During the past 106 years, the Sun has moved from the very hot ( 106 K) low-density “local bubble” environment into the cooler and denser ( 6900 K, n(H0 + H+) 0.3 cm–3) “local interstellar cloud” (Frisch, 2000; Frisch and Slavin, 2005, 2006). The pressure characteristics of these two environments are similar, so that the radii of the heliosphere would have been similar (120 – 130 AU; Frisch 2000). However, the heliospheric structure would shrink to ten percent of its present size if the Sun encountered molecular clouds, and such encounters are likely to have occurred several times in the past (Frisch, 2000).
Moving back in time, the evidence becomes indirect. The best “memory” of the earliest episodes of the Sun’s life is contained in meteorites (Section 6.6). The presence of live 60Fe in the early solar system (see Section 6.6) inferred from meteoritic trace elements (Tachibana and Huss, 2003) cannot be explained by local processes (such as stellar flares, see Section 6.6) but is thought to require supernova explosions (Meyer and Clayton, 2000). The local environment of the forming solar system was therefore likely reminiscent of a high-mass star-forming region like the Orion region; the young Sun and its circumstellar disk may have resided in a H ii region for a considerable amount of time (Hester et al., 2004); the intense ultraviolet radiation field from massive stars might have contributed to the evaporation of the molecular environment of the Sun (so-called proplyds in the Orion Nebula, O’Dell 2001, or “evaporating gaseous globules” [EGGs], Hester et al. 1996). These structures contain protostars that can be detected in X-rays (Kastner et al., 2005; Linsky et al., 2007), i.e., deterioration of the environment is due both to the larger-scale “interstellar” environment and the stellar magnetic activity itself.
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