Starship Asterisk*

Has it ever occurred to you that a lot of blue stars out there might be wimps? Comparatively speaking, of course.

Astronomers used to be extremely impressed with blue stars. That's probably because the early photographic films really only reacted to blue and ultraviolet light. When astronomers photographed the sky, the blue stars stood out like sore thumbs. And when astronomers photographed other galaxies, such as the Andromeda galaxy, let alone M33 and the Magellanic Clouds, the blue stars seems to blast them with their power.

So at least some astronomers believed that more or less all blue stars were incredible stellar powerhouses. It was hard to measure parallaxes, but since blue stars were assumed to be so bright, they were thought to be really far away and therefore incredibly luminous. It worked like this: Spectroscopy showed the star to belong to spectral class O or B, and at least some astronomers ascribed a distance to it that would give it its proper brilliance.
Take Alkaid as an example. Alkaid is the end star of the handle of the Big Dipper, and it is the only B-type star of the Big Dipper asterism. Alkaid is an early B-type star, at that: spectral class B3V. (How hot is a typical star of spectral class B3V? Judging from this paper, its temperature might be something like 18,000K.) You'd expect Alkaid to be both very blue and very bright, judging from its spectral class.

My first guide to the stars was Burnham's Celestial Handbook. Burnham was suitably impressed by Alkaid and wrote that

(it) has a computed distance of about 210 light years; the actual luminosity must be close to 630 suns, and the absolute magnitude -2.1.

Sky Catalogue 2000.0 from 1991 refrained from giving a distance and an absolute magnitude to Alkaid, but it said, on the other hand, that its (Johnson) B-V index is -0.19.

Well, the stark reality, revealed by probes Tycho and Hipparcos, was a lot less impressive! The Johnson B-V index of Alkaid is not -0.19 but -0.099, the distance to Alkaid is not 210 but 104 light-years, and the absolute V luminosity of Alkaid is not 630 suns but 154 suns. 154 suns is not bad, but if you expected 630 suns (and a B-V index of -0.19), Alkaid suddenly seemed both puny and insufficiently blue!

What's more, Alkaid suddenly "shrank" compared with the other stars in the Big Dipper, too. Burnham expected Alkaid to be the brightest "Big Dipper member" by far, even in V light. In reality, Alioth, of spectral class A0, is not very much fainter than Alkaid in V light (105 suns), and Dubhe is considerably brighter (223 suns) and also farther away (123 light years). Admittedly Jim Kaler wrote that the bolometric (total) luminosity of Alkaid was greater than that of Dubhe - 700 times the Sun versus 300 times the Sun. Still, I was really disappointed, and not only that: I felt cheated somehow!

I spent a summer comparing the B-V index and absolute V mag of blue stars estimated by Sky Catalogue 2000.0 with those measured by the Tycho and Hipparcos probes. I found, again and again, that the blue stars had been overestimated: they were neither as blue nor as bright as they had been thought to be. (In many cases, of course, the stars were simply reddened by a lot of intervening dust, and if they had been unreddened, they would have been a lot brighter and bluer.) Nevertheless, the blue stars seemed to be systematically "generally fainter" than previous generations of astronomers had expected them to be.

Yes. But there were a few true blue whoppers, which most certainly didn't disappoint. Here are a few of my favorites:
1) Upsilon Orionis (Thabit), spectral class B0V. Its B-V index is -0.26, which is very, very, very blue indeed!!! According to Hipparcos, its likely V luminosity is about 9,000 times the Sun, and according to Jim Kaler, its bolometric luminosity may be around 116,000-173,000 times the Sun.


2) Alnilam: For a supergiant star of spectral class B0, it is really pretty blue, -0.18. And it is incredibly bright, with a likely V magnitude of 64,000 Suns - that's a V magnitude, mind you!!! Its likely bolometric luminosity is 375,000 times the Sun, according to Jim Kaler.


3) Zeta Puppis, Naos: The latest Hipparcos "reduction" (as I think it is called) places Zeta Puppis at about 1,100 light years, making it a bit fainter than it was thought to be before. Jim Kaler still talks of if as if it was at its previously estimated, larger distance, so I won't quote him. Nevertheless, Zeta Puppis is a supergiant star of spectral class O5, giving it a temperature of about 42,000K!! (I got the temperature from Jim Kaler, so I'd better quote him all the same.) 42,000K is extremely hot! (And the Johnson B-V index of Naos is -0.27!) According to Hipparcos, the V luminosity of Naos is about 12,000 times the Sun. I can't believe that the bolometric luminosity of Naos is a lot less than the bolometric luminosity of Alnilam, about 375,000 times the Sun.
4) 68 Cygni: 68 Cygni is a particular favorite of mine. I "spotted it on my own" , as it were, when I noted a star in Cygnus that seemed to create its own huge (although faint) surrounding nebulous shell. The spectral class of this star is O8, and in spite of its huge distance (4,700 ± 1,500 light-years, according to Hipparcos) and a nebula of its own making, it still has a blue color index (-0.06). Its likely V magnitude may be around 16,000 Suns, though that is a "reddened" value, so the unreddened absolute V magnitude of 68 Cygni may well be more. According to Jim Kaler, the bolometric magnitude of 68 Cygni may be more than 800,000 times the Sun! Jim Kaler wrote:

Theory then suggests a mass that might be as high as 60 Suns, making 68 Cyg one of the most massive stars of the Galaxy.

Now that's impressive!!!

5) HD 5980: HD 5980 is the brightest star in the Small Magellanic Cloud. Despite being located about 200,000 light years away from us, it still has a blue color index of -0.17!!

http://en.wikipedia.org/wiki/HD_5980 wrote:
HD 5980 is the brightest star in the Small Magellanic Cloud (SMC) and is located in NGC 346. It has three components, all amongst the most luminous stars known: an unusual primary with a Wolf–Rayet spectrum that has produced a luminous blue variable (LBV) eruption; a secondary, also a Wolf–Rayet star, forming an eclipsing spectroscopic binary; and an O-type supergiant that may not be physically associated.[4]

At one stage in 1994, the spectrum was that of a blue hypergiant: B1.5Ia+.[5] The bolometric luminosity stayed approximately constant during the eruption, as is typical for LBVs, but the visual brightness increased by over two magnitude during the brief peak. One study suggests an increase of 3-6 times in luminosity to 10 000 000 times the sun at its peak[6] but this may simply be due to different analysis techniques, and others find a fairly consistent luminosity of a few million times the sun.[7] The eruption peaked in 1994 and by 2009 the star had returned to near "minimum" with a spectrum of WN4/5. This would make it the hottest LBV and as might be expected one of the most luminous.

... the LBV star is several million times as luminous as the sun and more luminous than any LBV except Eta Carinae

Now that's impressive!!!
How can we explain the huge differences in luminosity among B- and O-type stars? One short answer has to do with mass, because in order to become very bright, a blue star has to be very massive. Another short answer has to do with size, because in order to be very bright, any star has to be big. White dwarfs aren't very bright, even if some of them are tremendously bright. But the brightest blue stars have to be both very massive, very hot, and very big. They must be big fish indeed!

It goes without saying that Alkaid has to be smallish, at least compared with what astronomers of yesteryear expected it to be. Even among main sequence blue stars, the run of the mill ones are, well, run of the mill.

But the odd super-duper hot massive large early B and O-type stars are impressive!!!

Ann