HORSING AROUND
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.