By Jim Kaler

Among the most famed and charming sky-tales is that of the maiden Andromeda, who was rescued from the jaws of Cetus, the Sea Monster, by the hero Perseus, who spotted her from high on Pegasus, his Flying Horse, while returning home from slaying the Medusa. Oddly, the two constellations, Perseus and Pegasus (both symbolic of northern autumn), nowhere connect; it's as if Peg got tired of Per and tossed him off. But there is Andromeda between the two, carefully tucked in with Perseus, who is carrying her on horseback home to her royal parents, Cassiopeia and Cepheus. And if this part of the story is not quite how the ancients told it, that's ok, since such tales are hardly fixed, but evolve with the storyteller, and such has it always been and always will be.

Pegasus is perhaps best known to the casual stargazer for its Great Square, whose northeastern corner (Alpheratz) formally belongs to Andromeda as Alpha Andromedae, the star's alternative name "Delta Pegasi" never used. You can also admire it as the brightest of the "mercury-manganese" stars, which can elevate their mercury-to- hydrogen abundances by 100,000 times relative to solar, while at the same time depressing other elements like calcium, all as a result of gravitational settling and radiative lofting in quiet, undisturbed atmospheres. Moving around the Square to Beta Peg we encounter a class M red giant half-magnitude variable. But by far the most intriguing of the Greek-lettered stars is Enif (Epsilon Peg), a class K supergiant at the far southwestern corner of the classical figure. In 1972 it was seen by one observer to flare up as bright as Altair, suggesting that we may not understand such stars all that well.

The real winner in the Horse's stall, though, is probably faint-fifth magnitude 51 Peg, which lies alongside the Square about halfway between Beta and Alpha. It was the "discovery" star for orbiting exoplanets, one that showed us that planetary systems can be very different from our own, as through the Doppler technique it was found to have a "hot Jupiter" in a 4.2-day orbit just 13 percent Mercury's distance from the Sun. Oddly, practically right next to it lies HR 8799, which was among the first to have had its planets (three massive "jupiters") discovered through direct imaging.

Returning to Epsilon, it and Theta point northwesterly toward Messier 15. Though some 35,000 light years away, it's one of the heavens' best and most easily found globular clusters, compliments of its membership of nearly a million stars. It also has the distinction of being among the most centrally concentrated of such assemblies, with a host of extremely blue "horizontal branch" helium-fusing giants, the result of being very metal-poor (about one percent solar). Messier 15 also has the rare distinction of hosting a planetary nebula, K648 (or Pease-1), a low-excitation expanding cloud around a relatively cool (for planetary nebulae, 25,000 Kelvin) dying central star.

Looking vastly further afield, Pegasus is also home to one of the first compact group of interacting galaxies ever discovered (and perhaps still the best-known), Stephan's Quintet, which lies near the boundary with Lacerta and is pointed to by the Square's Gamma and Beta Peg. "Stephan's Quartet" might be better, as the southeastern "member" lies much closer than the other four (which are a hefty 300 million light years away), showing once again that mere proximity do not a family make. The tidal collisions that the others are going through promote active star formation. Just half a degree to the northeast lies the bright, 50-million-light-year distant spiral galaxy NGC 7331, which has a massive and active black hole at its core.

Gazing across Andromeda to the Horse's Rider brings us into the Milky Way with its many treasures, almost any one of which could be listed as most famed or best loved. Some might say it is the beautiful Double Cluster, which comes close to belonging to Cassiopeia (and is pointed to by Gamma and Delta Cas). The only known binary cluster, the two move through space together at a well-determined distance of 7650 light years. Young, just over 10 million years old, it presents the eye with several red supergiants sprinkled against a splash of hot blue dwarfs, providing a wonderful contrast with ancient M15 across the way. The argument over the individual names runs long and deep. In his Uranometria, Bayer listed them as "h" (the eastern one) and "Chi," though it is not clear whether he meant the clusters or stars or how he might have know that there were two different assemblies.

Other clusters abound. Among the better ones is Messier 34, which lies near the border with Andromeda. Sixteen hundred light years away, though with an age nearly 200 million years older than h and Chi, it still contains bunches of blue-white class B stars. Among those whose merits are least sung is the one named after the luminary, the Alpha Persei cluster, probably because (like the Hyades) of its sprawling nature, in part caused by proximity, the system just 600 light years away.

The Alpha Per group is loosely related to the vast "Cas-Tau Association," which, from its very name, spreads itself over a huge volume of space estimated to be 4000 light years across. Unlike clusters, which are gravitationally bound together, associations are not. More commonly called "OB associations" as a result of their relatively great numbers of hot, massive, blue O and B stars, they are individually falling apart, expanding away from common centers, their places of birth. The blue O and B stars, however, do not live very long, so they do not get far away before they expire, giving the associations great integrity and visibility. Along with the hot stars also go hosts of less-massive members.

Perseus has a number of such associations, beginning with the extended 50-million-year old Alpha Per association, also known as Perseus OB3. Among the others is Perseus OB2, sometimes called the Zeta Persei association after its main member, a class B1 supergiant 700 or so light years distant, a bit closer than the main group. Associations are known for their hierarchies, the result of supernovae that produce sequential star formation, one set of stars blowing up, which compresses the interstellar clouds that then generate more associations, and so on. Orion is famous for such descending sub-associations, the disintegrating Cas-Tau association producing the Alpha Per group.

Among the recently-rejected members of Per OB2 is X Per (not Greek "Chi," but "Ex"), an O9.5 variable of uncertain distance that oscillates erratically between sixth and seventh magnitude and that has a neutron star companion, the result of its once-mighty neighbor blowing up as a supernova leaving a spent cinder the size of a small town behind. Together the two make a "high mass X-ray binary" (HMXB), the X-rays caused by mass flowing from the visible star onto the neutron star. The latter is also an X-ray pulsar, with a pulse period of 835 seconds, the result of rapid rotation causing a beam of high-energy radiation to periodically sweep past Earth.

Much tamer is Perseus's famed California Nebula, which stretches three degrees from east to west. Apparently lit by the fourth magnitude 07.5 giant Xi Persei (another rejected member of Per OB2), the California Nebula, roughly 1500 light years away, fronts for a vast molecular cloud that is a seat of star formation. Xi itself is a "runaway" star that has been shot off at high speed either by a one-time companion that went supernova or through binary interaction.

At the quiet end of the evolutionary process, we find one of just three planetary nebulae obvious enough to Messier to have made his catalogue. The trio includes the Ring Nebula in Lyra (M57), the Dumbbell in Vulpecula (M27), and our focus here. Called the "Little Dumbbell," Messier 76 has such a strong bi-lobed structure that it received TWO NGC numbers, 650 and 651. Its form is the result of highly non-uniform mass loss in the one-time advanced giant star that created it. Expanding and now more than 1.5 light years across (the distance only poorly known), it is ionized and fluoresced by a 17th magnitude central star (the giant's exposed core) that has been heated to 170,000 Kelvin, making it among the hottest stars in the Galaxy.

But all the above are preliminaries, as we await that Star of Stars: Algol, Beta Persei, the Demon Star. It's the brightest (or at least most obvious) eclipsing binary. Every 2.867 days it drops from mid-second magnitude to dim third as a class K (or cool G) giant cuts partly in front of a much more luminous class B dwarf. (Midway between the major eclipses is a blip caused by the B dwarf passing in front of the extended companion.) It's fun to watch.

More importantly, Algol is telling us something of deep importance. The two stars are very close together. Analysis of the orbit shows us that the K giant is by far less massive than its dwarf companion. But high mass stars ae supposed to die before lower mass ones. That Algol B is a giant means that it should be the MORE massive. The famed "Algol paradox" is resolved through mass loss. Algol B USED to be the more massive, but as it lost its hydrogen core to become a giant, tidal forces began to force it to lose mass to the B dwarf, a process confirmed by emissions in the spectrum that show flowing matter. The B star is literally consuming its companion, which we might suppose is the ultimate in one star "horsing around" with another.

Copyright © James B. Kaler, all rights reserved. These contents are the property of the author and may not be reproduced in whole or in part without the author's consent except in fair use for educational purposes. First published in the January/July 2011 Newsletter of the Lowestoft and Great Yarmouth Regional Astronomers, who are gratefully acknowledged.