THE NAVIGATOR
Constellations reflect the times. And
the first people to invent them skim the best stars. Most of the
classical constellations from ancient days are made of bright
patterns that represent heroes and familiar animals. Look at Orion, the Hunter, composed of some of
the brightest stars of the sky, or at Ursa
Major, the Great Bear, which is centered on one of the most
beloved figures of the heavens, the Plough or Dipper, depending upon which side of the puddle you
live. (Of course there is spring's dim Crater, the Cup, which belies these rules, but it is
a distinct exception, and besides, it really does look
like a Cup.)
By the time of the next round of constellation-making, in the
seventeenth and eighteenth centuries, the bright patterns of the
northern hemisphere had been used up. The vast deep southern
hemisphere (which the Greeks and their predecessors could not
see) was loaded with them, however, and there were still plenty
of faint star patterns that the ancients had ignored, all waiting
to be organized and named. The culture had also changed. While
animals abound in this set of "modern" constellations, we now see
a variety of industrial artifacts, among them three similar naval
navigational devices (used to determine angles in the sky so as
to find terrestrial location): the quadrant, octant, and sextant,
in the sky honored as Quadrans, Sextans, and Octans. They
are remarkably well distributed: Quadrans lies in the far
northern hemisphere, Sextans straddles the equator, and Octans
surrounds the southern rotation pole.
Though made of dim stars, Sextans and Octans are reasonably
familiar. You will look in vain for Quadrans, however, as it was
long ago officially dropped, and thus appears on no modern star
chart. No matter, it still exists if you have a sky dark enough
to find it. Locate spring's Boötes with
brilliant Arcturus. The Herdsman's hand extends northward to a
long thin triangle just to the east of the Great Bear's tail.
Further east by a bit over an hour of time is a set of faint
fifth and sixth magnitude stars of no particular form that in the
latter 1600s still somehow reminded French astronomer Joseph
Lalande (1732-1807) of a quadrant. Its best-known star is
obscure BP Bootis, an "RS Canum Venaticorum" variable. Such
stars are made of close pairs whose rotations have been
synchronized to their short-period orbits, and thereby exhibit
strong solar-type magnetic behavior that
includes notable starspots.
While among the departed (many mercifully so) constellations, the
ghost of Quadrans survives through a meteor shower that was
discovered (in 1825) emanating from it before its official
demise, the marvelous Quadrantids. One of the best of the
regular showers of the year, it can produce up to 200 meteors per
hour, comparable to if not better than the much more famous
Perseids. Its lack of popularity is in its timing, as the shower
peaks around January 3 (near Earth's orbital perihelion, though
that has nothing to do with it), and it's hard to get people to
lie outdoors on their backs for hours in the snow.
Meteor showers are caused by bits of fluffy rock that break off
comets when they travel close enough to the Sun for their icy
matrices to be evaporated. The little meteoroids travel in the
comet's path, and if Earth passes close to it, we can be hit with
the debris, which burns up high in the air. The meteors' parent
body is identified by matching the particles' collective orbit
(found by tracing the resulting meteors backwards) with that of a
comet. Halley's, for example, produces May's Eta Aquarids and
October's Orionids. The originator of the Quadrantids was only
recently identified with a near-Earth "asteroid" called 2003 EH1,
which is not an asteroid at all, but a now-boiled-out dead
comet.
Moving southward, we come to Sextans, invented much earlier by Johannes Hevelius
(1611-1687). The luminary, such as it is, is a white class A
faint-fourth-magnitude giant 287 light years away. Consistent
with its placement within a navigational instrument, this
ordinary star, Alpha Sextantis, sits
but half a degree south of the celestial
equator almost directly south of Leo's Regulus.
Precession, the 26,000-year wobble of the Earth's axis, is slowly
moving this part of the equator to the north relative to the
stars, and therefore makes the star appear to move to the south.
A century ago Alpha Sextantis was NORTH of the equator, crossing
over near the beginning of 1924.
Sextans is perhaps best known for a wildly contrasting pair of
galaxies, one very bright, the other amazingly faint. NGC 3115,
discovered by William Herschel, is so obvious that it even has a
special name, the "Spindle Galaxy" (which it shares with others).
The ninth magnitude Spindle, 27 million light years distant, is a
class S0 disk galaxy seen edge-on that has no spiral arms. It
was once thought to harbor a supermassive black hole of a billion
solar masses that further examination reduced to a mere two
million solar, roughly equal to the one that resides at the
center of our Milky Way system. The constellation is also home
to three small members of the Local Group of galaxies. By far
the faintest is the "Sextans Dwarf." Only 280,000 light years
away, sprawling over a degree across, this "dwarf spheroidal"
galaxy is barely detectable against the night sky. In integrated
light it is fainter and less massive than the biggest of our own
Galaxy's globular clusters, and greatly under-studied. Such
systems are important in learning about star formation and the
history of galaxy encounters, in which galaxies collide, merge,
and tear tidal pieces from one another.
Now plunge far down to the South
Celestial Pole and to the last of our tools, Octans, the Octant (a product of the mind of the Abbé
Nicolas de La Caille, 1713-1762), where logic deserts the star
names. The
Alpha star is oddly only sixth brightest, being
beaten out (in brightening order) by Gamma, Theta, Delta, Beta,
and, at the top of the pile, of all things, fourth magnitude Nu Octantis, an interesting class K giant
69 light years away. With close to a solar mass, it is in the
act of brightening (over the next 100 million years) with a dead
helium core. The best known star (or best known of about
anything in the constellation) is nearly-sixth magnitude
Sigma
Octantis, a class F giant 270 light years off. Along with much
brighter Polaris (coincidentally a
class F supergiant), Sigma pins the sky's apparent rotation axis,
Polaris at the northern end, Sigma at the southern, from which
comes its little-used proper name, "Polaris Australis." Faint as
it is, Sigma, only a bit over a degree off the pole, is not a bad
pole star and (because of precession) just a century ago was
better at it than Polaris was (at that time) in the north.
Dim and mostly ignored the three may be. But lose your way, and
from almost anywhere on Earth you can find one of these venerable
devices to lead you home.
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 May/Augusg 2005 Newsletter of the Lowestoft and Great
Yarmouth Regional Astronomers, who are gratefully acknowledged.