The binary star BD+20 5391 has proven to be an extremely rare case of two red giants of almost identical mass evolving side by side. A new study led by a team from the University of Potsdam, in collaboration with experts from the ASU Stellar Department, provides a detailed look at their physical properties, orbit, and future evolution. The results suggest that matter will soon begin to spill out of both giants into their surroundings, which could lead either to their merger or to the formation of an extremely close pair of white dwarfs. In any case, it will be a unique laboratory case of the evolution of binary stars in the advanced stages of their lives.
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Direct observation of such a system is extremely difficult. That is why the BD+20 5391 system, discovered during research on red giants using ASU’s two-meter Perko telescope, attracted attention. A study accepted for publication in the journal Astronomy & Astrophysics presents a detailed analysis of this unique case and suggests that BD+20 5391 may be a model example of the double evolution of red giants—a phenomenon that we know more from theoretical models than from real data.
The binary star BD+20 5391 consists of two stars of spectral class K, i.e., cooler red giants located about 500 parsecs from Earth. It is a so-called double spectroscopic binary star – both stars can be distinguished by the fact that the lines of each component in the spectrum from their system shift alternately due to the Doppler effect. Thanks to these measurements, it was possible to derive the exact orbit and mass of both components.
The observations took place from 2020 to 2025 at the ASU Observatory in Ondřejov, and were supplemented by a high-dispersion spectrum obtained at the Mercator Telescope at the La Palma Observatory. A total of 15 spectra were obtained, the initial processing of which did not deviate from standard practice in the field. This resulted in a set of very accurate measurements of the radial velocities of both stars as a function of time.
The observations took place from 2020 to 2025 at the ASU Observatory in Ondřejov, supplemented by a high-dispersion spectrum obtained at the Mercator Telescope at the La Palma Observatory. A total of 15 spectra were obtained, the initial processing of which did not deviate from standard practice in the field. This resulted in a set of very accurate measurements of the radial velocities of both stars as a function of time.
More: ASU website (in Czech)
Image description
Development diagrams of the components of the binary star BD+20 5391. The first component is marked in blue, the second in red. The squares indicate values derived from modeling the total spectrum of both stars, thus corresponding to their current position. The triangles indicate where the Roche lobe will fill up for each component and matter will begin to overflow onto the other component. It is very clear that the evolutionary tracks and both highlighted moments are very similar for both components. The shading indicates the error interval.
Informations and contacts
- source: Na čem pracujeme: Vzácná dvojhvězda obrů a exotické typy horkých hvězd (M. Švanda, popular article in Czech)
- M. Kurpas, M. Dorsch, S. Geier, B. Kubátová a kol., The twin red giant branch system BD+20 5391 A case study of low-mass double-core evolution, Astronomy & Astrophysics in press, arXiv:2509.26132
- Dr. Brankica Kubátová, brankica.kubatova@asu.cas.cz
