The Scorpius-Centaurus Association (sometimes called Sco-Cen or Sco OB2) is the nearest OB Association to the Sun. This stellar association is composed of three subgroups (Upper Scorpius, Upper Centaurus-Lupus, and Lower Centaurus-Crux), whose mean distances range from 380 to 470 light years[1].
The Sco-Cen subgroups range in age from 5 million years (Upper Scorpius) to roughly 15 million years (Upper Centaurus-Lupus and Lower Centaurus-Crux). Many of the bright stars in the constellations Scorpius, Lupus, Centaurus, and Crux are members of the Sco-Cen association, including Antares (the most massive member of Upper Scorpius), and most of the stars in the Southern Cross[2]. Hundreds of stars have been identified as members of Sco-Cen, with masses ranging from roughly 15 solar masses (Antares) down to below the hydrogen-burning limit (i.e. brown dwarfs)[3], and the total stellar population in each of the three subgroups is probably of the order 1000-2000[4]. The Sco-Cen OB association appears to be the most pronounced part of a large complex of recent (<20 million years) and ongoing star-formation. The complex contains several star-forming molecular clouds in Sco-Cen's immediate vicinity—the Rho Oph, Pipe Nebula, Barnard 68, Chamaeleon, Lupus, Corona Australis, and Coalsack cloud complexes (all at distances of ~120-200 parsecs), and several less populous, young stellar groups on the periphery of Sco-Cen, including the ~3-5 million-year-old epsilon Cha group, ~7 million-year-old eta Chamaeleontis cluster (also called Mamajek 1), ~8 million-year-old TW Hydrae association, ~12 million-year-old Beta Pictoris moving group, and possibly the ~30-50 million-year-old IC 2602 open cluster[2].
The stellar members of the Sco-Cen association have convergent proper motions of approximately 0.02-0.04 arcseconds per year, indicative that the stars have nearly parallel velocity vectors, moving at about 20 km/s with respect to the Sun. The dispersion of the velocities within the subgroups are only of order 1–2 km/s[5], and the group is most likely gravitationally unbound. Several supernovae have exploded in Sco-Cen over the past 15 million years, leaving a network of expanding gas superbubbles around the group[6], including the Loop I Bubble. To explain the presence of radioactive 60Fe in deep ocean ferromanganese crusts, it has been hypothesized that a nearby supernova, possibly a member of Sco-Cen, exploded in the Sun's vicinity roughly 3 million years ago[7].