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The present Sun is a G2 V star with a surface effective temperature of approximately 5780 K. Stellar
evolution theory indicates, however, that the Sun has shifted in spectral type by several subclasses,
becoming hotter by a few hundred degrees and becoming more luminous (the bolometric luminosity of the
Sun in its zero-age main-sequence [ZAMS] phase amounted to only about 70% of the present-day output;
Siess et al. 2000
). In understanding the solar past, we must therefore also consider stars of mid-to-late
spectral class G. On the other hand, alternative evolutionary scenarios have suggested continuous mass loss
from the young Sun at a high rate that would require a somewhat earlier spectral classification of the
young Sun (Sackmann and Boothroyd, 2003). In any case, magnetic activity in the outer stellar
atmospheres is predominantly controlled by the depth of the stellar convection zone and stellar
rotation, both of which also evolve during stellar evolution. For our understanding of magnetic
activity, the precise spectral subclass is rather likely to play a minor role. When discussing “solar
analogs”, I will therefore concentrate on stars mostly of early-to-mid-G spectral types but will
occasionally also consider general information from outer atmospheres of somewhat lower-mass stars if
available.
The situation is more complex for stars in their PMS stage. The Sun spent much of its PMS life as a mid-K (K5 IV) star when it moved down the Hayashi track. But again, the precise spectral subtype matters even less for magnetic activity in this stage, the more important key parameters being the age of the star (controlling its total luminosity, its radius, and the development and therefore the depth of the convection zone), the presence and dispersal of a circumstellar disk (controlling mass accretion and, via magnetic fields, the spin of the star), and the presence and strength of outflows (controlling, together with accretion, the final evolution of the stellar mass). A somewhat more generous definition of “pre-main sequence solar analogs” is clearly in order, given that the Sun’s history of rotation, accretion, the circumsolar disk, and the solar mass loss cannot be precisely assessed. Quite generally, I will take solar-like stars in the PMS phase to be, from the perspective of “magnetic activity”, stars with masses of roughly 0.5–1.5 solar masses, covering spectral classes from early G to late K/early M.
The expression “solar twin” (Cayrel de Strobel and Bentolila, 1989) is occasionally used. This term
should be used solely in the context of a solar analog with an age close to the Sun’s, i.e., of order 4–6 Gyr,
an age range in which the internal structure and the rotation period of a 1 
(and therefore,
its activity level) evolve only insignificantly. Efforts toward identifying real solar twins have
been important in the context of putting our Sun into a wider stellar context; nearby solar
analogs that are essentially indistinguishable from the Sun with regard to spectral type, effective
temperature, gravity, luminosity, age, rotation, and magnetic activity (Porto de Mello and
da Silva, 1997) prove that the Sun can be robustly used as an anchor to calibrate evolutionary
trends – the Sun is not an exception but is representative of its age and mass, a conclusion
also reached by Gustafsson (1998) from a rather general comparison of the Sun with sun-like
stars.1
Table 1 gives a list of terms, symbols, and acronyms used throughout the text.
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| http://www.livingreviews.org/lrsp-2007-3 | 
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 2.0 Germany License. Problems/comments to |
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