Measuring the distances of distant objects in space is a difficult task for astronomers. Even determining the distance to our neighbors in the universe (the Small and Large Magellanic Clouds) poses some difficulties. However, scientists are now starting to find tools to measure these distances. One such tool is Cepheid variables, which are stars that change their brightness in a pattern. However, little is currently known about the physical properties of Cepheids, making their use as distance markers difficult. It would be much easier to study their properties with data from Cepheid binaries, but until recently only one Cepheid pair was known.

In a new paper, researchers from Europe, the United States and Chile present measurement results of nine more pairs of Cepheids. This is enough to start understanding the features of these valuable distance markers.

Like traditional binary stars, Cepheid binary systems form when two stars orbit each other. If the brightness changes of two stars occur at different rates and phases, the properties of these systems can differ significantly. Figuring out which star is moving in which direction, both relative to us and to each other, creates some difficulties. Observing these variables requires long follow-up periods, and that’s exactly what the new paper offers.

Researchers studied nine candidates for binary Cepheid systems, which could not be confirmed due to the difficulty of distinguishing these two stars. They obtained data from the database of the Optical Gravity Lens Experiment (OGLE), which has been run by the University of Warsaw for more than 30 years. Scientists have confirmed the existence of two separate stars in each of the proposed binary systems.

Studying the Small and Large Magellanic Clouds and these nine binary systems in the Milky Way is the first step. The closest of these is located in the Milky Way: only 11 kiloparsecs (about 3000 light-years).

A more detailed study of Cepheids may shed light on their origin, composition and life cycle. The authors of the article state that this is part of a long-term project. The same scientists were part of the team that first confirmed the existence of the binary Cepheid system in 2014. The OGLE project continues to collect new data and research is ongoing that may eventually lead to the discovery of even more Cepheid binaries.