LeDrew - Orion OB1

A Brief History of the Stars of Orion

The brightest stars of most constellations typically have a wide range of age and distance. Orion is one of the few exceptions. Its prominent stars are physically related.

Orion hosts one of the most-studied star-forming regions in the heavens: the Orion OB1 association. Located far from the band of the Milky Way means astronomers studying the association encounter little confusion with other galactic structures. And the relative nearness in space allows examination of the stars in great detail.

LeDrew - Orion chart
This chart plots stars to magnitude 11. All labelled objects, with the possible exception of Bellatrix, are known or suspected to be related to the Orion stellar association. For stars, the data in brackets are the M-K spectral classification (of the primary only, if binary or multiple). For the four stars not lying within an association boundary, the distance in light-years is also provided. For associations, as well as the sparse cluster Cr 69, the distance in light-years and the age in millions of years are given. An arrow beside Betelgeuse indicates its direction of motion across the sky. (Illustrations by Glenn LeDrew)

The leading players of the show are well known to any backyard skywatcher, especially Orion’s brilliant belt and sword region. In our examination of this rich celestial real estate, we highlight the brightest stars, the subgroups of the association and the observable nebulosities and star clusters. To start, some historical perspective on what we see.

How Orion Happened

The first of the bulk of Orion’s prominent stars were born very recently in astronomical terms, about 12 million years ago, when the expanding shock wave from the then 20-million-year-old Gould’s Belt stars reached the massive molecular clouds that ultimately gave birth to many of Orion’s stars. A probable exception is Bellatrix (γ Orionis), lying significantly nearer to us, which is most likely a Gould’s Belt star of a somewhat earlier generation. Thereafter, within a few million years, young massive stars began exploding as supernovas, sculpting out the huge Orion-Eridanus Bubble. The most prominent part of this bubble is Barnard’s Loop, made visible as the shock slams into the denser gas closer to the galactic equator. The opposite edge has expanded more rapidly away from the association, far beyond the right edge of the chart, in Eridanus and Taurus (the near edge is within 500 light-years of us). Deep images of Orion’s southern half reveal a tortured landscape of molecular cloud fragments possessing cometlike tails pointing away from the scene of the crime. By happy coincidence, one of the largest of these just happens to be made visible by the light of nearby Rigel (β), resulting in the huge reflection nebula IC2118, the famous Witch Head Nebula.

LeDrew - Orion OB1
Detail of chart showing the Orion OB1a and OB1b associations.

The core of the first bout of star formation is subgroup OB1a, located just to the west (right) of Orion’s belt. Current evidence suggests that this is the parent group of the runaway star Betelgeuse (α). Drifting away more slowly southward is η Orionis. A little nearer to us, and also showing a slow overall expansion, are the outliers Rigel and Saiph (κ). Some four million years ago, a supernova in subgroup OB1a sent the then companion (now runaway) star 53 in Aries on its way.

Around the time of the first supernova blast or two, subgroup OB1b formed. Its luminaries are the three belt stars and the in­teresting multiple σ Orionis, a backyard-telescope favourite. Other members make up the dispersed swarm of stars called open
cluster Cr 70, centred on Alnilam (ε). This subgroup abuts the western edge of the Orion molecular cloud complex, a small portion of which is being evaporated and ionized by σ Orionis, creating the emission nebula IC434. The superimposed stubby finger of dense, dark molecular gas universally known as the Horsehead Nebula stoutly resists ablation. Partially embedded within the molecular cloud is the Flame Nebula (NGC2024), whose ionizing source is hidden behind the dark band bisecting it.

Not long after the appearance of the belt stars, the stars of Orion’s sword, subgroup OB1c, were forged. The standout member here is ι Orionis, the source of the runaway stars AE Aurigae and μ Columbae, slingshot out in opposite directions 2.5 million years ago. Adding lustre to the sword are star cluster NGC1981 and the sparse group that is illuminating the reflection nebula NGC1973/5/7. As with any association that still contains a reservoir of molecular gas, a single age cannot strictly apply, because star formation is ongoing. Like the belt, this group is perched on the west edge of the Orion molecular cloud complex. The Trapezium cluster, brilliantly lighting up M42, is the latest adornment to appear, a few hundred thousand years ago. Indeed, the Trapezium group is referred to as subgroup OB1d.

HST Orion Nebula
This spectacular portrait of the Orion Nebula was taken in 2006 by the Hubble Space Telescope and is the most detailed image so far of the massive star cloud. The nebula is visible to the un- aided eye as the middle “star” in Orion’s sword (star formation subgroup OB1c). At a distance of 1,350 light-years, it is the closest region of massive star formation to Earth. The nebula is approximately 25 light-years across and about 2,000 times the mass of the Sun.

At the time of the creation of the sword, Orion’s head appeared on the stage. Meissa (λ) and the sparse cluster Cr 69 began evacuating a bubble of hot gas. This process was augmented 300,000 years ago when a supernova resulted in the neutron star Geminga. Hurtling east-northeastward at over 200 kilometres per second, it is located just across the border in Gemini. Before that, over one million years ago, a pair of runaways was dynamically ejected (perhaps by λ Orionis itself?) on opposite courses; they currently lie just outside the area of this chart. The interior of the bubble is illuminated as the vast nebula Sh2-264 (a.k.a. the Lambda Orionis Nebula). It is rimmed by numerous molecular clouds and fragments, a few of them catalogued by E.E. Barnard as dark nebulas.

With the possible exception of Bellatrix, all the stars labelled here (including the Trapezium members in M42) are destined to explode as supernovas; the first is expected to be well-evolved Betelgeuse. But the action will not peter out with the demise of these stars. Further rashes of star formation are yet to come, particularly in the million-solar-mass Orion molecular cloud complex, which occupies much of the east side of the heroic figure Orion. Early hints are revealed by reflection nebulas NGC2023, M78 and NGC2071, their illuminating stars having recently popped “blisters” on the near side of the vast, obscuring mass of dusty gas.Thus far, at least 10,000 stars have been minted over the past 12 million years, most below naked-eye visibility. Many more thousands are destined to be born, with new supergiants and their spectacular ends along with star clusters and nebulas awaiting who knows what eyes in the distant future.

For telescope explorers of Orion, here are a few of my own observations made in dark sky locations that may be of interest: I’ve seen the brighter northern half of IC2118 as a ghostly presence through 25×100 binoculars. Reflection nebula NGC1788 is small but has a reasonably high surface brightness that makes it readily visible through a moderate aperture. I’ve glimpsed the brightest portion of Barnard’s Loop, near M78, through 8×50 binoculars. The Flame Nebula, NGC2024, is visible under a reasonably dark sky with 15×70 binoculars. If not for the glare of nearby ζ Orionis, it would rank as one of the easier nebulas for small telescopes. The Lambda Orionis Nebula has reportedly been seen with just the unaided eye and a Hydrogen-beta filter (but not by me — not yet anyway).

Deep-sky expert Glenn LeDrew of Ottawa is a regular contributor to SkyNews.