Prototype variable star has a previously unknown companion

The uncertainty of science: The star that gave its name to the class of variable stars called Cepheid variables — used by astronomers to measure distances to nearby galaxies — has after more than 200 years of study been suddenly found to be a binary system.

Delta Cephei, prototype of the cepheids, which has given its name to all similar variable stars, was discovered 230 years ago by the English astronomer John Goodricke. Since the early 20th century, scientists have been interested in measuring cosmic distances using a relationship between these stars’ periods of pulsation and their luminosities (intrinsic brightness), discovered by the American Henrietta Leavitt. Today, researchers from the Astronomical Observatory of UNIGE, Johns Hopkins University and the ESA show that Delta Cephei is, in fact, a double star, made up of a cepheid-type variable star and a companion that had thus far escaped detection, probably because of its low luminosity. Yet, pairs of stars, called binaries, complicate the calibration of the period-luminosity relationship, and can bias the measurement of distances. This is a surprising discovery, since Delta Cephei is one of the most studied stars, of which scientists thought they knew almost everything. [emphasis mine]

This discovery illustrates the shaky foundation of a great deal of astronomical research. The presence of a companion is an additional variable that could very significantly skew the relationship between the stars’ pulses and its luminosity, thus making the use of this data to determine distance much less reliable. This in turn could have a significant effect on the present estimate of the expansion rate of the universe, which in turn could have a significant effect on the theories of dark energy. Moreover, if the past distance estimates to many objects are wrong than what we know about those objects is far less certain.