Free Astronomy Magazine July-August 2023

7 JULY-AUGUST 2023 ASTRO PUBLISHING U ltraviolet image of Betelgeuse taken by the Hubble Space Tele- scope. The bright spot in the image, one of the so-called hot spots, should be one of the poles of the star. [NASA/ESA] J ust how big is Betelgeuse in com- parison to other stars? This video shows the scales of known stars in our Milky Way galaxy and shows you just how big Betelgeuse really is. This short video was taken from the planetarium show, The Sun — Our Living Star. [ESO/L. Calçada/M. Kornmesser] subtends an angle of 40-50 milliarc- seconds, the complex variability of this star makes it impossible to calcu- late precisely the distance and a whole series of physical properties (absolute luminosity, radius, current mass, initial mass, internal rotational state and associated composition). If these properties were known, we could precisely match observations, data and models, defining the cur- rent evolutionary stage of Betel- geuse and therefore the expected times for the final event. Since it is not possible to unequivocally charac- terize the star, the models provide several possible scenarios, with the result that no researcher knows ex- actly what type of reaction is cur- rently taking place in the nucleus of Betelgeuse. The only certainty is that the phase of fusion of hydro- gen into helium ended some time ago and that the phase of helium into carbon began after the con- traction of the nucleus that, follow- ing the depletion of hydrogen, raised the star’s temperature (with consequent expansion of the outer layers of the star). However, it is far from certain whether the fusion of helium into carbon inside the nu- cleus is still ongoing, or if the he- lium has also run out and conditions have been reached to trigger the fusion of carbon into heavier ele- ments following a further contrac- tion. Unfortunately, direct observa- tion of Betelgeuse does not high- light anything that happens in the depths because, contrary to what happens in Sun-like stars, the outer layers of the red supergiants are not