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Figure 1.11 Central region of Taurus-Auriga in 12C16O, along with the locations of infrared stars (squares) and optically visible T Tauri stars (triangles).

deeply embedded sources seen here in the infrared from optically visible, low-mass objects. The latter belong to a class known as T Tauri stars, named for the prototypical object indicated in Figure 1.9. Stars of the T Tauri class are but one type of pre-main-sequence star. In regions like Orion, one also finds more massive pre-main-sequence objects known as Herbig Ae/Be stars. Finally, we have seen that Orion (but not Taurus-Auriga) contains a number of O and B stars, which are main-sequence objects. Note that a given source is classified as a T Tauri or Herbig Ae/Be star based on specific observational criteria, while the pre-main-sequence and main-sequence designations refer to the presumed evolutionary state of the star.

It is evident from Figure 1.11 that both the infrared and optically visible young stars of Taurus-Auriga are confined to the denser molecular gas. The same is true for Orion, but the stellar distributions are quite different. The Taurus Molecular Cloud harbors a T association, in which stars are spread out more uniformly. Although some degree of clumping is evident, we do not see the extreme crowding of Orion. Recall, for example, that the entire Trapezium Cluster shown in Figure 1.5 has a diameter of about 0.4 pc, while each of the sparser groupings in Figure 1.10 extends over about ten times that length.

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