THE KING
Not well known for his exploits, Cepheus the King is noted more as husband to Cassiopeia and father of Andromeda, who in myth was rescued from Cetus the Sea Monster by Perseus and his flying horse, Pegasus. Set into a faint part of the Milky Way, the five-sided figure can be
difficult to find. And why bother? There are no grand Andromedean
galaxies, no splash of
Cassiopean clusters. He
does, however, have his stars.
The constellation's luminary, Alderamin (Alpha Cephei), just barely
makes it to second magnitude. An ordinary class A7 dwarf much like Altair (though at 49 light years three times
farther), its repute lies more with Earth. Our rotation axis now
points close to Polaris. But 'twas not
always so as a result of precession, the 26,000-year wobble of
the Earth's axis that causes the celestial
pole to describe a circle of 23.5 degrees radius about the
orbital perpendicular. Around 2500 BC, Thuban (Alpha Draconis) was a fine pole
star. Polaris will keep getting
better for another century, and then (from our perspective) will
pull away. In 7300 AD, Alderamin will be within three degrees of
the pole. Which is hardly as good as Polaris, but we take what we
can get.
Beta Cephei (Alfirk), a third magnitude
hot blue B1 spectroscopic subgiant with a temperature of
27,000 Kelvin, shines with a total luminosity of 23,000 Suns and lends its name to a whole set of
variables. "Beta Cephei stars" are generally hot class B subgiants
and giants that vary by a few
hundredths of a magnitude with multiple periods, Alfirk with 4.72,
4.46, 4.43, 4.88, and 4.3 hours. Carrying a mass around 13 times
that of the Sun, and in its evolution actually a hydrogen-fusing
dwarf (such dichotomies in classification not unusual), it will
likely someday explode as a supernova. Accompanying it are a close B5
dwarf enclosed in a radiating disk and a class A dwarf set off from
the inner pair by 13 seconds of arc.
Gamma Cep (Errai), a third magnitude K1
giant 45 light years away, has a planet, one
of the few such to bear a proper name.
You'd not want to live there. With a mass at least 1.9 times that
of Jupiter, the planet takes 2.5 years to orbit two Astronomical
Units away from an evolving giant 11 times brighter than the Sun.
Gamma Cep will beat Alderamin in the "race to the pole," passing
about 2 degrees from it in 4000 AD.
At the pinnacle of the King's treasures lies Delta Cephei. A supergiant with a luminosity
around 2000 Suns and a mass of six solar, Delta changes smoothly
between magnitudes 3.5 and 4.3 over a period of 5.43 days, its
class going between F5 and G2. Found to vary in 1784, it's yet
another prototype. "Cepheids" are among the jewels
of the stellar sky. In 1912, Henrietta Leavitt of the Harvard
College Observatory discovered that the average apparent magnitudes
of the Cepheids in the Magellanic Clouds
(nearby satellites to our Galaxy) closely correlated with
the logarithms of their variation periods in days. Knowing the
distances to the Clouds or the distances to a few Cepheids in the
Galaxy gives absolute magnitude vs. period. Once a Cepheid is
found in another galaxy and its period and apparent magnitude
measured, Leavitt's very much refined relation yields the galaxy's
distance. In the 1920s, Edwin Hubble applied the method to the Andromeda Nebula, proving that it (and by
analogy similar "spiral nebulae") were not small bodies within our
own system, but were separate external galaxies similar to our own.
Identifying the Cepheids in the galaxies of the Virgo cluster and beyond was one of the Key Projects of
the Hubble Space Telescope. Using a variety of other distance
methods as well, astronomers then calibrated the peak absolute
luminosities of Type Ia (white dwarf) supernovae,
which led to the establishment of the Universe's expansion rate
(the Hubble constant), its age of 13.7 billion years, and to the
mysterious "dark energy" that
propels an acceleration of the expansion (and with normal and dark matter renders the Universe
"flat.") Parallax gives
a distance to Delta of 865 light years, the period-luminosity
relation a satisfying 900. Delta and the K supergiant Zeta Cephei are the main members of an OB association called Cepheus OB6,
whose central distance is an equally satisfying 880 light years.
An "OB association" is a collection of stars born pretty much in
the same place at the same time from the same parent giant
molecular cloud. Unlike a cluster, an association's members are
too far apart and moving too fast to be held together by gravity,
so given their natural motions at birth, they escape from their
centers in vast outbound flows. Associations are most easily
recognized when they are young and still have their brilliant O and
hot B stars.
Few constellations can match Cepheus in embracing OB associations.
Fifth magnitude Lambda Cephei lies
just to the northeast of Delta. A class O6 supergiant 2000 light
years away and just now evolving away from its hydrogen-fusing
state, Lambda Cep is the hottest O star you can more or less easily
see with the naked eye. Through the telescope, however, instead of
the blue-white sparkler you might expect for a temperature of over
36,000 Kelvin, it takes on a dull white glow, the result of 1.7
magnitudes of absorption and consequent reddening by the Milky
Way's interstellar dust. With a
luminosity of 450,000 Suns (mostly in ultraviolet light) and a mass
perhaps 45 times that of the Sun, it is certain to blow a hole in
its surroundings when it explodes. Moving swiftly, it appears to
be a "runaway" from the Cepheus OB3 association, which is centered
40 percent farther away. The star is moving fast enough that it
and its wind hammer a shock wave into the interstellar gases much as does a
speeding boat plowing through water.
With birth masses at least 15
or so times that of the Sun and extending beyond 100, when stars
run out of core hydrogen, they turn into supergiants, those under
about 40 Suns becoming immense red supergiants. Cepheus has a
magnificent naked eye pair. Just off the southern end of the
pentagon is Herschel's Garnet Star, Mu
Cephei, that from its purported membership in the Cepheus OB2
association lies perhaps 2500 light years away. As an M2 red
supergiant shining with the total light (most of it in the infrared) of 350,000 Suns, which
tells of a mass around 30 times solar, not only is it colorful in
its own right, but two magnitudes of absorption and consequent
reddening give it an even more lustrous color. Were the dust not
there, Mu Cep would shine at mid second magnitude and might have
been the Alpha star. Mu Cep is
so big that we can directly measure its angular size. At 2500
light years, a radius of 7.7 Astronomical Units would in our Solar
System bring it out halfway between the orbits of Jupiter and
Saturn. Not that radius means much for such stars. Unlike the
Sun, their "edges" are not sharply defined and depend a lot on
where in the spectrum we look. Herschel's very own is so big that
it cannot even find a stable radius, its pulsations causing it to
vary by almost a magnitude over a period of 800-1000 days.
Somewhat north of Mu Cep is fifth magnitude VV
Cephei, which from its very name varies in light, and not once
but twice. It's best known as a long-term eclipsing binary in which a
hot, blue class B star of uncertain kind disappears behind a
monster M2 red supergiant for 250 days every 20.4 years, causing
the combined light of the system to drop by 20 percent. The next
eclipse is scheduled for 2018. Relating it again to Cepheus OB2
makes it 2500 light years away (as does the parallax). Analysis of
the eclipse shows the star to be about the size of Mu, falling
between the orbits of Jupiter and Saturn. An uncertain temperature
in the mid-three-thousands gives it a luminosity and mass similar
to Mu Cep, perhaps more. Tides raised
in the supergiant by the smaller dense star distort the larger,
causing mass to flow from the larger to the smaller and making the
definition of "radius" even more questionable. However uncertain
the parameters may be, there is little doubt that the King hosts
two of the biggest stars in the Galaxy.
Less massive stars have their own places, including Cepheus's long-
period (Mira) variable, overlooked T Cephei
just southwest of Alfirk. Though most of time it's invisible to
the naked eye (bottoming out at 11th magnitude), during its 388 day
period it can reach naked-eye fifth. Just a third the adopted
distance of VV and Mu, T Cep is more than double the size of
Mercury's orbit and shines redly with the light of more than 3000
Suns.
In an advanced state of
evolution, Mira variables are brightening with dead carbon-oxygen
cores buried in fusing helium and hydrogen shells. Winds up to a billion times the
strength of the solar wind eventually sweep away the outer stellar
envelopes, leaving the nuclear burning cores behind as hot cinders,
nascent dense white
dwarfs. For a brief moment the fleeing dusty gases, compressed
by the cores' fast winds and lit by their harsh ultraviolet
radiation, produce more Herschellian favorites, planetary nebulae (the first
announced by Sir William in 1791 and named for its "planetary" or
disk-like appearance). Within the south central portion of the
constellation's main pattern, NGC 7139 appears as a near-perfect
ring a bit over a minute of arc across centered on a 19th magnitude
central star with a temperature of just over 100,000 Kelvin (near
double that of the hottest O dwarf). At a distance of 5000 light
years (but who knows?), it spans half a light year. Down toward
Cepheus's southeastern edge lies smaller but brighter NGC 7354. Expanding at around 20 kilometers
per second, the two stellar flowers will eventually fade into
nothingness, carrying composted stellar matter back into space to
help make new stars: and perhaps new Earths.
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 August/December 2013 Newsletter of the Lowestoft and Great
Yarmouth Regional Astronomers, who are gratefully acknowledged.