Massive stars are the primary architects and engines of cosmic evolution. Despite their brief lifespans, they fundamentally reshape their surroundings: their fierce radiation and powerful stellar winds trigger the birth of new stars and define the appearance of entire galaxies. Within their cores, like giant cosmic forges, the chemical foundations of our world are cast: from the oxygen we breathe to the iron in our blood. Studying these giants under conditions of extremely low metallicity is key to understanding how the universe functioned during the epoch of the first stars, currently a major focus for the James Webb Space Telescope (JWST).

    Luminous Blue Variables (LBVs) hold a unique place in this picture. These are supermassive stars in a brief and highly unstable final stage of evolution. Amplitudes of their optical variability, with characteristic time scales of several years, reach 1–2 stellar magnitudes. During such variations the temperature of their radiating layers varies substantially, however, the bolometric luminosity remains about constant. Namely during this period, they lose a significant fraction of their mass, shedding it into the surrounding space through powerful winds and catastrophic eruptions. Due to the very short time a massive star spends in this phase, fewer than a hundred such stars are known. The vast majority of them are found in our Galaxy and nearby massive galaxies. Therefore, their abundance of heavy elements (“metals”) is about the solar value. These metals determine the opacity of matter to radiation emanating from the star’s center and determine the force of its pressure on the outer layers and the effectiveness of the wind.

    Modern theoretical models predict that even with a deficit of heavy elements (“metals”), massive stars can exhibit the LBV-type activity. However, verifying these models has long been challenging due to the extreme rarity of such objects in the Local Universe. The object DDO68-V1, residing in the galaxy DDO68, located 40 million light-years away, was discovered at SAO in 2008 as LBV in the beginning of its “giant eruption”. It has become a unique natural laboratory: its metallicity is 40 times lower than that of the Sun, making it the closest local analog to the stars of the early cosmic epochs.

    An eight-year monitoring program of DDO68-V1 using the main Russian telescope, the 6-meter BTA, the Zeiss-1000, and the MSU 2.5-meter telescope has confirmed these theoretical predictions and yielded an unexpected result. Following a “giant eruption” in 2009–2014, the star entered a phase of unusually high variability. The amplitude of its brightness variations reached 3–3.5 magnitudes on the timescale of a year. This exceeds the values typical of standard LBVs several times and indicates that under low-metallicity conditions, physical mechanisms of instability can manifest much more radically than previously thought. It is also possible that the very unusual variations are related to how supermassive stars recover from catastrophic eruptions.

    These results provide unique data for calibrating evolutionary models of the first stars and help astrophysicists correctly interpret feedback processes in galaxies at the dawn of time.

Fig. 1. The original V-band light curve is shown of the whole region K3 (containing the LBV DDO68-V1), which is the difference of the real magnitude relative to the minimum level.

Fig. 2. The real V-band light curve of DDO68-V1 is shown. This is obtained by subtraction of the underlying constant (minimal) brightness from the original light curve. Light variations resembling those of “S Doradus” type, but with amplitudes of up to 3–3.5 mag, are seen during the years 2015–2023.

The work was carried out as part of the SAO RAS government contract approved by the Ministry of Science and Higher Education of the Russian Federation and supported by the Ministry of Science and Higher Education of the Russian Federation.

Published:
1. Pustilnik S.A., Perepelitsyna Y.A., Vinokurov A.S., Egorova E.S, Moskvitin A.S., Goransky V.P., Burenkov A.N., Maslennikova O.A., Spiridonova O.I. Monitoring of DDO68 “Northern Ring” SF Regions in 2016-2023, 2024, Astroph. Bulletin, v.79, 593-612.
2. Pustilnik S.A., Perepelitsyna Y.A. The variability of DDO68-V1, a unique, extremely metal-poor luminous blue variable, 2025, Astronomy & Astrophysics Letters, 695, L7.