Free Astronomy Magazine September-October 2020

7 SEPTEMBER-OCTOBER 2020 STELLAR EVOLUTION reached the orbit of Mars. To convey its enormousness, a curious illustration from the surface of Betelgeuse was in- serted into a popular work of 1923, entitled Hutchin- son’s Splendor of the Heav- ens . It shows a 14-year-old boy on its surface firing a bullet from a rifle; behind him is a 70-year-old man who is about to be hit by the same bullet. The young and the old in this odd illustra- tion are the same person − the bullet took 56 years to circle around Betelgeuse and return to its starting point. Today we know that that paradoxical suicide could not have taken place, as Betel- geuse is much larger than what Michelson and Pease calculated. In fact, the star is approximately as large as the orbit of Jupiter, although the diameter is still not known precisely. This is due to the fact that, despite a century of measurements, the dis- tance to the star is still very uncertain. About Betelgeuse, we know for sure that it is a red super- giant (more orange than red) near the end of its existence and that it will end with an explosion of a type II super- nova (core collapse). In the modern star classification sys- tem, developed starting in the 1930s by William Morgan and Philip Keenan, Betel- geuse oscillates between the spectral classes M1 and M2Ia- Iab, betraying its nature as a variable star. This variability has been known since an- cient times and is caused by the tumultuous reshuffling V irtual Hertzsprung- Russell diagram showing Betel- geuse's position with respect to other reference stars. Below, a curious illustra- tion inserted in Hutchinson's Splendor of the Heavens , which emphasizes the extraordinary size of Betelgeuse.