148 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



the mass of the sun. So if we consider the huge dimensions of our 

 galaxy, the amount of matter contributed by interstellar gases is by 

 no means negligible. 



An interesting and somewhat amusing subject is that of the tem- 

 perature of these gases in space. We are accustomed to dwell upon 

 the intense cold of outer space far removed from the heat of any 

 nearby star and we are quite right in doing so. A thermometer placed 

 in interstellar space would show a temperature of about 3° above 

 absolute zero on the Centigrade scale or about 455° below zero on the 

 usual Fahrenheit scale. But this is by no means the temperature of 

 the atoms or molecules of a highly diffuse gas. In such a gas the 

 effect of the radiation of a star which falls upon an atom is to drive 

 out electrons, or, to use a technical word, to ionize it. It is the same 

 process which happens when light falls upon a photoelectric cell: 

 electrons are driven out and the energy of these electrons when ampli- 

 fied rings a burglar alarm or opens a garage door. The electrons in 

 space have a temperature depending upon the mean energy with 

 which they are driven out of the atom, and when they collide with the 

 atom they raise its temperature. Thus the atoms and molecules of | 

 gas are lifted to a high temperature estimated at some 10,000° to j 

 20,000° on the Fahrenheit scale. The interesting feature about the 

 process, as Eddington has shown, is that it depends upon the quality 

 and not the quantity of the radiation, so that the temperature of a 

 gas far in space will be just as high as if it were near a star. The 

 rate of production of electrons will be slower but the temperature will 

 not be affected. So we may say that we have two kinds of tempera- 

 ture in space, one of space itself as registered by a thermometer, and 

 a very different one for the gases, which through their remarkable 

 structure are able to build up and maintain a temperature of thou- 

 sands of degrees in spite of the bitter cold of the medium which 

 surrounds them. 



There is one other interesting characteristic of the molecules and 

 atoms in our interstellar gases. Under ordinary conditions such as 

 in a physical laboratory they are in a wild state of excitement, flying :^ 

 about rapidly, colliding with one another and knocking off and 

 picking up electrons in a fraction of a millionth of a second. In the 

 extremely rarefied conditions of gas in space, however, the situation 

 is quite different. Collisions are extremely rare and the atoms and 

 molecules can remain for weeks and perhaps even months in the least 

 excited state which the state of their being will allow them to have. 

 To use a homely comparison, if we touch a sleeping cat the cat 

 responds with a a twitch of an ear or a leg which represents the least 

 possible disturbance to its equilibrium. So the lazy molecules of 

 space when disturbed by a ray of light or heat from a star seek to 



