Audio English interview with Anton Pannekoek on the temperature of the stars

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INTERVIEWER
...Another of the principal speakers was Dr Antonie Pannekoek, professor of astronomy in the University of Amsterdam, who delivered the most important lecture on the temperature scale of the stars.
In order that our American audience may have an opportunity to become somewhat acquainted with certain other distinguished scientists from abroad who come to this country for this special occasion, we have invited Dr Pannekoek to speak to you this evening. And he has very graciously consented to discuss with me, quite informally, some of the ideas that he presented in more technical fashion to his fellow astronomers this afternoon.
Just how do you go about the task of determining the temperatures of such remote objects as the stars, Dr Pannekoek?

PANNEKOEK
To find the temperature of a star, the first and most promising method is the use of the energy distribution in its spectrum. For black-body radiation, the formula of Planck gives a direct relation between the temperature and the intensity gradient of the spectrum. It is well known that the first measures by Wolfson and Rosenberg gave contradictory results about the temperature scale. The chief cause of the discrepancy is that the star does not radiate as black bodies. Of course, we have no reason to assume here black-body radiation, but there was no other means than this assumption to derive temperature from the spectrum gradient. So, we call the results 'colour temperature', it has a great practical importance because all the easily determinable data as colour index, [...] wavelengths, depend on the same gradient.

INTERVIEWER
I see! Just as for a piece of black metal, heated to a fiery red colour, or when it is white hot, the colour gives an accurate indication of its temperature, so for the stars. [CHANGE OF TAPE]
...the colour, especially determined by the study of spectrum, [...] the temperature.

PANNEKOEK
Yes, but there is one important difficulty: the stars are not radiating energy as the black bodies would radiate it, and therefore, the colour temperature may not be the real temperature of the star's surface.

INTERVIEWER
And how do you get around that difficulty?

PANNEKOEK
Firstly, by applying physical theory to our problem. Atomic physics has advanced so much in the last ten years that we are able to compute by theory the the absorption coefficients of the gases in the stellar atmosphere. And we determined the quality of the emitted light and how it is different from black-body radiation. I was working many years on applying this theory to our problem. The most striking discrepancies have been cleared up in this way. We understand now why the white A-stars show a colour temperature of 18,000 degrees, so the real temperature must be near to 10,000 degrees.

INTERVIEWER
What do you mean by 'real temperature'? Of course, the temperature in a star must be increased greatly in the deeper layers below its surface, or, do you mean the surface temperature?

PANNEKOEK
Not exactly. What I call the 'real temperature', it means more exactly 'effective temperature'. It means the temperature of black [surface?] must have to emit the same amount of energy as the star does. So, it indicates the intensity of the stream of radiation that flows through the solar atmosphere outward. And this is the fundamental difference for the stars?. It determines the conditions in its atmosphere, its spectrum, and everything. It is some average value of the temperatures of the visible areas, and so, it's somewhat higher than the surface temperature. Our sun, with an effective temperature of nearly 6,000 degrees, has a surface temperature of about 12,000 degrees.

INTERVIEWER
But, uh, when your theory allows you to compute the colour temperature of a star, from its real, effective temperature, then you must be able, conversely, to find, by theory, the effective temperature from the measured colour temperature.

PANNEKOEK
It might seem so, but methods are not so easy. These computations are very complicated and the results are only approximate. Much more theoretical must be done to make it precise, and a large number of outstanding [...] must be cleared a bit first, so that is seems that such measurements are rather a test for the theory than a good determination of temperature. And the same holds for those other methods which make use of line intensities in the spectroscopy of stars.

INTERVIEWER
Am I correct when I conclude that the several methods of measuring stellar temperatures, which you have mentioned, are indirect methods? Does that mean there is no way of measuring the temperature directly?

PANNEKOEK
Well, of course, it has long been possible to measure the effective temperature of the sun directly. We know its diameter and, therefore, the area it covers in the sky. We can measure the [...] radiation and know the [radius?] from which that radiation comes [CHANGE OF TAPE]
We can compute directly the temperature of its surface. That is the basis for the well-known statement that the effective temperature of the sun is 6,000 degrees centigrade.

INTERVIEWER
Such a method could, I suppose, be applied to any star whose diameter was accurately known.

PANNEKOEK
Yes, but only a very few red stars have an apparent diameter large enough to be measured even with the modern interferometer. Some of the red giant or supergiant stars have, however, been measured with the interferometer at the Mt. Wilson observatory. The total radiation must then be derived from the visual brightness by means of a reduction which can be computed exactly only by means of the theoretically derived spectral intensity curve. Thus, for the red supergiant Antares in the Scorpion, the effective temperature is found to be 3,310 degrees. The method remains somewhat unsatisfactory, because the total radiation is derived from a minute visual part. So, a better result may be expected from [barometric?] or radiometric measurements. Pettit and Nicholson derived in this way 3,270 degrees for Betelgeuse and for Antares. The accordance of the two results is very satisfactory, so that the effective temperature derived in this way for these red supergiants, 3,300 degrees, must be considered as a well-determined and fixed point in the scale of stellar temperatures.

INTERVIEWER
But how about the hotter, white stars, which I understand are smaller than the red giants? Is there any way to measure the diameter of stars too small or too far away to permit direct measurement by the interferometer?

PANNEKOEK
Yes, it can be determined sometimes in an indirect way. As for example, when the star is an eclipsing variable. I-

INTERVIEWER
Pardon me, Dr. Pannekoek! Uh, just what is an eclipsing variable?

PANNEKOEK
When a pair of stars is revolving around one another in a plane directed towards us, so that once in each revolution one star covers part of the other, then the total light is diminished and recovers periodically. Just as when the moon partly covers the sun, we call this an eclipse. Because the variations of light from six stars are due to eclipses, we call them eclipsing variables.

INTERVIEWER
Ah, yes, I understand now.

PANNEKOEK
When all the data about the eclipses of one of these pairs of twin stars are known, the ratio of the stellar diameters to the orbit can be determined, and then the radial velocity gives the diameters, which, together with the observant radiation, gives the temperatures. There is only one among the white stars, Beta Aquarii [Pannekoek may be mistaken and means 40 Eridani B], for which all these data are determined without uncertain assumptions. The computation of its temperature was made 10 years ago by Russell, Dugan, and Stewart in their textbook and was only to be corrected for a better reduction of visual brightness to total radiation than was adopted at that time in accordance with black-body distribution of energy. Making the computation for the two hypotheses treated by Shapley of uniform disks and of complete darkening at the limb, the resulting values for temperature are 10,300 degrees and 10,740 degrees. [CHANGE OF TAPE]
Hence, 10,500 degrees may be assumed as a fair approximation for the effective temperature of this star. It fixes another well-marked point, the temperature of class A0 in the scale of the stars' temperatures. So, two points in the temperature scale have been fixed. 10,500 degrees for white stars and 3,300 degrees for red giant stars, with limits of uncertainty in both scales well below 5% of their value. They have been found by direct methods of determining the effective temperature itself.

INTERVIEWER
That is certainly a triumph for modern astronomy. And now tell me just one thing more, Dr Pannekoek, what is the use of all this astrophysical research? Of course, such discoveries are very gratifying to the scientists who make them, and who are eagerly trying to learn more and more about the universe around us, but will mankind in general profit from such studies? Will this knowledge enable people to live better lives in a more satisfactory way?

PANNEKOEK
Well, Dr Mather, the pleasure of understanding the world is not only for scientists, but, at the rate that science diffuses, for an increasing number of people. But, I will not stick to that point. My opinion is that all science is described not only to give the intellectual pleasure, but also to make our material lives better and richer. Rescued free from want and hardship, by always more complete domination of the forces of Nature, by always better access to the sources of energy in Nature. Nature possesses enormously richer sources of energy than we are using now, especially [...] atomic energy, and she displays them most profusely in the stars. We do not yet know how she does, but we may think that if ever this problem of opening these richest energy sources to mankind will be solved. This problem, of foremost importance for the development of mankind in future, that the study of astrophysics in our days will be remembered then as the first step on this way. And, for the moment, it cannot be doubted that the study of the stars by laboratories, where Nature is experimental, is of the greatest importance as an addition to the study of physics in our own laboratories.

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