NOVA is an example of picturesque but inaccurate terminology. The word means new, and refers to the appearance of a ‘new’ star which was apparently not present before. In fact, a nova is an eruption on a star in the last stages of its evolution. A nova, as we shall see, requires the presence of a companion in orbit around the old star. In this sense, a nova is an extrinsic rather than an intrinsic variable. The eruption occurs, however, because of an intrinsic instability which arises in the old star. This is yet another example of the difficulty of classifying variable stars.

Novae can be meaningfully subdivided into ORDINARY novae, RECURRENT novae and DWARF novae (also called U Geminorum stars). An ordinary nova brightens by as much as 18 magnitudes in a few days, then slowly fades (figure 4.20). A recurrent nova is an ordinary nova which erupts more than once. A dwarf nova brightens by only a few magnitudes, but does so repeatedly.

The behaviour of a typical nova can best be illustrated by a specific example – Nova Cygni 1975 – which rose suddenly to prominence on the night of 29 August 1975. Like many novae, it was seen first by amateur observers. Some amateurs make systematic searches for novae, scanning the skies with binoculars, looking for changes in carefully-memorized patterns of stars. One such amateur is the Englishman, G.D.Alcock, who searched patiently for many years before discovering a nova in Delphinus in 1967; a few months later, he discovered a second nova, in Vulpecula. Nova Cygni required considerably less effort. It was so conspicuous that it could be recognized by anyone reasonably familiar with the constellations.

Japanese observer, Kentaro Osada, was officially credited with the discovery, and within an hour, dozens of other observers in Japan had confirmed the discovery. They immediately notified the director of the Tokyo Observatory, who in turn notified the International Astronomical Union’s Central Bureau for Astronomical Telegrams, in Cambridge, USA : from there, the news was relayed by telegram to observatories all over the world. Intensive studies of the nova, using optical, radio, infrared and X-ray telescopes, began even before the nova had completed its rapid rise to maximum light.

The light curve has been assembled from hundreds of photoelectric and visual measurements. Novae can be classified as fast, medium and slow, according to how rapidly they fade from maximum light. Nova Cygni was a fast nova! While some astronomers carried on the intensive study of Nova Cygni, others searched archival records for previous evidence of the nova. Routine patrol photographs, taken at the Harvard Observatory since 1898, showed no object brighter than magnitude 15.5 at the position of the nova at-any time. Photographs in the Palomar Sky Survey s VTVM no object brighter than magnitude 21. The total range of the nova outburst must have exceeded 19 magnitudes! The outburst apparently occurred in two steps, how ever because on Russian patrol photographs taken earlier in August 1975. the nova appeared as an object of magnitude 16.

The spectacular changes in brightness were accompanied by equally spectacular changes in the spectrum Before maximum, the spectrum was that of a SUPERGIANT A-TYPE STAR, with broad, weak absorption lines greatly shifted to the violet. Soon thereafter, the spectrum developed strong emission and absorption lines of hydrogen and other elements; the emission lines were very broad, and the absorption lines were Doppler-shifted to shorter wavelengths by as much as l000 kms-1. As the nova faded in brightness, the spectrum was dominated by these features. Additional sets of emission and absorption lines appeared, shifted by even greater amounts. Finally, the spectrum became that of a typical emission nebula, with emission lines of hydrogen, helium, oxygen, nitrogen and neon. Among these are forbidden lines, characteristic of a gas of very low density.

The light and spectrum variations are consistent with the follow¬ing simple picture of the nova eruption. The star suddenly swells and ejects a spherical shell of material. The shell is small, but thick enough to mimic the distended atmosphere of a supergiant when the nova is at maximum. As the shell expands, we see Doppler-shifted absorption lines from the part of the shell between us and the star, and broad emission lines from the sides of the shell. Eventually, the shell dissipates, but is still faintly illuminated by the star Nova 31

A dozen or more novae occur in our Galaxy each year, but most of them are never observed because of great distance, obscuration by dust, or both. In nearby galaxies, they can be seen more easily. From these novae, astronomers have discovered an interesting relationship between the absolute magnitude of a nova at maxi-mum, and the time taken for the nova to decline in brightness by three magnitudes Fast novae are brightest at maxi¬mum. Nova Cygni. for instance, had an absolute magnitude of —10 at maximum. This relation, when ‘calibrated’ using novae of known distance and absolute magnitude, provides a useful tool for measuring distances to nearby galaxies.

DWARF NOVAE brighten by two to five magnitudes, at roughly constant intervals of ten to several hundred days (figure 4.25). Stars which have the largest eruptions tend to have the longest intervals between them; this tendency seems to extend to recur-rent novae, which suggests that these two kinds of novae are closely related. There is a subclass of dwarf novae named after Z Camelopardalis: members of this subclass have long ‘hesitations’ in light, at levels intermediate between maximum and minimum. The cause of this phenomenon is not known.

The essential clue to the cause of the nova eruption was dis¬covered in the 1950s by Robert Kraft and Merle Walker: every post-nova was a close binary system, consisting of a white dwarf and a relatively normal cool star. The two stars were so close that tidal forces exerted by the white dwarf pulled material from the outer layers of the cool star. This material formed a ring around the white dwarf. Astronomers already knew that a white dwarf was a dying star, which had exhausted its supply of nuclear fuel; it consists of a dense core of helium (or some heavier element) surrounded by a thin layer of hydrogen-rich material. The addition of any more hydrogen-rich material to this layer (from the ring, for instance) creates a highly unstable situation, and an eruption can well occur.

Brian Warner and R.E.Nather have added many important details to this close binary model, using the, technique of high¬ speed photometry which they developed at the University of Texas. Kraft, Walker and others had already discovered that many post-novae were eclipsing variables, Warner and Nather showed that, in addition, there was rapid, irregular flickering, which disappeared during the eclipse. They attributed this to a HOT SPOT on the ring, at the place where material was flowing onto the ring from the cool star. Shows the light curve of the dwarf nova Z Chamaeleontis, The flickering is quite intense, but disappears during the eclipse. The hump in the curve is due to the light from the hot spot when it faces the observer. The trough is the eclipse of the hot spot. In this system, the white dwarf supplies very little of the total light of the system

When Z Chamaeleontis undergoes an eruption, the nature of the light curve changes: the eclipse becomes shallower and wider, indicating that the hot spot and /or the white dwarf have swelled This is consistent with our simple picture of a nova eruption.

Astronomers have discovered a few stars that, although they have not been observed to erupt, have many properties in com¬mon with post –novae, such as emission lines (as from a gaseous ring) and intense flickering. Some of these NOVA LIKE VARIABLES have other interesting properties. One such star is associated with Scorpio X1. a strong source of X-rays. These X-rays may he produced by material Mourns flowing from a normal star onto a ring deep within the gravitational field of a compact white dwarf. Other X ray sources have been Identified with binary stars in which the compact member is a neutron star or even a black hole

SUPERNOVAE are without doubt the most spectacular of all variable stars. In a supernova eruption, a single star brightens until it is as luminous as a billion suns. It ejects a shell of gas at velocities exceeding lOOOOkms-1, and in so doing, transforms itself irreversibly into a gaseous remnant and a tiny neutron star. In the course of this transformation, supernovae play a crucial role in the chemical evolution of a galaxy, because the gas which they eject into the interstellar medium has been enriched in heavy. elements by thermonuclear reactions within the star.

Supernovae may perhaps occur as often as once a decade in our own Galaxy, but they are rarely detected, because they usually inhabit the regions of our Galaxy that are thick with obscuring dust Supernovae were recorded in 1572 (by Tycho Brahe) and 1604 (by Johannes Kepler), but none has been recorded since. Astronomers therefore search for supernovae in other galaxies . Several observatories carry out systematic surveys of a few hundred nearby galaxies, using wide-field Schmidt telescopes, and together they discover 10-20 supernovae each year. Most of these are fainter than magnitude ten, but even these can be studied in detail, using sensitive photoelectric photometers, spectrum scanners and image-tube spectrographs.

There are at least two, and possibly five different types of super¬nova. Type I occurs among old, low-mass stars of Population II; type II occurs among young, massive stars of Population I. (Again, note the problems of classification and terminology!) The light curve of a typical type-I supernova is shown in figure 4.29; note the characteristic hump at maximum, and the exactly linear decline in magnitude. The light curve of a type-II supernova is rather similar, except that the decline portion is more convex. Type-I supernovae attain an absolute magnitude of about —18.6 at maximum; type-II supernovae are about a magnitude fainter. Both types are useful for distance determination, because they can be seen in very distant galaxies.

The spectra of supernovae consist of very broad absorption and emission lines. In this respect, type-II supernovae are the better understood. Their spectra are similar to those of ordinary novae, but with expansion velocities of thousands rather than hundreds of kms-1. From the expansion velocity and the strength of the lines, we can calculate the mass of material ejected by the supernova. We can check our calculation by measuring the mass of a supernova remnant such as the Crab Nebula. In each case, we arrive at a figure of between one and ten solar masses. Clearly, a star that ejects so much material will be profoundly changed.

The nuclear processes which precede a supernova eruption are described elsewhere, we only summarize them here. In the core of a highly-evolved star, there are several processes which can absorb energy, and therefore trigger a GRAVITATIONAL COLLAPSE; these processes were understood in the 1950s. The problem was to con-vert this collapse into the observed expansion of the supernova and its gaseous remnant. Clearly, Nature could effect this conversion, and the consequences for nucleosynthesis were discussed at length by E.M. and G.R.Burbidge, W.A.Fowler and Fred Hoyle in the late 1950s. At that time, astronomers assumed that the supernova eruption would leave no stellar remnant. This assumption was toppled in the late 1960s by the discovery of pulsars, and it soon became evident that pulsars must be rapidly-rotating neutron stars: collapsed stars with densities exceeding 1018kgm~3. The only known way of producing such stars was in the collapse processes which had been implicated in the production of a supernova.

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