A Sky Without Betelgeuse?

Orion's premier red supergiant was the first star ever to have its diameter measured by interferometry, and the first to be directly imaged as a disk, by the Hubble in 1996. Only a handful of extrasolar stars can be thus observed, Betelgeuse and Antares among them. This image of Betelgeuse is a new one, reported in Sky & Telescope a couple of weeks ago. (Yes, I'm playing catch-up here.)

Betelgeuse's stellar stats are widely available but always worth repeating. At 640 light years away it is one of the nearer supergiants, with a total luminosity (mostly in the IR) about 100,000 times brighter than the Sun. The measured diameter depends on the wavelength you use - the star probably has no sharply defined surface - but comes to about 900 solar diameters, comparable to the size of the asteroid belt.

And astronomically speaking, it is not long for our sky. An artist's conception shows it boiling ominously, with bubbles the size of the orbit of Mars. Betelgeuse is blowing off an Earth mass or so every year, and - also rather ominous - its diameter seems to have decreased about 15 percent in the last 15 years.

In fact, Betelgeuse is in a pre-supernova state ... but just how pre-? I initially misread one of the researchers' linked press releases, saying that it expected to blow 'in the next few thousand to hundred thousand years' as 'in the next thousand years.' It sort of jolted me. But even the correct statement amounts, in astrophysical terms, to any time now.

Whenever Betelgeuse goes off, it will be hard to miss. No supernova has been observed in the Milky Way since 1604, and the brightest recorded one, in the year 1004, was of a star 7100 light years away - more then 10 times the distance to Betelgeuse. The supernova of 1054, that formed the Crab Nebula, was 4000 light years away. The Geminga supernova was only some 550 light years away, but it went off 300,000 years ago.

So the Betelgeuse supernova will be at least dozens of times brighter than any observed in the historical era, perhaps a hundred times brighter, nearly as bright as a full moon. Wow!

Yet afterwards, as the first commenter on the Sky & Telescope article noted, it will be rather strange not to have the familiar figure of noble Orion in the sky anymore. It is the first constellation I ever recognized, and because at the start of each season I see it rising in the southeast, on its side, I think of its figure as representing not a giant but an exotic stringed instrument, its strings running from Betelgeuse to Rigel, with the Belt stars as pegs on the soundboard. Without Betelgeuse the instrument's symmetry will be broken, the music of the spheres altered.

But if you gotta go, what a spectacular exit!


Related links: I previously wrote about the pleasures of both sky observing and, more recently, armchair astronomy.