To get a feel for the tremendous size of that star:
https://en.wikipedia.org/wiki/File:Star-sizes.jpg
"In the outer reaches of the photosphere the density is extremely low, yet the total mass of the star is believed to be no more than 20 M☉. Consequently, the average density is less than twelve parts per billion (1.119 × 10−8) that of the Sun. Such star matter is so tenuous that Betelgeuse has often been called a "red-hot vacuum"."
> astronomers have the worst conventions in the world (for largely “historical reasons”)[2]
You don't say.
[1] http://en.wikipedia.org/wiki/Solar_mass
[2] article