Desktop heavenly discoveriesThe history of science abounds in examples of objects and processes the existence of which had been first predicted by theory or intuition and corroborated later by actual discoveries. This is how biologists found the "missing links" in the evolution chain, how physicists derived and still derive the existence of new elementary particles, etc. Astronomy is no different in this respect.
The history of Neptune's discovery is quite well known. It can be found in any astronomy textbook and in many books for the general public as well. So, let's turn to an old description of the fact, contained in Henry Kiddle's A Manual for Astronomy published in New York in 1863, mere 17 years after Neptune's discovery.
Neptune is the most distant planet known in the Solar System. It was first observed, in 1846, by Dr. Galle, of Berlin. The position of this planet was very nearly ascertained by Leverrier, a French mathematician, before its actual discovery, by observing its action upon the planet Uranus. (...) The planet was discovered under circumstances, which give a greater triumph to modern science, than any other discovery recorded in the annals of human knowledge. During several years previous, Uranus had been observed to deviate, in a mysterious manner, from the path assigned to it by the most careful calculations. Nothing but the supposition of another planet, existing somewhere in its vicinity, could account for the disturbance. Accordingly, two mathematicians, Mr Adams, of England, and M. Leverrier, of Paris, undertook to calculate the position of this unknown planet. Both, though unknown to each other, arrived at conclusions differing but slightly. M. Leverrier, however, wrote to Dr. Galle, of Berlin, and requested him to direct his telescope to a certain point of the heavens. He did so, and the new planet was found within only one degree of the point specified by the mathematician.The account is surprisingly matter-of-fact. The only missing fact is J.C.Adams's failure to convince the directors of the observatories in Cambridge and Greenwich to undertake necessary sky observations. Nevertheless, both then and today the tryumph of theory is attributed to both Leverrier and Adams.
There is, however, another surprising sentence in the account: "It is believed that Neptune has two satellites". This is indeed true, but the first of them, Triton, being discovered by Lassell already in 1846, the second, Nereid, was discovered by Kuiper as late as 1949! Forcefully, this reminds the story of the satellites of Mars described by Jonathan Swift in his Gulliver's Travels long before they became known. Kiddle says nothing about satellites of Mars. However, he assigns 6 moons to Uranus, while only 4 were known at the time. Was it just his imagination?
Let's return to the 20th century. In 1967 The Astrophysical Journal, a very serious medium indeed, hosted a long discussion on the supposed pygmy stars. Fritz Zwicky announced the discovery of several objects much weaker than the white dwarfs and possessing some peculiar properties. In a passionate polemic Allan Sandage and Olin Eggen proved that the supposed pygmies are white dwarfs. Many astronomers believe that Zwicky's was a missed attempt at the discovery of neutron stars. In fact, their existence had been theoretically established in the thirties, shortly after the discovery of a new elementary particle, the neutron, by Chadwick in 1932. Oppenheimer and Volkoff and independently Landau worked out the first models of stars consisting of neutron matter alone. Neutron stars discovered at the desk would be very small, a dozen kilometres at most, although they would have unheard-of high densities. Their models showed some similarity to those of the white dwarfs. The Indian astrophysicist Subrahmanyan Chandrasekhar, very young at that time, had just elaborated a theory for the latter. In both cases an increase in mass results in a decrease in dimensions of the objects and in both cases there is an upper bound on the mass. For the white dwarfs, built mainly of helium, the bound is about 1.4 times the mass of sun; for the neutron stars the exact value of the bound is yet to be found. The doubt is due to uncertainties in the knowledge of the neutron gas state equation at very high densities. Different forms of this equation yield values ranging from about 1 to about 3 - or even 4 - times the mass of the sun. Finally, it is now beyond doubt that both the white dwarfs and the neutron stars (as well as the black holes) represent the final stage of the star evolution process.
Almost simultaneously with the failed discovery of pygmy stars another revelation was made. Jocelyn Bell Burnell, a young PhD student of Anthony Hewish, was working at the radio observatory at Cambridge on a program aimed at the investigation of radio wave scintillation in interplanetary matter. The phenomenon is in fact very close to that of the twinkling of stars, the only difference being that in the latter case it is the earth atmosphere which is responsible for the effect. In the radio case the twinkling only occurs for point sources of radio emission. These were precisely the main investigation objects at the observatory, the observation being performed with a relatively simple and cheap system of aerials. At the end of 1967 Jocelyn Bell Burnell observed some astonishing changes in the radiosources. The first of them was pulsing very regularly with a period of 1 and 1/3 of a second, while the second exhibited a shorter period of 1.2 seconds; even shorter periods were discovered later. One of the many hypotheses intended to explain the strange phenomenon went as far as to attribute these signs to distant cosmic civilizations! Nevertheless, it soon became evident that the pulsars, as the newly discovered objects were called, could not be but neutron stars.
The particulars of the radio wave pulsation process are still being investigated. However, the identification with neutron stars stems from much more elementary considerations. A strictly periodic phenomenon is usually due to either pulsation or rotation of the object involved. In both cases the smaller the object, the shorter the period - in full analogy with the pendulum: the shorter its length, the shorter its period. Thus the solution was to be sought among the smallest astronomical objects. Periods of the order of a second seemed to be in character with white dwarfs as well as with neutron stars, but when pulsars with periods of a few hundredths of a second became known, only the latter would remain.
Further considerations excluded the possibility of pulsation and finally the only fitting model was that of a lighthouse, i.e. a neutron star rotating at great speed and emitting a narrow beam of radio waves. There was a particularly spectacular corroboration of this hypothesis. In fact, it turned out that the nucleus of the Crab nebula, a left-over of a supernova explosion in 1054, was a pulsar. It was known before in the theory of star evolution that the nucleus of a supernova becomes a neutron star after the star's explosion. Moreover, the flare-ups of the pulsar in Crab were the first to be observed in the optical domain and then in the entire spectrum interval.
A few years later Hewish received the Nobel prize in physics for the discovery of the pulsars. Some commentators held the view that his doctoral student merited the prize in the same degree. In fact, there would be even more candidates to the prize for the discovery of neutron stars. It was established in the course of the sixties that neutron stars are to be encountered in double systems emitting X radiation.
In these systems, as well as in many other double systems, the flow of matter from one component to the other can be observed. The fall of matter on the receiving component is accompanied by some energy output. Mechanical energy is being transformed into radiation. The amount of that energy depends on both the speed of matter flow and the progress of the "falling" process. This second factor is also responsible for the type of radiation emitted. It is quite understandable that the process will be very violent when the receiving component is relatively small (assuming the differences in the object's mass are not too big; let's take it that the mass is about that of the sun). To put it in simple formulation, matter falling on a star comparable to sun, i.e. of dimensions of the order of several hundred thousand kilometers, cannot reach very high speed. On the other hand, the same matter falling on a neutron star of dimensions of the order of a dozen kilometers attains much greater velocity and the loss of speed is much more violent. It turns out that in the latter case high energy X radiation must be generated.
Double systems with X radiation form an important link in the evolution chain of some types of close double systems. They are evidence, besides objects like the Crab nebula, of explosions of supernovae. Had all this variety of forms and richness of phenomena been anticipated by those who discovered neutron stars at their desks about sixty years ago?