Astronomers have discovered a new explanation for the eclipses of red supergiants.

In the autumn of 2019, one of the brightest stars in the constellation Orion — Betelgeuse — significantly dimmed. The dimming lasted until the first half of 2020, and astronomers proposed two hypotheses to explain the star's brightness decrease of 35 percent: cooling of part of the supergiant's surface and a cloud of dust expelled due to mass loss.

In particular, a study published in the journal Nature focused primarily on the scenario of a dust cloud that formed close to the surface. Conversely, another article states that a temporary decrease in Betelgeuse's brightness could be attributed to "cold" (but not necessarily dusty) gas. In short, scientists are still exploring the possibilities of dimming events characteristic of such stars, as they are unstable and at the final stage of their life cycle.

Recently, a team of astronomers led by Bernd Freytag from Uppsala University (Sweden) presented the results of global 3D modeling, confirming the role of cold molecular gas and convection processes in the formation of temporary dimming events. The text of the scientific paper is published in the journal Astronomy & Astrophysics.

After considering several scenarios, including cooling of the star's surface due to heat loss and abundant radiation, the formation of bubbling "bubbles" due to convection, and the creation of a "haze" of molecular gas, researchers concluded that for stars of AGB type, the "pulsation" mode more frequently produces "cocoons" of gas and dust that cover nearly the entire photosphere of the star. Such dimming events occur periodically and appear spectacular from the outside.

For more massive supergiants (like Betelgeuse), brightness fluctuations are less regular, making dimming events more episodic; however, they can be very noticeable under certain combinations of convection and pulsations.

Notably, in all cases, gas can cool rapidly, leading to the formation of a large number of molecules (TiO, H₂O, and others), which significantly increase the opacity of the gas (its ability to absorb light). This is why these "clouds" appear as dark spots on the stellar disk.

Thus, astronomers concluded that sufficiently cool gas may accumulate in the upper layers of the atmospheres of red supergiants, blocking light from the depths of the photosphere and forming dark regions. Further observations combined with computer modeling will help scientists learn more about the nature of temporary dimming events and the ability of such stars to "hide" part of their luminous layer.