside our own, not just a patch of nearby gas, and the Milky Way shrank to a sand grain, with ourselves a subatomic particle inside it.

Recently, four scientists have suggested that a sufficiently advanced civilization could use Cepheid variables as a method of very-long-range communication. In an arXiv preprint, Learned, Kudritzki, Pakvasa and Zee propose that a jolt of extra energy could influence a Cepheid's oscillation, provoking an outburst earlier than normal. A Cepheid builds up ionized helium gas in its outer layers, and this ionized gas blocks the starlight radiating out from the stellar core, making the star go dim. Eventually, the Cepheid's atmosphere expands and the helium ions pick up electrons, neutralizing themselves and making the atmosphere go transparent. Thus, the star goes through a "sawtooth" oscillation, slowly dimming before quickly brightening and then gradually dimming again. This curve, measured for the star δ Cephei (the prototype after which the class is named) shows the pattern:

(Image shamelessly filched from HyperPhysics.)

So, our hypothetical Sufficiently Advanced Aliens could send a beam of neutrinos into a handy Cepheid variable. Neutrinos pass through matter remarkably well, so they could easily reach the star's core; however, they do have a chance of interacting with atoms they pass through, so the beam will deposit a portion of its energy within the star, and pumping extra energy into an unstable system is always a recipe for fun.

Unfortunately, the kind of effect such perturbations might have on Cepheid oscillations wouldn't be immediately obvious. Learned, Kudritzki, Pakvasa and Zee argue that the normal Fourier transforms and whatnot through which variable-star observations are traditionally stuffed would miss the tell-tale effects of Cepheid Streaming Video modulation, and they propose another technique (looking at phase residuals) which might let us observe the green-skinned space teenagers singing into webcams on Andromeda.

John G. Learned, R-P. Kudritzki, Sandip Pakvasa, A. Zee, "The Cepheid Galactic Internet" (arXiv:0809.0339).

We propose that a sufficiently advanced civilization may employ Cepheid variable stars as beacons to transmit all-call information throughout the galaxy and beyond. One can construct many scenarios wherein it would be desirable for such a civilization of star ticklers to transmit data to anyone else within viewing range. The beauty of employing Cepheids is that these stars can be seen from afar (we monitor them out through the Virgo cluster), and any developing technological society would seem to be likely to closely observe them as distance markers. Records exist of Cepheids for well over one hundred years. We propose that these (and other regularly variable types of stars) be searched for signs of phase modulation (in the regime of short pulse duration) and patterns, which could be indicative of intentional signaling.