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.
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