SELF

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O.N. Karavashkina and S.B. Karavashkin

Now let us complement a little this representation. Consider a region of interstellar medium satisfying the above condition (2.1) of self-compression (see Fig. 2.8.1).

Fig. 2.8.1 - 2.8.2

As the primary nebula densifies, the collisions of atom of neutral hydrogen of which this nebula mainly consists become more frequent. First collisions are elastic, but as the substance densifies (or due to some external affection - heating by a near hot star or above mentioned compression wave) the collisions become inelastic. In such collisions the hydrogen atoms partly transit into molecular state and partly ionise and emit quanta of infrared radiation. Until the nebula is rarefied and transparent, the radiation easy scatters into the outer space (see Fig. 2.8.1 where the compressive force of cloud is denoted by green arrows). However as the gas compresses, the collisions become more intensive and gas more opaque for radiation. Consequently, the medium heats; in its turn, it makes the collisions in gas more frequent and beefed-up. As the process develops, we notice within the clouds of ionised hydrogen an infrared source (see Fig. 2.8.2 where the infrared radiation is denoted by red arrows) which can be seen as a point either a spot, dependently on the distance.  

Fig. 2.9. Radio isophotes of the CO line in the Orion nebula [1, p. 97, Fig. 31].

We have available the examples of such stage. In Fig. 2.5 shown above, we saw the isophotes of light absorption by the dark cloud in Serpens and the accumulation in it the infrared sources just in the region of maximal absorption. Absolutely so in Orion (Fig. 2.9) where some time ago (by calculations, about 50 000 years) there also happened a near explosion, we see a bright compact region of neutral hydrogen as if cleft by a dark, dense molecular cloud (5*10 ⁴ cm ^{- 3}  with general mass of the cloud  2000 M ). "In the region of two maximums of brightness of the CO line corresponding to the most dense parts of molecular cloud ( n_H2 2*10 ⁶ cm ^{- 3}   with the mass  200 M ), we observe the sources of long-wave infrared radiation" [1, p. 97]. To the north-west from this cloud we see a path of young hot stars whose age grows to the north-west. Apparently, some time ago this cloud was larger, but under affection of the near burst, its north-west part has already concentrated into stars, and we are observing the going on process.

We should mark, the denser is cloud the longer new sources of infrared radiation will be invisible for observer. Consequently, the sources seen from without are already enough powerful - accumulation and density of ionised hydrogen are already very large. These are the developed protostars.