The Supergiants of Orion

The two brightest stars in Orion look very different in the sky: very orangey-red Betelgeuse as Orion’s shoulder, blue-white Rigel as his knee. But they’re more alike than you think! Both are supergiants: starting out as hot massive stars, quickly burning their hydrogen fuel and swelling up to their enormous present-day sizes.

BetelgeuseRigel
Mass15x Sun21x Sun
Size700x Sun74x Sun
Age10 Myr8 Myr
Distance700 ly~850 ly
The difference is that Betelgeuse had a bit of a head start by a few million years; it was briefly a blue supergiant star, and has cool to its red supergiant phase, now burning carbon (after depleting its helium). Rigel is following the same evolutionary path, but it still burning helium, and has yet to cool like Betelgeuse (but it’ll get there in a few million years). Both, however, share the same fate: after burning through their carbon and what little remains of heavier elements, they will go supernova, leaving either a neutron star or a black hole. By the time we’d be seeing a very red Rigel in the sky, Betelgeuse will just be a memory.
EvolutionBetelgeuseRigel
Started AsHot O StarHot B Star
Blue Supergiant8-10 Myr agoToday
Red SupergiantTodayfew Myr from now
FusingCarbonHelium
Supernova~100 kyr?few Myr

The Galactic “Anti-Center”

Whenever the Milky Way is above the horizon, there are plenty of things to discover! Stretching from Perseus and Auriga almost overhead, and all the way to the Southern horizon in Canis Major and Puppis, you don’t need elaborate equipment and careful planning to casually discover many open clusters and brighter nebulae with binoculars or a small telescope: just point in that direction and casually scan!

In contrast to the Summer Milky Way we’re now looking β€œout” and away from the Galactic Center (at the spout of the β€œTeapot” of Sagittarius); in fact the Galactic β€œAnti-Center” is in Auriga. Here it’s the outer β€œPerseus” spiral arm we see and while it seems as if we’re just peering into a much thinner section of our Galaxy, you wouldn’t know it looking at the myriad of objects in Auriga, Gemini, Monoceros, Canis Major and Puppis. Orion β€” for all its magnificent objects β€” is really in the foreground, much closer to the Sun; both are in a minor spiral arm called the β€œOrion Spur”. Still, no matter what part of our Galaxy you observe (even the outer halo with its globular clusters) - there are fantastic objects to discover and appreciate!

This Month’s Image

Bob Donahue, NBAS

The Pleiades is one of the oldest known deep-sky objects even though its stars are very young. But which star is the 7th star of the β€œSeven Sisters?” With simple imaging, the bright stars of the cluster are embedded in dust: unlike emission nebulae like M42 in Orion, the wispy structures are dust that the cluster stars are traveling through and illuminating through reflection, and not a part of the stellar nursery that spawned the cluster itself.

M 45 is important because accurate measurements of its distance (423 ly) is a fundamental rung in the β€œdistance ladder” constructed to determine distances of objects in our Galaxy and far beyond!