Skyward for February 2021.
David H. Levy.
Orion in Winter.
As twilight deepens these evenings, Orion is just clearing the eastern horizon. Robert Frost wrote eloquently in his famous poem “The Star Splitter”
“You know Orion always comes up sideways,
Throwing a leg up over our fence of mountains.”
Whenever I see Orion rising, which is almost every night from fall to midwinter, I am reminded of how poets like Robert Frost saw the mighty hunter as it entered the sky to take command of winter. Even if you have difficulty finding some constellations, the three stars in a row that form Orion’s belt are a giveaway. And if you have a telescope, as Frost did, the view is even better. Just below the belt lies a fainter set of three stars. Surrounding the middle one is a gigantic cloud of hydrogen gas which is the Great Nebula in Orion. It is one of the richest star forming regions in our whole galaxy.
During that first winter I enjoyed watching lots of the fainter stars within the nebula change their brightness over time scales of days, hours, or in one case, minutes. According to Janet Mattei, the late director of the American Association of Variable Star Observers, these variable stars can “flicker” as they go through their carefree cycles of stellar youth.
Near the top of Orion, marking his left shoulder, is a much older, grandfather star. Named Betelgeuse, this star is at the other end of the stellar life cycle. An old, very large and massive sun, Betelgeuse varies lazily from being almost as bright as Rigel, the star marking Orion’s lower right knee, to not much brighter than Bellatrix, the star marking Orion’s right shoulder.
Last winter Betelgeuse faded more than usual, and throughout 2020 it was setting off alarms that it was about to explode as a supernova. Probably not now, though it will likely happen within the next hundred thousand years or so. In the spring Betelgeuse began to brighten again, but when I saw it rising above the eastern horizon in late August, it had faded once more. Around that same time, the Hubble Space Telescope, observing in ultraviolet light, provided data that suggested that the unusual dimming was caused by an ejection of some very high temperature gas from within the star into space.
When Betelgeuse is finally done being the star we love, its core will collapse almost instantaneously, within a few seconds. Betelgeuse will increase exponentially in brightness. It will shine as brightly as the first quarter Moon and will be easily visible in daylight for three months or more. It will be brighter than Tycho’s great exploding star of 1572, and brighter even than the brilliant supernova of 1006. As large as it is, Betelgeuse is probably not massive enough that its core will shrink to a black hole. Instead, it will probably form a new neutron star, small, dark, very dense, and cold.
Stars are people too. They age just as we do. They enjoy the carefree times of youth, go through a long middle age like our Sun, and then get strange again as they grow old. Please go out and enjoy Orion rising over the eastern horizon these evenings. It is time to settle back and enjoy this magnificent king of the winter sky. As you look, imagine how young stars like those in the nebula, and old ones like Betelgeuse, tell their beautiful story of the life cycle of distant suns.