L

Temperature log (reff) (K)

Figure 1.18 Pre-main-sequence evolutionary tracks. Each track is labeled by the stellar mass in units of Mq. The grey curves are isochrones, labeled in yr. The t = 0 isochrone coincides with the birthline, the lighter solid curve at the top. Note that the t = 1 x 108 yr isochrone nearly matches the ZAMS, the lighter solid curve at the bottom.

The pre-main-sequence phase of evolution is not the first. At an earlier epoch, stars are still forming out of the gravitational collapse of their parent dense cores. Such protostars are even more luminous than pre-main-sequence stars, but are so obscured by interstellar dust that their emitted radiation lies entirely in the infrared and longer wavelengths. Under these circumstances, Teff cannot be found by the traditional methods. Indeed, the character of the observed spectrum reflects more the properties of the dust surrounding the star than the stellar surface. Protostars therefore cannot be placed in a conventional HR diagram, and their identification within molecular clouds is still not secure.

Returning to pre-main-sequence evolution, we noted earlier that the contraction of any star causes its central temperature to rise. As long as the stellar mass exceeds 0.08 Mq, the temperature eventually reaches a value (near 107 K) where hydrogen fusion begins. Just at this point, the star is said to be on the zero-age main sequence (ZAMS), and the corresponding relation between L* (or Lboi) and Teff marks a rather precise locus in the HR diagram. The ZAMS is the curve actually shown in both Figures 1.15 and 1.18, and its properties are also listed in Table 1.1. The reader should bear in mind that the table represents a one-parameter family, where the basic physical variable is the stellar mass. Note also that astronomers sometimes use, in place of Lboi, the bolometric magnitude:

where the constant m0 is +4.75. The difference Mbol — MV for a main-sequence star of any spectral type is known as the bolometric correction, here evaluated in the V -waveband.

Table 1.1 Properties of the Main Sequence

Mass

Sp. Type

My

log ¿bei

log TeS

¿MS

(M0)

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