Anthony Barreiro wrote:
Sigma Ori appears less bright that the three brightest stars of Orion's belt simply because it is further away. In reality it's hella bright and going to give future astronomers a very interesting supernova to observe.
I only partly agree. Yes, the brightest components of Sigma Orionis are very bright, and they will almost certianly produce a supernova in the future - or two!! But I don't agree that Sigma Orionis looks fainter than the Belt stars simply because it is farther away than Alnitak, Alnilam and Mintaka. In particular, I am not at all sure that it is necessarily farther away than Alnilam.
Stars grow brighter as they age, and that is true even for hot stars, before they turn into swollen giants or supergiants. Sigma Orionis is - as you pointed out - a young cluster, indeed so young that even its most massive member, Sigma Orionis A, is unevolved. It is still on the main sequence, fusing hydrogen in its core. And while it is an O-type star, it is about as cool as an O-type star can be, with a spectral classification of O9.5.
I have taken an intense interest in the properties of blue stars, and one thing that I think I have learnt is that you just can't guess the brightness of hot star from its spectral classification or B-V index. The best way to find out about its brightness is indeed to measure the exact distance to it. The distance to Sigma Orionis has not been well measured at all, for two reasons: The small cluster is far away, and the two brightest components are sufficiently well separated to be clearly resolved, and sufficiently close to one another to play hell with one another's parallaxes.
So we don't really know how far away Sigma Orionis is, although it is probably farther away than Alnitak and Mintaka. But in my opinion, Sigma Orionis is certainly fainter (and possibly no farther away) than Alnilam, at least. Alnilam appears to be an absolutely hugely bright and massive star. Unlike main sequence stars Sigma Orionis, Alnilam is evolved and classified as a supergiant of spectral class B0Ia. According to its (uncertain) Hipparcos parallax, Alnilam might be as bright as 64,000 times the Sun in visual
light. That is an enormous brightness for a star as hot as spectral class B0, since it will radiate most of its energy as ultraviolet light. It must be admitted that Alnilam is not as hot as Sigma Orionis A. Jim Kaler estimates the temperature of Alnilam at 25,000 K, while Sigma Orionis A has a likely temperature of 32,000 K. However, Alnilam is certainly a much bigger star than Sigma Orionis A.
And we may compare Alnilam with Rigel. We are often taught that Rigel is a fantastically bright and massive blue supergiant. Yes, Rigel is impressive, but it pales when compared with Alnilam. The V magnitude of Rigel (11,500 K) may be some 49,000 times that of the Sun, compared with perhaps 64,000 times that of the Sun for Alnilam. But because Alnilam is more than twice as hot as Rigel, it emits many times more ultraviolet light than Rigel.
Alnilam, as far as I can understand, is one of the truly, truly bright hot stars that are moderately close to the Earth. Not many other nearby very hot stars stand out because of their high output of V light on top of their fantastic production of ultraviolet light. As for Sigma Orionis, it may well be quite bright (even in V light) for its spectral class, but I don't think that it will ever turn into an "Alnilam".
A final note on Sigma Orionis. David Malin claimed that it is Sigma Orionis that ionizes the red "curtain" of luminosity that provides a backdrop to the Horsehead Nebula. That's as good a claim to fame as anything - for Sigma Orionis, that is - if you ask me!