Molecular Clouds

While our galaxy has a mass of about 1011 MQ, and typical stars possess masses in the range 0.1-120 Mq, giant molecular clouds have masses of up to 106 Mq. They are the most massive objects in our galaxy and there are large numbers of them. Their name comes from the many molecules identified within them by radio astronomy, some of which are listed in Table 1.1. In addition to hydrogen in the form of H2, the most abundant molecules are OH, H2O, CO, and NH3. But they even harbor organic compounds, albeit much less complicated ones than those found in living organisms. Although more than 99% of the mass in molecular clouds is made up of gas, they also contain large quantities of interstellar dust, which cools through infrared radiation and serves to shield the cloud's interior from heating by stellar radiation. As a result, the cloud cores become very cold, with temperatures as low as 5-10 K, and resulting densities as large as 103-105 particles per cm3. These cloud cores are the seats of star formation, the separate stages of which are shown in Fig. 1.4.

Table 1.1. Molecules detected in molecular clouds (Wootten 2002)

Simple hydrides, oxides, sulfides, halides, and related molecules

Simple hydrides, oxides, sulfides, halides, and related molecules

Table 1.1. Molecules detected in molecular clouds (Wootten 2002)

h2

co

nh3

CS

NaCl

HCl

SiO

SiH4

SiS

A1C1

h2o

S02

CC

h2s

KCl

ocs

ch4

PN

A1F

Nitriles, acetylene derivatives, and related molecules

hcn

hc3n

ch3c3n

ch3ch2cn

CH2CH2

H3CCN

hc5n

ch3cch

ch2chcn

chch

ccco

hc7n

CH3C4H

hnc

cccs

HCgN

hnco

hcccho

HC11N

hncs

H3CNC

Aldehydes, alcohols, ethers, ketones, amides, and related molecul

H2CO

ch3oh

hcooh

CH2NH

ch2s

CH3CH2OH

ch3cooh

ch2nh2

CH3CHO

ch3sh

CH30CH3

nh2cn

NH2CHO

ch2co

Cyclic molecules

C3H2

SiC2

c3h

Ions

CH

HCO

hcnh

nh2

HCOO

so

HCS

Radicals

OH

C3H

CN

HCO

C2S

CH

c4h

C3N

NO

NS

c2h

CbH

ch2cn

so

c6h

HSiCC

hscc

When the density wave of a galactic spiral arm rushes over a molecular cloud, or a neighboring supernova explosion ejects its material against it, fragments of the cloud become compressed, and start to collapse under their own gravity. Figure 1.4a shows, for example, the molecular cloud L1152 (Hartmann 1998), which has been mapped using the radio lines emitted from its molecules CO, CS, and NH3. Giant molecular clouds are lengthy objects, a.

Fig. 1.4. The collapse of a molecular cloud core and the formation of a solar system. a. The molecular cloud core; the indicated scale is 1 Ly. b. Collapsing cloud core. c. Fragmentation. d. Precursor of a solar system with an accretion disk (seen from the side). There is a large difference in scale between each of these four stages

Fig. 1.4. The collapse of a molecular cloud core and the formation of a solar system. a. The molecular cloud core; the indicated scale is 1 Ly. b. Collapsing cloud core. c. Fragmentation. d. Precursor of a solar system with an accretion disk (seen from the side). There is a large difference in scale between each of these four stages which contain many cloud cores. Because the collapsing cloud material converges from a very extended volume, the small amount of rotation that it originally possessed becomes strongly enhanced and, due to conservation of angular momentum, the collapsing core region starts to rotate rapidly (Fig. 1.4b). As the collapse proceeds in a chaotic and nonradial fashion, the rotating regions break up into fragments, and their rotation is converted into orbital motion (see Fig. 1.4c).

This process of collapse, increased rotation, and fragmentation repeats itself several times, until small enough flattened sub-fragments (the predecessors of solar systems) are generated (see Fig. 1.4d), in the center of which an accretion disk is formed. Note that in going from the stage of Fig. 1.4a to that of Fig. 1.4d the size is reduced by about a factor of 2000. The flattened shape of the fragment and the accretion disk derives from the fact that the collapse is much easier parallel to the rotation axis than perpendicular to it, where direct collapse violates the conservation law of angular momentum. The rate of rotation determines whether a solar system ends up as a multiple stellar system (70% of the cases), or as a system with a single central star (30%). It is important to note that this entire process does not lead to the formation of individual stars. Instead, whole star clusters with many hundreds or even thousands of stars are effectively created simultaneously.

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