Section 3–4 Astronomy

(Stars / An astronomer / Nuclear energy)

Feynman states that astronomy got physics started by showing the beautiful simplicity of the motion of the stars and planets. In this section, the three interesting points mentioned are stars, an astronomer, and nuclear energy.

1. Stars:

“What goes on inside a star is better understood than one might guess from the difficulty of having to look at a little dot of light through a telescope… (Feynman et al., 1963, section 3.4 Astronomy).”

According to Feynman, we understand the distribution of matter in the interior of the sun far better than the interior of the earth by just analyzing a small little dot of light through a telescope. This is because atoms emit light which has definite frequencies of light waves, just like the timbre of a musical instrument which has definite frequencies of sound. By using a spectroscope, we can analyze the frequencies of light waves and deduce various atomic compositions that are in different stars. By using statistical mechanics, we can analyze the behavior of the stellar objects under conditions of high temperature and not very high density. Interestingly, the most remarkable discovery in astronomy is that the stars are made of atoms of the same kind as those on the earth.

It is potentially misleading when Feynman mentions that technetium and helium were discovered in stars before they were discovered on earth. Initially, the discovery of technetium was confirmed in a December 1936 experiment at the University of Palermo in Sicily by Carlo Perrier and Emilio Segrè. Next, Paul Merrill in California detected the spectral signature of technetium in 1952 in light waves from S-type red giants (stars). To be precise, Perrier and Segrè discovered artificially produced technetium. Furthermore (or theoretically speaking), there is no natural technetium that has been discovered on earth. (If you listen to the audio CD of this lecture, Feynman only said helium.)

Note: Feynman could have made fun of Auguste Comte, the father of “positive philosophy.” Historically, Comte predicted that we can never know the atomic composition of stars. In The Positive Philosophy, Comte (1896) writes that “[w]e may obtain positive knowledge of their geometrical and mechanical phenomena; but all physical, chemical, physiological, and social researches, for which our powers fit us on our own earth, are out of the question in regards to the planets. Whatever knowledge is obtainable by means of the sense of Sight, we may hope to attain with regard to the stars, whether we at present see the method or not; and whatever knowledge requires the aid of other senses, we must at once exclude from our expectations, in spite of any appearances to the contrary (p. 148).”

2. An astronomer:

“… One of the men who discovered this was out with his girlfriend the night after he realized that nuclear reactions must be going on in the stars in order to make them shine (Feynman et al., 1963, section 3.4 Astronomy).”

In a section titled Feynman the sexist pig in his autobiography, Feynman identifies the astronomer as Eddington. In Feynman’s (1988) words, “[t]he other story they objected to was told by the great astronomer Arthur Eddington, who had just figured out that the stars get their power from burning hydrogen in a nuclear reaction producing helium. He recounted how, on the night after his discovery, he was sitting on a bench with his girlfriend. She said, ‘Look how pretty the stars shine!’ To which he replied, ‘Yes, and right now, I’m the only man in the world who knows how they shine.’ He was describing a kind of wonderful loneliness you have when you make a discovery (p. 72).” However, Feynman was accused of being anti-woman for this part of the lecture. A feminist group claimed that Feynman was saying a woman is unable to understand nuclear reactions.

Feynman mentions that Eddington’s girlfriend was not impressed with the only man who knew why stars shine. Curiously, this might not be true because Eddington was a Quaker, that is, he was a religious person and he was never married. On the other hand, shortly after the 1957 Rochester Conference, Feynman (1997) expresses that “[i]t was the first time, and the only time, in my career that I knew a law of nature that nobody else knew (p. 250).” Note that Feynman was separated from Mary Louise Bell in 1956 (for working calculus problems continuously) and he was married to Gweneth Howarth in 1960. Thus, in a sense, Feynman could be referring to himself instead of Eddington when Feynman says “it is sad to be alone, but that is the way it is in this world.”

Note: In his 1920 presidential address to the British Association for the Advancement of Science, Eddington suggests that the solar energy is due to the conversion of hydrogen atoms to helium by citing Aston's measurement of the mass difference between hydrogen and helium. In an article titled The source of stellar energy, he elaborates that “[i]t has, for example, been objected that the temperature of the stars is not great enough for the transmutation of hydrogen into helium – so ruling out one possible source of energy. But helium exists, and it is not much use for the critic to urge that the stars are not hot enough for its formation unless he is prepared to show us a hotter place (Eddington, 1926, p. 30).”

3. Nuclear energy:

“… It is the nuclear ‘burning’ of hydrogen which supplies the energy of the sun; the hydrogen is converted into helium (Feynman et al., 1963, section 3. 4 Astronomy).”

It is the not simply nuclear “burning” of hydrogen which supplies the energy of the sun. Essentially, there is a continuously making of various chemical elements in the centers of the stars from hydrogen and other atoms. Thus, it is debatable whether Eddington exactly understood how the stars shine. More important, the making of atoms such as C¹², C¹³ or Technetium is a result of nuclear reactions instead of chemical reactions. Interestingly, Feynman elaborates that our elements were “made” in the stars and spit out in the explosions which we call novae and supernovae.

Hans Bethe provides an in-depth explanation of nuclear energy in stars. In 1967, Bethe was awarded Nobel prize in Physics for his contributions to the theory of nuclear reactions, especially his research on the energy production in stars. In short, he conceptualizes the carbon-nitrogen-oxygen (CNO) cycle by which stars convert hydrogen into helium and heavier elements. In general, the CNO cycle may have the following nuclear reactions: ¹²C + H → ¹³N + g ; ¹³N → ¹³C + e⁺ + n ; ¹³C + H → ¹⁴N + g ; ¹⁴N + H → ¹⁵O + g ; ¹⁵O → ¹⁵N + e⁺ + n ; ¹⁵N + H → ¹²C + ⁴He.

Note: In his Nobel Lecture, Bethe (1967) says that “Eddington, in the 1920’s, investigated very thoroughly the interior constitution of the sun and other stars, and was much concerned about the sources of stellar energy. His favorite hypothesis was the complete annihilation of matter, changing nuclei and electrons into radiation (p. 216).” (Feynman was assigned to Bethe’s Theoretical Division in Los Alamos during World War II.) However, according to Jeans (1926), “I find that some of the most fruitful ideas which I have introduced into astronomical physics – e.g., the annihilation of matter as a source of stellar energy, and highly dissociated atoms and free electrons as the substance of the stars – are by now fairly generally attributed to Prof. Eddington (p. 335).”

Questions for discussion:

1. Is it still true that we understand the distribution of matter in the interior of the sun far better than we understand the interior of the earth by looking at a little dot of light through a telescope?

2. Should Eddington be recognized as the first person that has a correct understanding of nuclear reactions that make stars shine?

3. How does the nuclear “burning” of hydrogen supply the energy of the sun?

The moral of the lesson: the stars are made of atoms of the same kind as those on the earth because our elements were initially “made” in the stars and spit out in the explosions which we call novae or supernovae.

References:

1. Bethe, H. A. (1968). Energy production in stars. In Nobel Lectures in Physics 1963-1970. Singapore: World Scientific.

2. Comte, A. (1896). Positive philosophy (Vol. 1). London: Bell.

3. Eddington, A. S. (1926). The source of stellar energy. Nature, 117(2948), 25-32.

4. Feynman, R. P. (1988). What Do You Care What Other People Think? New York: W W Norton.

5. Feynman, R. P. (1997). Surely, you’re Joking, Mr. Feynman. New York: Norton.

6. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley.

7. Jeans, J. H. (1926). Diffuse matter in interstellar space. The Observatory, 49, 333-335.