STELLAR STORIES

HUNTING ORION

By Jim Kaler

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 Sword of 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!"

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 nebula.

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 stars."

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 another.

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.