The Scorpius–Centaurus association (also known as Sco–Cen or Sco OB2) is the closest OB association to the Solar System, composed of three subgroups (Upper Scorpius, Upper Centaurus–Lupus, and Lower Centaurus–Crux) and located at a distance of 420 light-years (130 parsecs) from the Sun. Analysis using improved Hipparcos data has brought the number of known members to 436. The cluster shows a continuous spread of stars with no apparent need for subclassification.
The Sco–Cen subgroups range in age from 11 million years (Upper Scorpius) to roughly 15 million years (Upper Centaurus–Lupus and Lower Centaurus–Crux). Many of the bright stars in the constellations of Scorpius, Lupus, Centaurus, and Crux are in fact the brightest members of the Sco–Cen association, including the red supergiant star Antares (the most massive member of Upper Scorpius), and most of the stars in the Southern Cross. The total stellar population in each of the three subgroups is probably of the order 1000–2000, and the total number of stars in the association exceeds 10,000.
The stellar members of the Sco–Cen association have convergent proper motions of approximately 0.02–0.04 arcseconds per year, indicating 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, 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, including the Loop I Bubble.
To 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, causing the Pliocene–Pleistocene boundary marine extinction and leaving radioactive 60Fe in deep ocean ferromanganese crusts and in biogenic magnetite crystals within Pacific Ocean sediments. 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 an ultra-violet B (UV-B) catastrophe. 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.
Observation
The Scorpius-Centaurus Association is one of the most conspicuous and easily recognizable galactic structures in the entire celestial vault, as well as, in terms of apparent size, the largest OB association visible from Earth. It extends for over fifty degrees and includes a large number of very bright blue stars, whose apparent magnitudes can reach first magnitude, making them visible even from the centers of large cities. Generally speaking, it can be said that almost all the bright stars that make up the constellations of Lupus, the northern part of Scorpius, Centaurus and the Southern Cross belong to this association; only a few stars are exceptions, such as Alpha Centauri, Gacrux, Theta Centauri and Iota Centauri, which appear in this region of the sky only due to visual perspective, as they are located in the foreground with respect to the association. The association appears tangent to the luminous trail of the Milky Way, inclined, with respect to it, by more than 20°.
The Scorpius-Centaurus Association lies entirely in the Southern Celestial Hemisphere. The northernmost part, coinciding with the Antares region, lies at an average declination of −23° and is visible without excessive difficulty—even from much of the Northern Hemisphere. The central segments, coinciding with the constellations of Lupus and Northern Centaurus, lie on average at 40°S and can only be observed from lower temperate, Mediterranean, and subtropical latitudes. The southern section extends into the southernmost part of Centaurus and the Southern Cross, until it touches Musca and ends in Carina with the bright open cluster of IC 2602 (the Southern Pleiades). The southern section is only easily observable from tropical regions, and it is no coincidence that from the Southern Hemisphere it appears to be circumpolar and extend throughout the extratropical belt.
Due to its wide extension across the starry sky, a complete and clear view of the association can only be obtained from regions in the Southern Hemisphere, where the portion of the Milky Way affected by its presence, which coincides with the southernmost part, appears high on the horizon. Global views are also possible north of the equator, in the lower tropical zone, provided the southern horizon is completely clear of obstacles. The ideal time for its observation in the evening hours is between March and June, its presence high in the sky during southern nights indicates the advance of the autumn season, while in the Northern Hemisphere, the northern parts of Centaurus and Scorpius appearing in the southeast indicate the imminent arrival of the summer season.
Due to the precession of the equinoxes, the south celestial pole is slowly moving towards the southwestern part of the association, between the Southern Cross and Carina. Within a few thousand years, when the south celestial pole will point towards the Milky Way and the False Cross asterism, the stars of the Scorpius-Centaurus association will have reached their southernmost point. As the Earth's axis moves away from that region of the sky, the constellations of Scorpius and Centaurus will assume increasingly northerly declinations, until they even move partly north of the celestial equator.
Characteristics The Scorpius–Centaurus (Sco–Cen) association region exemplifies a medium-scale star formation process, wherein giant molecular clouds produce stars across a broad mass spectrum, before disintegrating under the influence of stellar winds and supernova explosions from its progeny—which accumulate, compact, and erode residual gas and dust to trigger additional, often marginal, star formation episodes. As the most prominent component of a vast complex of recent (<20 million years old) and ongoing star formation, the Sco–Cen OB association encompasses numerous nearby molecular clouds at distances of approximately 120–200 parsecs, including the Rho Ophiuchi, Pipe Nebula, Barnard 68, Chamaeleon, Lupus, Corona Australis, and Coalsack complexes, these peripheral clouds, aligned along the association's inclination relative to the Milky Way, host relatively subdued low-mass star formation and form the edges of the broader Scorpius–Centaurus complex, featuring western structures like the Chamaeleon and Coalsack nebulas, and eastern ones such as the Lupus, Corona Australis, Rho Ophiuchi, and Pipe nebulas, oriented toward the galactic bulge. Surrounding Sco–Cen are several less dense young stellar groups, including the ~3–5 million-year-old Epsilon Chamaeleontis moving 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 ~30–50 million-year-old IC 2602 open cluster.
The three large subregions of the association are almost completely devoid of interstellar gas, which has been completely swept away. The stars present here also show a very low extinction rate, an indication of low obscuration due to dust. A complex ring-like structure has formed around the association, a low-density bubble whose edges are composed of molecular hydrogen (H I), whose mass is around 300,000 and could coincide with what remains of the large molecular cloud from which the association itself originated.
Compared to other regions of the Galaxy of more or less contemporary origin, the Scorpius-Centaurus association hosts a higher percentage of double or multiple systems, up to a rate 1.16 times higher than the average, and this percentage increases significantly if low-mass pre-main sequence stars, such as red dwarfs, are considered. A study conducted on almost 200 members of the first spectral classes (i.e. the blue-white stars of class B and A) located mainly in the northernmost part of the association, showed the presence of 176 companion stars, of which at least eighty are physically linked to the major stars around which they have been observed. On average, the masses of these minor stellar and substellar components identified vary from 0.03 to 1.2 .
Evolution
The dynamics that led to the formation of the various subgroups of the association are very complex and partly still unknown. Specifically, the processes that took place in the northern part of the association, visible in the direction of Scorpius, are known, while those that generated the two southernmost subgroups remain almost completely unknown. It is known that the age of the stars in the association increases as one proceeds in a westerly direction, a sign that the oldest star formation phenomena took place mainly in the region occupied by the constellation of Centaurus, it is also known that the stars located north of the galactic equator have a younger age (about 12 million years) than those located to the south (about 17 million years). Furthermore, the southern part appears to be at a distance of 109 parsecs, slightly closer than the northern part, located at 123 parsecs.
According to some very simplified models, star formation would have initially taken place in the northern part of the upper Centaurus-Lupus group, about 17 million years ago, and would have then extended southwards, to the lower Centaurus-Crux group, reaching its peak about 12 million years ago; the new stars would have been initially concentrated in small clusters and filaments surrounded by gas, containing tens or hundreds of stars. The residual gas of the progenitor molecular cloud would have subsequently been swept away by the combined action of the stellar wind and the supernova explosions of the most massive components, which completed their life cycle very quickly. About 6 million years ago these generative processes extended to the clouds located south of the galactic equator, in particular in the regions of the Chameleon Cloud and stars Epsilon and Eta Chamaeleontis.
Starting from 12 million years ago, the bubble formed by the wind of the young stars of the upper Centaurus-Lupus group began its expansion, perhaps further accelerated by the explosion of some supernova at a later time. About 5 million years ago, the expansion front of the bubble generated pressure and compressed the molecular cloud corresponding to the current group of stars that form the head of Scorpius, generating the youngest part of the association, the upper Scorpius group, also catalogued as Scorpius OB2. Through the knowledge of the physical distance between the two groups of the association, equal to about 60 parsecs, it has been calculated that the expansion velocity of this bubble was about 25 km/s.
The burst of star formation in the Northern Scorpius Cloud generated approximately 2,500 stars, including some particularly massive ones with masses exceeding 10 . These stars rapidly evolved and subsequently exploded as supernovae, generating, among others, the pulsar PSR J1932+1059. The powerful shock waves generated by these explosions almost completely swept away the residual gas of the ancient molecular cloud, whose remnants are visible in the delicate filaments known as Sh 2-1 and Sh 2-7. The shock waves also impacted the adjacent Rho Ophiuchi Cloud over the last million years, fueling the intense star formation activity in the region that can still be observed today. The same shock waves may also be responsible for the initiation of stellar genesis phenomena in the Lupus Molecular Cloud, where the oldest stellar components show an age less than 1 million years.
Structure