Free Astronomy Magazine July-August 2023

6 JULY-AUGUST 2023 ASTRO PUBLISHING view of researchers on the current evolutionary stage of Betelgeuse and on the consequent possibility that the star could explode in an un- expectedly near future. Among the recently published works on this topic, the most dis- cussed is undoubtedly the one signed by Hideyuki Saio, of Tohoku University, Japan, and three of his colleagues at the Université de Genéve, Switzerland. In early June, the team published an article in the Monthly Notices of the Royal As- tronomy Society that essentially states that Betelgeuse could ex- plode within a few decades. Saio and colleagues came to this conclu- sion (one of many admitted by cur- rent models and knowledge) by reinterpreting the importance of Betelgeuse’s two longest periods of variability. As readers will remem- ber, this star is a semiregular pulsat- ing variable, characterized by four different periods that overlap, gen- erating a rather complex light curve. The most relevant variation is asso- ciated with a pulsation with a pe- riod of 400-420 days, during which the star expands radially and then contracts. The other three periods, considered secondary, have dura- tions of approximately 2200, 230 and 185 days. Models of red super- giants such as Betelgeuse interpret these pulsations quite well and show that periods of a few hundred days are typically due to the fundamental pulsation, while the others are con- sidered harmonics of the fundamen- tal. By accepting the 400-420 day period as the fundamental period, the models place restrictions on the maximum size that Betelgeuse can reach before contracting again. Knowing theoretically the size ex- pected in reality and then measuring the apparent one in the sky, it should be easy to obtain a reliable estimate of the distance of the star. Unfortunately, although Betelgeuse T he Hertzsprung-Russell diagram (HR diagram) is one of the most important tools in the study of stellar evolution. Developed independently in the early 1900s by Ejnar Hertzsprung and Henry Norris Russell, it plots the temperature of stars against their luminosity (the theoretical HR diagram), or the color of stars (or spectral type) against their absolute magnitude (the observational HR dia- gram, also known as a color-magnitude diagram). Red giant and red supergiant stars fall in the top-right of the chart. This tells us they are brighter than main sequence stars, but also redder and cooler. This is because they expand and cool as they reach the final stages of their lives. However, because of their large size, they remain very bright. [Liverpool John Moores University]