On the Relics of Supernovae

W. H. RAMSEY D e p a r t m e n t of As t ronomy, Un ive r s i t y of Manches te r

SUMMARY I t is sugges ted t h a t the l uminos i t y of the r e m n a n t of a supernova , such as the Crab Nebula , is m a i n t a i n e d m a i n l y by the r ad ioac t ive decay of uns t ab l e nuc lear isotopes formed dur ing the explosion of the p a r e n t s t a r and no t by the r ad i a t i on of a h y p o t h e t i c a l cen t ra l s t a r as has h i the r to been supposed. The mos t i m p o r t a n t r ad ioac t ive isotope is C ~4, and i t is shown t h a t 0.05 per cent by mass of th i s isotope can m a i n t a i n the r ad i a t i on from the Crab Nebula . Some impl ica t ions of th is hypo thes i s are discussed.

IT is known from observations on the extra-Galactic nebulae that there is approxi- mately one supernova per galaxy every 500 years. There have been only three Galactic supernovae in recorded history, and all three occurred prior to the invention of the telescope and the development of precise astronomical measurements. Never- theless, the positions of the supernovae in the sky and their decay in brightness after maximum are well-defined by the ancient observations. The light curves are, within the observational uncertainties, identical with those of the extra-Galactic supernovae observed by modern telescopes. In the present paper the principal interest lies in the remnants of the supernovae rather than in the supernovae themselves. The most recent Galactic supernova occurred in 1604 A.D. and is usually associated with the name of KEPLER. The remnant of this supernova is a diffuse cloud of gas with a rather characteristic spectrum. There was a supernova a generation earlier, in 1572 A.D., which is usually known as Tycho's Star. There is no visible remnant of this exploded star, but there is in the same position a radio star. But the most striking of all supernovae occurred in 1054 A.D. and was observed and studied by Far Eastern astronomers. In this position there is one of the most remarkable objects in the whole sky, the Crab Nebula. This is an enormous mass of expanding gas, the core of which is amorphous and exhibits no spectral lines. The diffuse outer filaments, however, do exhibit spectral lines, and the spectrum is similar to that of the remnants of Kepler's Star. The Crab Nebula, like Tycho's Star, is also a radio star. The rate of expansion of the Crab Nebula indicates that it is about 900 years old, and there is no doubt that the star observed by the Chinese was its parent. Our knowledge about the Crab Nebula greatly exceeds that about the remnants of the other two supernovae, and therefore will be the centre of interest in this paper.*

At maximum brilliance the supernova observed by the Chinese outshone even Venus, and it must have attained an absolute magnitude in the neighbourhood of -- 16.5. Its maximum luminosity, therefore, exceeded the total of all other stars in the Galaxy. But its brilliance was short-lived, and after a few years it was invisible to the naked eye. Nevertheless, the energy released in these few years must have been comparable to, if not approximately equal to, the total thermal energy of the parent star. To-day the absolute magnitude of the Crab Nebula is about -- 1.35 and this has not changed perceptibly in the past thir ty years. The luminosity of the

* The available information about tile three galactic supernovae and modern spectroscopic data about their relics have been surveyed in an excellent series of publications by i~AA1)E (I 942, 1943, 1945) and M[NKOWSKI (1942, 1943).

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]508 ()n the relies of supernovae

Nebula, even in its present quiescent state, is thereibre 300 times as great as that of the Sun. The amorphous central core of the Nebula accounts for about 80 per cent of the total light, and the outer filaments for about 20 per cent.

The Crab Nebula, as it now is, presents a number of difficulties. First, there is no known source of energy which could enable the Nebula to radiate with its present brilliance for nine centuries. I t has often been supposed that there is a central star, a relic of the parent star which exploded, which is responsbile for the luminosity. But the Nebula is comparatively transparent and stars both within and beyond the Nebula are easily observed. All but one of these stars can be excluded as their proper motions differ significantly from that of the Nebula. The absolute magnitude of this star is about 5.6 and so its luminosity is less than that of the Nebula by a factor of about 600. The second difficulty presented by the Crab Nebula is one of temperature. The gas temperature, as judged by the colour, is about 7000°C. But the temperature of the free electrons in the amorphous core must be at ]east 50,000°C ; only very fast free electrons can completely obliterate the spectral lines as is observed. The third difficulty presented by the Nebula is the relative strengths of the spectral lines observed in the outer filaments. The lines of hydrogen are comparatively weak, but this is not usually regarded as a difficulty, as CHANDRASEKHAR (1941) has sug- gested that a supernova is the result of a transition of a star to a white dwarf con- figuration after its ordinary atomic fuel has been nearly exhausted. But the lines 6548 A and 6584 A of singly ionized nitrogen do present a difficulty, as they are the strongest in the whole spectrum ; the singly ionized sulphur lines 6711 A and 6728 A are also surprisingly strong. These lines are also the strongest lines observed in the remnant of Kepler's Star.

MINKOWSKI (1942, 1943) has attempted to overcome the first and second diffi- culties mentioned above by supposing that the central star, which exhibits no striking abnormalities, is indeed a very remarkable star. Though it appears to be faint he assumes that its luminosity is indeed very large. Though its colour appears to be typical of a star of moderate surface temperature he assumes that its surface temperature is extremely high. The abnormal properties of this central star are supposed to be masked from us by the gaseous material of the Nebula in such a way that it appears to be quite normal. MINKOWSKI has made detailed calculations on the basis of this model and has found that the luminosity of the central star must exceed that of the Sun by a factor of 30,000, even though its absolute visual magnitude appears to be less than that of the Sun. The surface temperature of the central star is not a few thousands of degrees, as would appear, but of the order of half a million degrees, and its radius is only about 2 per cent of that of the Sun. This star would be an even more remarkable object that the Crab Nebula itself, and it is postulated merely to explain the properties of the Nebula. There is, after all, no reason to believe that the star is even within the Nebula. Furthermore, this model does not explain the properties of the Crab Nebula; it gives no clue to the reason for the abnormally strong lines of ionized nitrogen and sulphur. I t will be assumed in the present paper that either there is no central star in the Nebula or, if there is, it makes only a minor contribution to the maintenance of the radiation from the Nebula.

I t will be postulated in this paper that the luminosity of the Crab Nebula and of the relics of other old supernovae is maintained mainly by the radioactive decay of unstable nuclear isotopes formed during the explosion of the parent star. This

W. H . RAMSEY 1509

important r61e of radioactivity is a consequence of a particular mechanism of the formation of supernovae which the writer is developing, and which will be described elsewhere. The conclusions of this paper are, however, being presented separately as they do not depend inherently on the writer's special mechanism. Indeed, they follow from any reasonable theory of supernovae, such as that of HOYLE (1946). The extreme internal pressures and temperatures necessary to blow a star completely to pieces are sufficient to maintain very rapid nuclear transformations which supply the energy required to disintegrate the star. Indeed, only nuclear sources could supply the energy required. The exploding star is probably converted into an enormous "helium bomb" and the energy is released mainly through the synthesis of heavier elements from helium; CttANDRASEKHAR'S view, which is generally accepted, is that the parent star has already exhaused most of its hydrogen. Both stable and unstable isotopes of the heavier elements will be formed during the explosion, but most of the unstable isotopes will be short-lived. There remains, how- ever, the possibility that the more slowly decaying isotopes cannot be ignored in studying the remnant of a supernova such as the Crab Nebula.

As the Crab Nebula is nine centuries old, isotopes with lifetimes less than, say, twenty years will have long since decayed, and so will have no bearing on the present state of the Nebula. The longer-lived unstable isotopes are comparatively rare; a table of nuclear species and their properties will be found in ROSENFELD'S book (1948). The heavy elements with atomic weights greater than about 70 are almost certainly rare both before and after the explosion; they are therefore very unlikely to make an important contribution to the properties of the Nebula. Thus, only isotopes of lifetime greater than twenty years and atomic weight less than 70 remain of interest. These are four in number and they are listed in Table 1 ; the half- life of the isotope is expressed in years and the energy released by fl-disintegration in mega-electronvolts.

Table 1

Half-life Energy Released Isotope (Years) (MeV)

Be 1° . 2.5 × 106 0.56 C 14 5.1 × 10 a 0.16 C136 106 O' 16 K 4° 2.4 × l06 1 '35

The first and most important feature of the Crab Nebula to be elucidated by the above considerations is the nature of its source of energy. I t will now be shown that the luminosity of the Nebula can be accounted for by radioactivity without the hypothesis of an abnormal central star. Nearly all the radioactive energy must be supplied by the isotope of carbon. Not only is this isotope expected to be by far the most abundant listed in Table 1, but its lifetime is less than those of the other isotopes by a factor of the order of a thousand. According to MI•KOWSKI (1942, 1943) the mass of the Crab Nebula is about fifteen times that of the Sun, and its absolute magnitude is -- 1.35. The proportion of C 14 necessary to maintain the radiation of the Nebula is therefore about 0.05 per cent by mass. This figure is reasonable, but it must be remembered that the uncertainties in the observational data permit only

1510 ( ) n th(~ I'~qics ()[' sup(!rlto'~:a(~

an order of magnitude estimate. In solar material carbon, oxygen, and nitrogen each constitute about one-third of I per cent el" the Vot.al mass. But the proportions of these elements in tim (~rat) Nebula must be considerably greater, as the Nebula is known to be markedly deficient in hydrogen and probably also in helium. The admission of 0.05 per cent of C la by mass is therefore acceptable, in contrast to the hypothetical attributes of the central star. The constancy of the magnitude of the Nebula over the past thirty years is also understandable. The Nebula will decay in brightness with ~he same half-lifetime as (~4. It will therefore alter by only about one magnitude in 7000 years; that is, by about 0.005 magnitude in thir ty years, which is undetectable, gadioactivi ty also, of course, accounts for the radiation fl'om the remnant of Kepler's Star.

The second peculiar feature of the Crab Nebula is the temperature of the fl'ee electrons in the central amorphous core. The gas temperature of the core, as indi- cated by its colour, is only about 7000°C. But the temperature of the fl'ee electrons must be at least 50,000°C in order to obliterate all traces of spectral lines. The abnormally high "temperature" of the electrons is understandable as they are of radioactive origin. The decay electrons of C 14 have an energy of 0.16 MeV, which corresponds to a "temperature" of the order of 109°C. The concept "temperature" is not strictly applicable to the free electrons as their distribution is not Maxwellian ; the velocity distribution will be determined by the inelastic collision cross-sections with the atoms of the Nebula. However, so far as the obliteration of the spectral lines is concerned, the only requirement is that the electrons should be fast.

The third feature of the Crab Nebula which needs elucidation is the relative intensities of the spectral lines observed in the outer filaments. This aspect of the problem would require a detailed treatment beyond the scope of this contribution, but the writer would like to draw attention to one point. The atom resulting fl'om the decay of one of the isotopes in Table 1 will, in general, be left in an excited state and its emission lines may be enhanced to a detectable degree in the spectrum of the Nebula. The lightest isotope Be 1°, like all isotopes intermediate between helium and carbon in atomic weight, is expected to be very rare; it is therefore not surprising that its decay product boron has not been detected in the spectrum of the Nebula. The isotope K 4° is also expected to be rare and its lifetime is very long; one would not therefore expect to detect its decay products argon and calcium. The most abundant and shortest-lived isotope in Table 1 is C 14, which is fi-active and decays into nitrogen ; the lines of singly ionized nitrogen are very strong in the spectra of both the Crab Nebula and the relic of' Kepler's Star. The remaining unstable isotope in Table 1 is C1 a6 which can decay by fi-disintegration into argon or by K-capture into sulphur. The former type of nuclear transformation is much more probable but it is the lines of singly ionized sulphur which are conspicuous in the spectra of the relics of supernovae. The absence of argon lines in the spectra is not surprising as the most intense lines would be in the ultraviolet. But it seems difficult to attribute the abnormal intensities of the sulphur lines directly to radioactive chlorine. The detailed structure of the spectra should, however, be investigated in greater detail, and it is hoped that this will be undertaken at University College, London.

The next point arising from the above considerations is that it should be possible to detect the remnants of prehistoric supernovae. The Crab Nebula could be detected with available equipment even if it were fainter by more than 12 magnitudes. I t

~V. H . I{AMSEY 1511

has been shown above that the Nebula is expected to f~de by only one magnitude every 7000 years, and so similar relics of supernovae of the past 80,000 years should be detectable. In this time these have been of the order of 200 Galactic supernovae, but not all will have left relics as conspicuous as the Crab Nebula. Some, like Tycho's Star, will have left no visible remains. The chance of the remnant being conspicuous is unknown, but unless it is very small some ancient remnants should be visible. OORT (1946, 1951) has suggested that the great loop nebula in Cygnus is such a relic of a supernova of some tens of thousands of years ago. The expanding renmant has been slowed down by interstellar gas and is now presumably a roughly spherical shell about seven parsecs in diameter. The source of the loop's luminosity is unknown. The hypothesis of a central star leads to the same difficulties as with the Crab Nebula; moreover, there is no remarkable star within the loop. OORT attri- buted the luminosity to the interaction between the expanding loop and interstellar gas. This explanation is, however, untenable as HUMASON (1952) has shown that the loop is now expanding only very slowly if at all. The luminosity of the loop, as with the Crab Nebula, is probably maintained by radioactivity. The spectrum of the loop has been photographed by HUMASO~ (1952) and it is a line spectrum somewhat similar to that of the filaments of the Crab Nebula. Unfortunately HUMASOZCS observations do not extend to sufficiently long wavelengths to reveal the lines of ionized nitrogen and sulphur so prominent in spectra of the Crab Nebula and the relic of Kepler's Star. Another possible relic of a prehistoric supernova is the filamentary object recently discovered by BAADE (1952, 1954) in the position of the strong radio source in Cassiopeia.*

Some, though probably not all, radio stars seem to be remnants of old supernovae. I t has been known for some time that there is a radio star in the position of the Crab Nebula, and BRowz~ and HAZARD (1952) have recently shown that there is also one in the position of Tycho's Star. As mentioned above, there is another possible relic of a supernova in the position of the strong radio source in Cassiopeia.~ I t therefore seems likely that there is some connection between radio stars and the relics of old supernovae. And so it seems worth while to search for visible objects in the positions of the radio stars and, if possible, to examine their spectra which may indicate whether they are remnants of old supernovae ; if so, the lines of ionized nitrogen, and possibly also of ionized sulphur, should be prominent. The connection between radio stars and the relics of old supernovae is not yet clear, but the presence of electrons from radioactive decay may constitute an important link. The decay period of the radio emission may therefore be comparable to that of the radioactivity and so hundreds of old supernovae may be detectable as radio stars.

In conclusion, the writer would like to thank Professor KOPAL and Professor BLACKETT for the interest they have shown in this work.

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[{AADE, W . . . . . . . . . . . . . 1942

1943 1945 1952

Ap . J . , 96, 188. Ap. J . , 97, 119. Ap . J . , 102, 309. I n t e r n a t i o n a l A s t r o n o m i c a l U n i o n , M e e t -

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* See also Vol. 1 of Vistas in Astronomy, pp. 565-566 1955. ¢ Also the remnant of KEPLER'S supernova of 1604 ~ igh t now be identified with a rather intense radio source (position:

17 h 26 m 24 s, -- 21 ° 22'), which is No. 2C.1485 in the Cambridge Catalogue "A Survey of Radio Sources between Declinations -- 38 ° and + 83°, ' ' published by ft. R. SHAKESHAFT, ~[. RYLE J. E. BALDWIN, B. ELSMORE, and ft. H. THOMSON in Mem. Roy. Astron. Sot., vol. LXVII , Part IH, p. 139, London, 1955. ,%e, also Vistas in Astronomy, vol. 1, p. 541 (note by RYLE). 1955.

1512 0 n the relies of superm>vae

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