Enif , Sadalsuud & Sadalmelik:

A Tale of Three Supergiants

By Steven C. Raine

 

Dedicated to Michael O’Leary who introduced me to all the stars.

 

When does a star have the same temperature and spectral class as the Sun yet shine thousands times as bright? The answer is when it’s a yellow supergiant and thus far larger and more luminous! These golden behemoths are among the rarest types of star visible in our skies -  yet  oddly enough in a small span of space, around the nose and the quite unofficial ”ears” of Pegasus you’ll find three medium-bright examples; Sadalmelik (Alpha Aquarii), Sadalsuud (Beta Aquarii) and Enif (Epsilon Pegasi) fairly close together in our sky and, indeed, most likely, closely related. (Figure 1: The author Steven C. Raine)

 

Despite their similarity, each of this remarkable trio has its own distinctive points. Enif, Epsilon Pegasi, is the only one of the trio in Pegasus, in fact it’s the brightest star in that constellation where it marks the nose of the flying horse. This was the key to one of the great stories the late Michael O’Leary told us: as a boy he was learning the patterns of the skies from a book and noticed that the nose of the horse, which was tucked in against the crease between the pages appeared to be the very apt “Sniff” only later discovering the initial letter was an ‘E’  not an ‘S’. This may have been a fortunate error – I’ve never forgotten the story or the star name since  - and it does seem apt.  Even  its actual name is not too far off that “sniff” mark as it arises from ‘Al Anf’, Arabic for “the nose” – and Epsilon Pegasi also went by the Arabic ‘Fum Al Faras’ situating it as the horses mouth instead. Flamstead too took a name from the “horses mouth” calling Enif “Os Pegasi” the Latin equivalent to “Fum al Faras’ although this name is virtually never used today.  Which leads to the question - if the ‘nose’ won out then what might Enif or Pegasus be sniffing? Perhaps it’s the nearby (at least as seen in our skies)  bright globular star cluster M15 (mag. 6.5) located only 4 degrees north-westwards in celestial co-ordinates but a whopping 33,000 light years distant in space! (Figure 2: Michael O'Leary 1932-2008)

 

As well as being in Pegasus not Aquarius, Enif is also the odd one out of our trio in another regard; as you can see from Figure 1 (at bottom), at class K2 Enif is technically, an orange rather than yellow supergiant - although most sources still describe it as “yellow” rather than orange. This makes Enif the coolest and largest of the three, with Sadalsuud being the hottest, smallest and intrinsically least luminous and Sadalmelik taking the middle spot.

 

Sadalsuud, despite being the Beta star of Aquarius, however, appears brighter than Sadalmelik, the alpha star in our sky. This probably indicates a misjudgment by Bayer in his ordering of this zodiacal constellation although it is an excusable one given the very small difference between Alpha and Beta Aquarii - just a tenth of a magnitude. Nor is it that exceptional, given numerous other cases of constellations violating the “Alpha equals brightest star” rule. For instance, Pegasus where, as noted,  Enif is the brightest yet is labelled Epsilon - is another example, along with Orion, Sagittarius and Gemini to name just a few.

 

Still, being deposed from its “rightful” Alpha ranking would however seem to belie Beta Aquarii’s proper name which is derived from the Arabic ‘Al Sa’ad al Su’ud’  meaning “Luckiest of them all!” This in turn may have some link with the ancient Euphratean name for Sadalsuud as their “Star of Mighty Destiny.” Echoes of this Arabic superstitious labelling were adopted by medieval Western astrologers who corrupted Beta Aquarii’s astrological moniker into ‘Fortuna Fortunarum.’ Yet there’s irony here because the likely cause for this “luckiness” association was connected with Sadalsuud’s helical rising at the beginning of the rainy season for that particular time and region. However, for us, the appearance of Sadalsuud and its companions marks the opposite event – the cessation of Adelaide’s main winter rainfall and so, logically, its name should (to us anyway) perhaps be the opposite!  

 

Either way, the “luck” theme in Sadalsuud’s naming flows on to the other stars of Aquarius too, with Sadalmelik  being derived from the Arabic “Al Sa’d al Malik” meaning “The Lucky One of the King”; Gamma Aquarii’s name, Sadachiba, from “Al Sa’d al Ahbiyah” or “The Lucky Star of Hidden Things” and Epsilon Aquarii’s name of Al Bali coming from “Al Sa’d al Bula” or “the Good Fortune of the Swallower” – presumably the mythical drinker of the water carried in the zodiacal figure’s water jar! There is one respect though in which Sadalsuud is, indeed, lucky – it is well positioned in our sky to serve as a marker for M2 (NGC 7089) a bright and concentrated globular cluster which was the second objet on Messier’s list after the Crab Nebula. M2’s massive sphere of stars, which at apparent magnitude 6.5 is easily visible in binoculars, is located 5 degrees north from Sadalsuud in our sky although from its vast distance of 37,000 light years away it is roughly as far from Beta Aquarii as we are!

 

This illustrates the scale of distances in our galaxy somewhat as from Sadalsuud, the other members of this supergiant trio would both be “zero magnitude stars” shining as or more brilliantly than Sirius and Canopus are here, whereas in Earthly skies all three are only moderately bright. In terms of Galactic structure, both Sadalsuud and Sadalmelik now lie on the outskirts of our “local neighbourhood” in a feature known as Loop III  towards the Aquila Rift  a dark cloud of gas and dust with Enif sitting nearer inside the “bubble” contained by Loop III , a ’bubble’ which adjoins our own smaller Local Bubble. The fact that Sadalsuud seems brighter than Sadalmelik is due to this geography and, specifically, that Sadalsuud is one hundred and forty light years nearer to us! 

 

Sadalmelik’s position in our sky  though has a very rare distinction; Alpha Aquarii is one of just two bright stars that lie within 1 degree of the celestial equator - the other being Mintaka or Delta Orionis. Also notable is that as renowned stellar expert James Kaler notes on his web page for this star; “Sadalmelik is also a rare "hybrid star." Less luminous stars of solar temperature tend to have magnetic fields that produce hot surrounding "coronas," the Sun's seen at the time of a solar eclipse. Very luminous stars, however, do not, instead having much cooler winds. Sadalmelik falls in between, with both characteristics.”

 

At class G0 Sadalsuud is the hottest of our gargantuan golden triplets and barely squeezing into the G-spectral type – any hotter and it would be an F-type star instead. But Sadalmelik, the Alpha star of Aquarius has the distinction of sharing our Sun’s spectral class – it too is a G2 star. Perhaps its name would’ve been better with its syllables scrambled and slightly altered to “Da-Sun-like-Me!” However, while its spectral class is the identical to our Suns, Sadalmelik’s luminosity class or basic star type (ie. dwarf, giant, supergiant) of ‘Ib’ reveals it to be vastly different. How much so? Reflect on this The next sunny day, its worth looking around and reflecting that if Sadalmelik replaced the Sun then given its identical spectral type, it would likely appear the same as our Sun in brightness and general appearance. We’d see the same white–yellow orb casting the same strong light and heat  – but only if our Earth was orbiting Sadalmelik from a distance of fifty-six Astronomical Units rather than our present location a single AU away! That is way beyond Pluto’s orbit, indeed, it is roughly equivalent to the orbit of that most distant ice dwarf planet Eris! Any closer and put simply we’d fry! This is because while our Sun is a normal dwarf star, Alpha Aquarii and its companions are “less bright” supergiants. Don’t let the “less bright” part fool you though as these are still true supergiants and thus unimaginably brilliant and colossal! In Sadalmelik’s case, it is about sixty times our Sun’s girth, somewhere around between six to ten times its mass and radiates an astounding three thousand times its light!

 

The numbers are stark, Alpha Aquarii’s diameter measures 95 million miles versus our Sun’s diameter of a mere 864 thousand miles. If it replaced our Sun it’s surface would extend out well beyond the Earth’s orbit which is about 93 million miles. Comparisons with our solar system are one thing but even compared to ordinary yellow giants, these supergiants stand out. The twin yellow giant stars of Capella are incredibly impressive but are “only” giants not supergiants and consequently have luminosities of a relatively meager eighty-two and fifty times greater than our Sun compared with the luminosities of two thousand two hundred (Sadalsuud) to six thousand seven hundred times (Enif) greater than our Sun’s brightness. But remarkable as such individual statistics are, possibly the most amazing thing is that these titans of our night probably have a shared history. Astronomers think their story begun about 15 million years ago with being born together around the same time in the same broad area as part of an O-B star association – a looser grouping than an open star cluster. All having around six to ten solar masses these trio lived their main-sequence years as hot B-type dwarfs similar to the stars of the Pleiades, Achernar and Spica. Over all this time they have drifted apart to their present separation of over 100 light years from each other. As their hydrogen fusion main-sequence lives drew to a close, their core supply of hydrogen ran low and forced them to switch to fusing helium and begin the transformation that will eventually result in them becoming red supergiants. As this transition takes place, their outer layers cool and swell, shifting through the spectral classes like a cooling metal bar from the searing blue-heat at the beginning through being white-hot to their present yellow and orange glow before the final molten red heat stage.

 

This means our three stars are all currently, to use Kaler’s words again : “bouncing around the ‘yellow evolutionary void’ in which stars become unstable and do not like to linger.” This void explains why these stars are quite rare, the process of transition being quick  - at least in astronomical terms. Its during this stage that stars evolve across what’s called the instability strip where significant stellar variability occurs – notably becoming Cepheids which none of these stars are - yet.

 

Why not is a mystery given Sadalmelik at least theoretically, should be a Cepheid. So is Sadalmelik a Cepheid that has temporarily ceased activity like Polaris has done? Or has this star yet to begin its Cepheid career? Or will it for some unknown reason never become a Cepheid in contrast to other yellow supergiants? We’re not sure and these are only a few of the many questions such stars raise. Another mystery is the question of super-flares, Kaler – once again – noting a remarkable incident that took place with Enif :

 

“In 1972, an observer in Florida saw Enif as bright as Altair in Aquila, five times brighter than normal, whereupon it faded. For over 10 minutes it appeared to pop some kind of enormous flare, one vastly brighter than the small ones that occur frequently on the Sun. Such events are rare -- only two dozen or so are known -- and not well documented, nor is there any theory for them. We apparently do not understand supergiants -- or any other kind of star for that matter -- quite as well as we think we do.”

 

There is considerable uncertainty in exactly how our trio of yellow supergiants stars will conclude their brilliant careers too. All depends on their exact mass and how much mass they lose before their nuclear fusing lives finish. With just six solar masses, Sadalsuud falls two full solar masses short of the limit for going supernova and will eventually produce the usual planetary nebula and white dwarf - although a fairly heavy one, possibly even a rare neon-oxygen white dwarf variety.

 

Enif with ten solar masses is right at the upper end of the 8-10 solar mass borderline where going supernova is a real possibility and even the likelihood. If it retains enough mass to fuse iron, it will depart the scene in the ultimate stellar blaze of glory as a type II “core collapse” supernova.  If not, however, if it loses too much  mass as a red supergiant then it too, like our Sun and nearly all other stars, will become a massive white dwarf. Yet there is a final twist though. Being so near the boundary of the possible supernova-white dwarf endings the type of white dwarf that Enif (and its two stellar cradle-mates) may  leave behind may well be very near the critical mass to produce a type I “white dwarf” supernova. Unfortunately, for those longing for supernova fireworks, such an event requires a nearby companion to shed matter on to it and drive it over that limit but whether suitable stellar companions exist is unclear. As for Sadalmelik its mass is somewhere close to its siblings and thus it too has an uncertain fate perhaps detonating a supernova or maybe just sloughing off its outer layers to fade  away as a white dwarf.

 

Whatever their final fate these stars - being relatively short-lived, fast-evolving, high mass stellar leviathans - are heading towards it fast. We’re glimpsing them in a rare transition stage taking place in an astronomical eye-blink. So however skeptical we are of the supposed astrological “luckiness” associated with these stars ancient names we are, indeed, lucky to catch these three closely related magnificent rare golden stars, Enif, Sadalmelik and Sadalsuud, in their astronomically brief present state simultaneously.

 

 

Figure 1 – The yellow Supergiant trio compared :

 

Compiled from sources listed.

 

KEY : Luminosity (lum) is bolometric, ie. including all wavelengths. Type = Luminosity type (eg. dwarf, giant, supergiant, Ib =less bright supergiant, V = main-sequence dwarf)  Distance is in light-years - except for the Sun. Magnitude : Mag1 =  apparent magnitude, Mag2 = Absolute Magnitude. Luminosity, radius and mass are measured relative to the Sun ie. Sun =1. 

 

Star    Bayer letter     Mag1  Spec. Type  Dist.   Lum.   Radius   Mass   Mag2

 

Enif            Epsilon Pegasi   2.4     K2  Ib     670     6,700   150      10       - 4.19

 

Sadalmelik Alpha  Aquarii   3.0    G2  Ib      760    3,000    60       ?       -  3.88

 

Sadalsuud  Beta Aquarii    2.9    G0  Ib        610     2,200    50       6        - 3.47

 

Sun             NA                - 27     G2 V           1 AU          1      1       1       +  4.82

 

 

references:

Frazier, Kendrick & Editors of Time-Life, 1985,‘Solar System’, Time Life Books.

Henbest, Nigel & Couper, Heather, 1994, The Guide To The Galaxy, Cambridge University Press.

Kaler, James B., 2002,'The Hundred Greatest Stars’, Copernicus Books.

Schaaf, Fred, 2008, ‘The Brightest Stars’, Wiley.

Motz, Lloyd & Nathanson, Carol, 1991, Constellations, Aurum Press.

Ridpath, Ian & Tirion, Wil, 1988, Collins Guide to Stars & Planets, Collins.

Zimmerman, Robert, Astronomy magazine, March 1995, "Polaris, the code-blue star,” Kalmbach publishing co.

 

‘Celestia’ computer program, accessed Sept. 2008.

Kaler, James B.,  stars website – Enif : http://www.astro.uiuc.edu/~kaler/sow/enif.html , accessed Aug-Sept. 2008.

Kaler, James B.,  stars website – Sadalmelik : http://www.astro.uiuc.edu/~kaler/sow/sadalmelik.html , accessed Aug-Sept 2008.

Kaler, James B.,  stars website – Sadalsuud :

http://www.astro.uiuc.edu/~kaler/sow/sadalsuud.html , accessed Aug-Sept-2008.

 

 

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Updated 4th of May 2012