Orion star chart7/2/2023 ![]() ![]() As with any association that still contains a reservoir of molecular gas, a single age cannot strictly apply, because star formation is ongoing. Adding lustre to the sword are star cluster NGC1981 and the sparse group that is illuminating the reflection nebula NGC. 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. Not long after the appearance of the belt stars, the stars of Orion’s sword, subgroup OB1c, were forged. Partially embedded within the molecular cloud is the Flame Nebula (NGC2024), whose ionizing source is hidden behind the dark band bisecting it. The superimposed stubby finger of dense, dark molecular gas universally known as the Horsehead Nebula stoutly resists ablation. 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. Other members make up the dispersed swarm of stars called openĬluster Cr 70, centred on Alnilam (ε). Its luminaries are the three belt stars and the interesting multiple σ Orionis, a backyard-telescope favourite. Some four million years ago, a supernova in subgroup OB1a sent the then companion (now runaway) star 53 in Aries on its way.Īround the time of the first supernova blast or two, subgroup OB1b formed. A little nearer to us, and also showing a slow overall expansion, are the outliers Rigel and Saiph (κ). Drifting away more slowly southward is η Orionis. Current evidence suggests that this is the parent group of the runaway star Betelgeuse (α). The core of the first bout of star formation is subgroup OB1a, located just to the west (right) of Orion’s belt. 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.ĭetail of chart showing the Orion OB1a and OB1b associations. 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. 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). 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. Thereafter, within a few million years, young massive stars began exploding as supernovas, sculpting out the huge Orion-Eridanus Bubble. 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. 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. ![]() To start, some historical perspective on what we see. 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. The leading players of the show are well known to any backyard skywatcher, especially Orion’s brilliant belt and sword region. An arrow beside Betelgeuse indicates its direction of motion across the sky. For associations, as well as the sparse cluster Cr 69, the distance in light-years and the age in millions of years are given. For the four stars not lying within an association boundary, the distance in light-years is also provided. For stars, the data in brackets are the M-K spectral classification (of the primary only, if binary or multiple). All labelled objects, with the possible exception of Bellatrix, are known or suspected to be related to the Orion stellar association. ![]()
0 Comments
Leave a Reply. |