San José State University
Thayer Watkins
Silicon Valley
& Tornado Alley

As part of a project to reveal the detailed history of the concept of a neutron star the following is presented as the first published statement concerning neutron stars.

First Postulation of Neutron Stars*

Walter Baade and Fritz Zwicky,
"Remarks on Super-Novae and Cosmic Rays,"
Letters to the Editor of the Physical Review
Vol. 46, 1934, pp. 76-77

We have recently called attention to a remarkable type of giant novae.1

1W. Baade and F. Zwicky, Proc. Nat. Acad. Sci. May 1934.

As the subject of super-novae is probably very unfamiliar we give here a few more details which are not contained in our original articles.

1. Distribution of super-novae

In our calculations we made use of the assumption that on the average one super-nova appears in each galaxy every thousand years. This estimate is based on the occurrence of super-novae in the following galaxies,

Our own galaxy    in1572
Messier 1011907

These three systems are located within a sphere of radius 12×105 light-years.

In the Virgo cluster, which contains about 500 nebulae, six super-novae were found on plates taken during the last thirty years. As a curiosity we mention that in N.G.C. 4321, which is a member of Virgo, two super-novae have appeared in 1901 and 1914, respectively.

In the same interval of 30 years six additional super-novae were found in isolated nebulae.

We wish to emphasize that all of these finds are chance finds since a systematic search for super-novae has been organized only recently.

From the estimate of one super-nova per galaxy per thousand years it follows that 107 super-novae appear per year in the 1010 nebulae which are contained in a sphere of 2×109 light-years radius (critical distance derived from the red-shift of nebulae). If cosmic rays come from the super-novae their intensity in points far away from any individual super-nova will be essentially independent of time.

2. Comparison with the lifetime of stars

The lifetime of stars is supposed to be on the order of at least 1012 light years. A nebula contains about 109 stars. These estimates, combined with the frequency of occurrence of one super-nova per galaxy per 103 years suggest that the super-nova process might occur to every star once in its lifetime, marking perhaps the cessation of its existence as an ordinary star. We realize that this suggestion is highly speculative in view of the possibility that the frequency of occurrence of super-novae may depend on time and in view of our complete ignorance with respect to the evolution of the universe.

3. Ions in super-novae

If super-novae are giant analogues of ordinary novae we may expect that ionized gas shells are expelled from them at great speeds. If this assumption is correct, part of the cosmic rays should consist of protons and heavier ions. Direct tests by cloud chamber experiments at high altitudes are desirable in order to test this conclusion. Also the problem suggests itself to investigate how much energy corpuscular particles lose on their long journey through space. On the picture of an expanding universe this loss has been computed by R.C. Tolman.

Fluctuations of cosmic rays

In our original papers we have calculated the change in intensity of cosmic rays caused by flare-ups on super-novae in nearby galaxies. The estimates given are perhaps too optimistic in view of the fact that the velocities of different particles are different. If various particles are ejected simultaneously at the time t=0 from a galaxy which is 106 light-years away the times t of arrival on the earth are

t= 106 yearsfor light if its velocity does not
depend on the frequency.
t= 106 years + 410 secondsfor 1011 volt electrons
t= 106 years + 47.6 daysfor 109 volt electrons
t= 106 years + 44 yearsfor 1011 volt protons

These time lags ti−t would tend to smear out the change of intensity caused by the flare-ups of individual super-novae. Dr. R.M. Langer in one of our seminars was the first to call attention to the straggling of simultaneously ejected particles.

5. The super-nova process

We have tentatively suggested that the super-nova process represents the transition of an ordinary star into a neutron star. If the neutrons are produced on the surface of an ordinary star they will "rain" down towards the center if we assume that the light pressure on neutrons is practically zero. This view explains the speed of the star's transformation into a neutron star. We are fully aware that our suggestion carries with it grave implications regarding the ordinary views about the constitution of stars and therefore will require further careful studies.



Mt. Wilson Observatory and
California Institute of Technology, Pasadena.

May 28, 1934

*Lev Landau is said to have anticipated the concept of a neutron star even before the discovery of neutron in 1931. He wrote about stars where "atomic nuclei come in close contact, forming one gigantic nucleus" (published in 1932: Landau L.D.. "On the theory of stars". Phys. Z. Sowjetunion 1: 285288.). However the belief that Landau predicted neutron stars is not correct. See P. Haensel, A. Y. Potekhin, & D. G. Yakovlev (2007). Neutron Stars 1: Equation of State and Structure (New York).

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