Most of the constellations and asterisms we see in the night’s sky are not, in reality, collections of objections with any particular close connection to each other. They are mostly just a set of stars that appear in the same part of the celestial sphere as viewed from Earth, but could be spread out over thousands of light years. The bright stars of Orion, for instance, include members that are less than 30 light years away, and others over 1300 light years away.
However, there are also genuine associations of stars: large clusters that are relatively compact that share a common history. One of the most easily seen of these is the Scorpius-Centaurus Association (Sco-Cen).
Sco-Cen is an OB association: that is to say, it is a collection of large young hot stars of the O and B spectral classes. O-class stars burn fast, blue and bright, with surface temperatures around 35000 K, and last only a million years or so before going supernova. B-class stars are a little cooler, from 10000 to 30000 K, but are still very bright and will last 3 to 20 million years, and they too will end up as supernovas.
OB associations tend to form in dense molecular clouds, in small clusters of stars like sacs of eggs, before the stars spread and “pop” in supernovas.
Sco-Cen is centred about 400 light years away, which is fairly nearby on a galactic scale, and is approximately 100 light years across. It is the closest OB association to us. Although the cluster as a group has fairly consistent direction of motion with respect to our solar system, it is also continuing to spread slowly, at a few hundred thousand km per year.
There have been a few dozen supernovas in this association over the last ten million years, which can be determined from supernova remnants, and from compact supershells/superbubbles (cavities). These have been matched to fine geological strata here on Earth using elemental or isotope analysis.
The original molecular cloud that spawned Sco-Cen is no longer intact but there are several little molecular clouds around the area, some of which are continuing to form their own little OB associations. These molecular clouds (“dark nebulae) include the popular Coalsack Nebula that I photographed the other day, and Barnard 68, which is so black that even the white bits are black.
All up there are more than 400 stars in Scorpius Centaurus Association, including most of the bright stars in that part of the sky, several of them more than 10000 times brighter than the Sun, intrinsically. However, this total also includes dozens of dimmer stars right down to brown dwarfs. It’s particularly rich in multiple star systems.
The set holds most of the bright stars in Crux, Centaurus, Lupus and Scorpius, as well as several stars in Norma, Musca, Libra, Sagittarius, Ophiuchus, Ara, Circinus and Corona Australis. All up the association includes 138 stars brighter than apparent magnitude 6.0, and 84 brighter than 5.0.
To illustrate, I’ve taken a snap of that region from the Stellarium app, and have coloured purple all of the Sco-Cen stars above mag 5.0. It would be a pretty bare area were it not for this association.
I’ve long regarded that part of the sky as being very rich in easy targets for amateur astronomy: plenty of dark nebula, bright nebula, small clusters. It was interesting and pleasing to learn that so many of those targets are actually causally and physically connected in a relatively compact area in actual 3D space.
https://zenodo.org/record/895414#.X0CvjnV95hE
Observation of 23 Supernovae That Exploded
https://en.wikipedia.org/wiki/Loop_I_Bubble
Loop I Bubble: a prominent superbubble in the Sco-Cen area.
https://arxiv.org/abs/1801.08540
The Kinematics of the Scorpius-Centaurus OB Association from Gaia DR1
https://arxiv.org/pdf/astro-ph/9809227.pdf
