The ScorpiusâCentaurus association (sometimes called ScoâCen or Sco OB2) is the nearest
OB association to the
Sun. This
stellar association is composed of three subgroups (Upper Scorpius, Upper CentaurusâLupus, and Lower CentaurusâCrux) and its distance is about 130
parsecs or 420
light-years.[1] Using improved
Hipparcos data, Rizzuto and colleagues analysed nearby stars more closely, bringing the number of known members to 436. They doubt the need to add a subclassification because they found a more continuous spread of stars.[2]
The ScoâCen subgroups range in age from 11 million years (Upper Scorpius)[3] to roughly 15 million years (Upper CentaurusâLupus and Lower CentaurusâCrux). Many of the bright
stars in the
constellationsScorpius,
Lupus,
Centaurus, and
Crux are members of the ScoâCen association, including
Antares (the most massive member of Upper Scorpius), and most of the stars in the
Southern Cross.[4] 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),[5] and the total stellar population in each of the three subgroups is probably of the order 1000â2000.[6]
The ScoâCen 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 ScoâCen's immediate vicinityâthe
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 ScoâCen, including the ~3â5 million-year-old
Epsilon Chamaeleontis group, ~7 million-year-old
Eta Chamaeleontis moving group, ~8 million-year-old
TW Hydrae association, ~12 million-year-old
Beta Pictoris moving group, and possibly the ~30â50 million-year-old
IC 2602 open cluster.[4]
The stellar members of the ScoâCen association have convergent
proper motions of approximately 0.02â0.04
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 1â2 km/s,[7] and the group is most likely gravitationally unbound. Several
supernovae have exploded in ScoâCen over the past 15 million years, leaving a network of expanding gas
superbubbles around the group,[8] including the
Loop I Bubble.
To explain the presence of
radioactive60Fe in deep ocean ferromanganese crusts and in biogenic magnetite crystals within Pacific Ocean sediments[9] it has been hypothesized that a nearby
supernova, possibly a member of ScoâCen, exploded in the
Sun's vicinity roughly 3 million years ago,[10] causing the
PlioceneâPleistocene boundary marine extinction.[11] However, other findings cite the distance at which this supernova occurred at more than 100 parsec, maintaining that it is not likely not to have contributed to this extinction through the mechanism of what is known as the ultra-violet B (UV-B) catastrophe.[9][12]
In 2019, researchers found interstellar iron in Antarctica which they relate to the
Local Interstellar Cloud, which might have been formed near the Sco-Cen association.[13]
In December 2021, around 70 new
rogue planets were discovered in the Upper Scorpius association.[14]