To the ancient Arabs, she was -- loosely translated -- the
"central one" ("al jauza"), the one in the middle, and yes, "she"
is correct. From ancient Greece we call the figure Orion, now a male and representative of the great
Hunter. No culture can ignore this magnificent constellation. Why
"central" to Arabians, no one knows, though we might speculate that
it comes from Orion's neighbors, as he is paid homage to by a
remarkable set of brilliant constellations that surround him (or
her): Canis Major, Canis Minor, Gemini, Auriga, Taurus, all with first magnitude stars. Even ancient
Eridanus the River laps at his feet, while
below him cowers Lepus, the Hare.
As if absorbing energy from this celestial ring, Orion blazes
forth with two first magnitude stars, the red and blue supergiants
Betelgeuse (Alpha Orionis) and Rigel (Beta), the "hand" and "foot" of al
jauza herself, who still rings within these stellar names
(pronounce them to see). Between the two stars lies the stunning
trio that makes the Hunter's Belt,
but to the Arabs was a string of
pearls, from right to left (nearly straddling the celestial
equator) Mintaka, Alnilam, and Alnitak (Delta, Epsilon, and Zeta). At the
upper right corner is the "womanly warrior" Bellatrix (Gamma), while at lower left is
"Saiph" for "sword," the true
Orion not here but hanging from the Belt, the The Hundred Greatest Stars" contains
four of Orion's and refers to one other, the constellation thus
accounting for five percent of the set.
We tell our students and friends that constellations are no
more than random groupings of stars, which is not quite correct.
Orion is instead made of a grand assembly of hot, blue-white,
massive class O and B stars called Orion OB1. Such "
OB associations" are loosely organized, gravitationally unbound
systems whose stars were born more or less contemporaneously from
the same giant dark interstellar molecular cloud. (Perseus, Scorpius, and Centaurus are other good examples.)
O and B stars do not live long, so OB associations are all
young; we see the Hunter in the full flame of youth! Orion must
therefore also be a hotbed of star formation. We can even follow
his progression. The more massive the star, the shorter its
lifetime (the relation calculated through theory). We can
therefore determine the age of any stellar group by finding the
most massive star remaining. In decreasing order of age, Orion's
association breaks down into four subgroups:
To these remarkable sets add the Lambda Ori association that
centers on Meissa (Lamda Ori itself),
which makes Orion's head (1400 light years, 5 million years old).
Orion is hardly a "random grouping!"
- Ia (stars up and to the right of the Belt), 1100 light years
away, 10 million years old;
- Ib (the Belt and neighbors, including the wonderful multiple
Sigma Ori), 1550 light years, 2 to 5 million years old;
- Ic (the true Sword), 1650 light years, 2 million years
- Id (the Orion Nebula and the
cluster), 1500 light years, less than one
million years old.
Orion stands in front of the current action, which takes place
in the dark, dusty Orion Molecular Cloud ("OMC"), a northern branch
stretching from the Belt to Betelgeuse, a southern from the Belt
through the Sword to Saiph. In the middle of the southern
extension is the focus of the backyard telescope, the Orion Nebula, the classic "diffuse
nebula." Within this cloud, some 20 light years across, we see
complex swirls of glowing gas punctuated by cold dark clouds that
lie near and in front of it. The Orion Nebula shines by a form of
fluorescence, its radiating gases ionized by the ultraviolet light
from a set of four hot stars that appear in the middle collectively
known as the Trapezium (Theta-1 Ori), most of the energy coming
from Theta-1 C, a 40-solar-mass O6 star heated to 40,000 Kelvin.
Though the stars look to be embedded in the nebula, they are
instead located in front of it, the Orion Nebula actually a blister
on the surface of the dark backdrop of the OMC. Buried within the
OMC are yet more young massive stars known only through their
penetrating infrared radiation, stars even now just forming, stars
that will someday eat away even more of the cloud and that maybe
will form another visible nebula. The Trapezium (all of which but
"C" are double or multiple) is actually at the core of a vast,
dense true cluster of fainter stars that swarm in front of the
To the east, associated with Sigma
Ori lies another bright cloud, one that highlights the famed
dark "Horsehead Nebula," in which stars are being born. Everywhere
among these dark clouds we see the signatures of the formation
process. As stars condense gravitationally from their cold cloudy
birthplaces, some of the infalling matter spins out into a disk,
from which emerge opposing jets. Indeed, in front of the Orion
Nebula we see numerous dark disks that hold embedded forming stars.
Such disks -- which surround all budding stars -- are reminiscent
of the disk that once surrounded the early Sun and that produced
(by accumulation of dust grains) the planets. Unless something
arrests their development, planets seem to be a natural by-product
of star formation.
The Trapezium region reveals the violence of stellar
encounters. Not far from it is Iota
Orionis, a visual triple, the bright member of which is itself
a tight spectroscopic class O
binary. Speeding away from the Trapezium in opposite directions at
a separation velocity of 200 kilometers per second are AE Aurigae (to the north) and Mu Columbae (to the south). Tracing the movements of
the stars back in time, they all cross paths. Around 2.5 million
years ago, two massive binaries smashed into each other, exchanged
stars, and hurled two others outward to become classic "runaway
While the focus is on star birth, Orion, filled with massive
stars, must also be a place for stellar death, and not quiet death
either. Stars are internally supported by energy released by
nuclear fusion, the creation of heavier atoms from lighter ones.
All the massive stars of Orion's OB associations, those above 10 or
so solar masses, are destined to fuse their central hydrogen cores
first to helium (going on now), then to carbon, finally to heavier
elements leading to iron. Iron, however, cannot fuse to anything
and produce energy at the same time. Once formed, an iron core
(typically the size of Earth with a mass half again that of the
Sun) will collapse catastrophically into a ball of neutrons (a neutron star!) about the
size of a small town. The sudden release of gravitational energy
explodes the rest of the star away, and a supernova graces the sky.
Supernovae send violent shock waves into their surroundings that
can trigger star formation, and we are back to the beginning.
Orion is thus a seat of "sequential" star formation, one group of
stars causing another to be created, one OB association following
Amidst all of this unseen action, rest your eyes again on Betelgeuse, which singularly illustrates
the power of a supernova. Here is a star (along with Rigel) in a
more advanced state of evolution, one most likely fusing helium
into carbon and oxygen. Closer to us than the blue-white OB
associations, Betelgeuse is about as big as the Solar System's
asteroid belt, and has an extended atmosphere that is nearly the
size of the orbit of Jupiter. Losing mass, Betelgeuse has created
a dusty shell with a radius some 300 times the size of the orbit of
Pluto. With a mass estimated at around 15 times that of the Sun, Betelgeuse should someday explode. If it
were to go now, it would light the land with the radiant power of
a gibbous Moon. What wonder it would be if we could step out of
time and watch Orion's future speeded up, as one after another of
these massive stars pops off, leaving both devastation and new
stellar life behind.
Orion, as in the myths of the ancient Hunter, is doomed, his
stars exploding him into non-existence (stellar motions doing that
anyway). Enjoy him now, but weep for him not, as new stellar
generations will be born within the dark clouds that will recreate
him -- or her -- for the generations of ourselves yet unborn.
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/April 2005 Newsletter of the Lowestoft and Great
Yarmouth Regional Astronomers, who are gratefully acknowledged.