All stars are mere points, we say. All but the Sun are so far away that for our eyes they are light sources with no width. That’s why they twinkle. Dust motes or ripples in the air can momentarily change them, whereas planets twinkle not, being near enough to have some angular width. (Planets’ apparent widths in arcseconds range from Venus’s more than 60 to Pluto’s 0.1.)
But it’s not quite true of one star: Betelgeuse. This bully red supergiant has a combination of size and distance that makes it for us the star with an angular width just large enough to measure, and this was attempted as early as 1920. It wasn’t easy, and the figures given have ranged from about 0.04 to 0.07 arcseconds. A much smaller dot even than Pluto; yet wider than the dots of all other stars in the sky, including thousands that are nearer to us.
Statistics about Betelgeuse are wobbly because it is in the variable stage that massive stars reach as they swell in the last million years of their relatively short lives. It varies irregularly and more widely than any others of the twenty brightest stars: between magnitudes 0.2 and 1.2, which means a factor of 2.5 in output of light. It pulses in and out like a huge pinkish bubble, and has expelled an irregular cloud of matter, which increases the difficulty of measuring it.
Its distance may be around 500 light-years, but estimates have ranged between 180 and 815, and these combined with the angular sizes could give a range for its real size from between 1 and 9 astronomical units (the Sun-Earth distance). In other words, if Betelgeuse were our Sun, we could be at or just inside its hazy surface, or its surface could be beyond Jupiter and almost to Saturn!
Yet it has probably only 4 or 5 times as much mass as the Sun. So the huge pulsing pinkish bubble has low density and, though putting out a huge total of light, has a relatively low surface brightness.
There it is, up in the middle of your winter evening sky – the pink navel of the winter sky. Betelgeuse is the center of an asterism we call the Winter Hexagon, the six corners around it being Rigel, Aldebaran, Capella, Pollux, Procyon, and Sirius, all in different constellations. Except that Betelgeuse is slightly dimmer than Rigel, the more distant blue supergiant in its own constellation.
I find this blog post becoming longer than I expected and I haven’t even got to beetroot juice and the further indelicate things I meant to mention, so – to be continued in our next.