The appearance of a bright star-like point of light in the sky over only a few nights of observation, at a position where no star had been previously known, was a matter of great interest in ancient times. The phenomenon tended to be ignored by European chroniclers in the Middle Ages because the prevailing cultural belief was the heavens represented a perfect and therefor unchangeable creation by God. On the imperfect world alone was change thought to be permitted, and when change did in fact occur in the sky the matter was dealt with by ensuring no written record of it was made, a practice unknown in modern times. When astronomers today are interested in what occurred a thousand years ago they perforce must consult the records of cultures other than the Europeans, notably of the Chinese, who held a very different but equally erroneous belief, that by noting changes in the heavens they could predict the course of events on Earth.

CLASSIFICATION OF EXPLOSIVE STARS
Such flare-ups in the sky are known nowadays to be of two distinct types with very different mechanisms, Nova and Supernova. We will deal here exclusively with Nova.

Since astronomers today have photographic plates of the whole sky on which even exceedingly faint stars can be seen, it is often possible by examining previously taken plates to find what was there at exactly the same position before the outburst of the nova. An even greater amount of information may be obtained by the spectral features of a nova through its sudden outburst into a decline in brightness over a time interval of weeks and months. The outcome of such investigations is that nova are in the main, if not wholly, white dwarf stars that happen to be members of a binary system.

Nova can be observed in galaxies other than our own, and attempts have been made to use such observations as n indicator of the distances of other nearby galaxies. But the great majority of observed cases are within our own galaxy, where for the limited region of the solar neighborhood they are found to occur at a rate of two a year. The solar neighborhood is determined by the fogging effect of interstellar dust, which cuts down the range of effective observations five percent over the whole galaxy, for which the rate of occurrence is estimated to be about 40 per year.

There are three large classes of nova: fast nova (Na), in which the decline starts immediately after the maximum; slow nova (Nb) which remain a long time near their maximum; and recurrent nova (Nr), which are stars of small amplitude but with nova-like out bursts that recur again and again.

Fast Nova
These stars are characterized by an extremely abrupt rise to maximum, some increasing more than ten magnitudes n one day. The very speedy rise means that it is not often observed and even then only in its upper part. The decline is also fairly rapid. An arbitrary but practical criterion to characterize them has been created: this is based on the time T(3) or the nova to lose three magnitudes with respect to the maximum. Oscillations have shown up in some nova during their decline, some as long as several months induration and over a magnitude in amplitude. There has been a large number of fast nova: out of 121 galactic nova whose type has been determined, 82 belong to type Na.

Slow Nova
The nova Del 1967 (HR Del) is a good example of this type. The star passed from magnitude 12 to one of six in one month and then continued to increase slowly with the maximum magnitude of 3.7 being reached five months later. The decline was even slower. These stars are less numerous than fast nova, only 32 being known out of the 121 known nova of all types. The difference between slow nova and ultra-slow nova is not at al clear-cut but ultra-slow nova are usually classed on their own as novoids.

Recurrent Nova
Recurrent nova are stars that show repeated nova-like variations. The brightest recurrent nova is the brightest with to maxima of magnitude 2. The most active has been T Pyx, whose five maxima were separated by 12, 18, 24, and 23 years.

It is important to point out that the maxima of a given star are the same as regards both the amplitude and the shape of the light curve. This is clearly shows, as we have seen, that the occurrence of the nova, in spite of its violence, does not appreciably alter the structure of the star.

Sometimes the spectrum of the star shows that it is other than it appears to be. WZ Sagitta is such a case. Long considered a recurrent nova, with three explosions 1913 (mag 8.5), 1946 (mag 8.7), 1978 (mag 8.7). However spectroscopic observations made during its last maximum showed that it is not to be classified with the recurrent nova but with the dwarf nova. In addition, it exhibits a spectroscopic oscillation common to dwarf nova. Finally, its absolute magnitude of approx. +10 is different from that of nova.

It is interesting to see how nova are distributed through the Galaxy. The distribution of longitudes is remarkable: out of 161 certain galactic nova, 74 occur between longitudes 345 degrees and 15 degrees, in other words , at least 15 degrees from the galactic center. Nova are thus particularly numerous in the direction of the galactic halo. As regards latitude, nova situated in the direction of the center are never more very far from the galactic plane. In other directions, several nova are known that are quite a long way from the galactic plane, but very few are more than 1500 parsec from it.

All his leads to the conclusion that nova form an intermediate Population II. However, they do not seem to form a homogeneous population. Two nova are known in globular clusters that we know form a typical Population II. In addition they have been found in three elliptical galaxies, which also form a typical Population II.

Many nova have been observed in nearby galaxies: 200 have been found in M 31 alone, which is as many if not more than are own Galaxy (they are on average of magnitude 16-17). Four nova are known in the Small Megallanic Cloud, six in the Large Megallanic Cloud, and a significant number in other galaxies. In all over 500 galactic and extra-galactic nova are now known.

Finally it should be pointed out that the nova observed in other galaxies are roughly of the same magnitude (-6 to -9) and have the same type of light curves as those in our galaxy. These nova are sometimes used to determine the distance of the galaxies in which they are observed. The results are uncertain, however, since the relationship between the speed of decline and the absolute magnitude is not rigorous one.

OBSERVATIONS
Our subjective perception of a nova is confined to light in the visual and photographic ranges of wavelength, omitting the ultraviolet. Since the ultraviolet is of varying importance, probably being of greater importance before the outburst than during it, our subjective perception tends to exaggerate the contrast between the pre-nova stage and the maximum emission of light during the outburst. The observed contrast for the visible light is usually about 10,000 to 1, but if all wavelengths are included the contrast would probably be 100 to 1.

The emission of visible light in the pre-nova stars is of a similar order to the emission of our Sun, whereas the emission at maximum outburst is of an order similar to an F8 super-giant star. A typical nova rises to its maximum in a few days and thereafter declines in brightness by a factor of about 10 in 40 days, although cases of both slower and more rapid declines are known and studies.

Clouds of gas are ejected at high speeds during outbursts, speeds typically of 930 miles/sec, which is more than sufficient for the expelled gases to become entirely lost into interstellar space, together with myriad fine dust particles that condense within the gases as they cool during their outward motion. The total amount of material thus lost is estimated to be about one part in ten thousand of the total mass of the parent white dwarf star, although the amount in especially violent cases is almost surely significantly larger than this.

AMPLITUDE AND ABSOLUTE MAGNITUDE
Nova have varied amplitudes that range from 7 to more than 19 magnitudes, but the value cannot always be determined since the star is often very faint at its minimum. Nevertheless, the amplitudes are known for 76 stars. There are two peaks in the frequencies with which they occur, one for an amplitude of nine magnitudes and the other one of 12. However, it is probable that large amplitudes are more common but are not known.

Recurrent nova have very small amplitudes ranging from eight to ten magnitudes. Slow nova can have large amplitudes, but nova with amplitudes greater than 13 magnitudes are mostly fast nova. Note that tis figure does not include all nova and omits the largest ever Nova Cyg 1975, which is greater than 19 magnitudes. This star is considered by many to be an exception – intermediate between a nova and a supernova.

Much work has been done towards the establishment of absolute magnitudes. Standard methods for determining distances cannot be used at the distances of nova. Other measurements, such as the intensity of interstellar lines, (intensity increasing as the distance of the object increases) and secondly by obtaining the apparent velocity of expansion of the nebulosity, (enabling the distance to be known if radial velocity of the gases has been determined).

Two groups can also be detected by considering the maxima, one with absolute magnitudes around -6 and the other around -9, and these correspond to the two groups in the distribution of amplitudes. This shows that there is a correlation between the absolute magnitude at the maximum, the speed of decline T(3), and the amplitude; the very fast nova and this of large amplitude are also those with the greatest luminosity at maximum.

All these results are corroborated by the observation of nova in the Andromeda galaxy and in the Megallanic Clouds. They also have two peaks in their frequencies of occurrence around -6 and -9, and there is also a correlation observed between T(3) and the absolute magnitude. There are therefore no different from the galactic nova.

VARIATIONS AT THE MINIMA
The pre-nova are generally not very well known, this is not surprising since it is not possible to predict which stars will become nova. However some nova had been known to be variables and so there is a pre-nova history on some stars.

These stars are obviously followed more closely in their post-nova phase. Some f them have fluctuations that are occasionally appreciable, with some sort of small secondary maxima of short duration but which may exceed one magnitude.

High precision photometry has revealed another type of variation that we shall find in many eruptive variables: this is “flickering,” which consists of small rapidly varying flares following each other without interruption.

RS OPh shows a semi-regular variation (P = 70 days) at an amplitude of 0.6 magnitudes. This confirms that there is an M giant in the system linked to a blue star.

DOUBLING
In 1954 it was shown that DQ Her (nova 1934) is an eclipsing binary with a very short period of 4h 39m. Since then all the nova bright enough to be clearly observed have been shown to be double. In this case, therefor, doubling is a general feature.

These binaries are formed from a red star that is large but no very massive and a blue star of high density, which resembles a white dwarf. This dissimilar pair is generally closely bound and has a very short orbital period, usually a few hours.

There are several exceptions to this structure, some pairs have a red component that is a giant star and other cases it is a sub-giant; the pair containing a sub-giant having a much longer rotational period.

In some of these binaries small changes in period which arise from variations in the two stars has been detected. The most interesting case is that of V1500 Cyg. The period has changed from 0.1410 days at the beginning of September 975 (the time of the explosion) to 0.1399 days at the end of October and to 0.1384 days in May-June 1976. The period of the binary system may thus have been changed by the violence of the explosion.

CAUSE OF NOVA
Much thought has been given over the last few decades to the cause of nova. The consensus of opinion is that the basic process of the explosion is the same as that for man-made nuclear weapons. To begin with, energy is produced at a comparatively gentle rate in material that is covered by a layer of relatively inactive other materials, tamped as one says. Because of the tamping materials the energy produces by nuclear reactions cannot escape and must therefore accumulate within the reacting material itself. Provided the tamping effect is strong enough, the rising temperature and pressure causes the nuclear reactions to become less and less gentle as the process proceeds. The same cycle of events is repeated and repeated again with the energy released and repeated again with the energy released from the nuclear reactions accelerating at an ever-increasing rate, until eventually the situation gets quite out of hand. Or at any rate until the tamping effect of overlaying material fails at last and the material is blown entirely out of the strong gravitational field of the white dwarf star, its chemical composition, and the amounts of the reactions and the tamping materials, the details of explosion can vary in ways that are subject to mathematical calculation, and which have been found to agree with many of the observed features of nova.

The mechanism causing a star to become a nova can be broadly describes as follows.

The mass of the ejected gas is relatively small: 0.0001 to 0.00001 of a solar mass. On the scale of the stars involved, this is extremely minute and the structure of the star is not affected by the explosion, yet it is as much as several times or several dozen times the Earth’s mass. This is the reason that recurrent nova are possible.

The white-dwarf stage represents the end point of a star’s evolution. Subsequently, the star simply cools, eventually becoming a black dwarf—a burned-out ember in interstellar space. This scenario is quite correct for an isolated star, such as our Sun. However, should the star be part of a binary system, an important new possibility exists. Consider a pair of stars one of them a large reddish star of low density which has reached or exceeded its Roche Lob, and the other a very dense white dwarf. Some of the material of the red star escapes and is transferred to the white dwarf. The transfer takes place in two stages: the material first falls into a kind of disc or ring which surrounds the white dwarf known as an accretion disc. At a later time, the matter which is attracted by the strong gravitational force from the whole dwarf leaves the disk and falls on to the dwarf at high speeds and with great turbulence. If the distance between the two stars is small enough, then the dwarf’s tidal gravitational field can pull matter—primarily hydrogen and helium—away from the surface of its main-sequence or giant companion. A stream of gas leaves the companion through the inner (L1) Lagrangian point and flows onto the dwarf.

The light curve of a typical nova. The rapid rise and slow decline in the light received from the star, as well as the maximum brightness attained, are in good agreement with the explanation of the nova as a nuclear flash on a white dwarf’s surface.

As it builds up on the white dwarf’s surface, the stolen gas becomes hotter and denser. Eventually its temperature exceeds 107 K, and the hydrogen ignites, fusing into helium at a furious rate. The arrival of the gas at the accretion disc, already hot and becoming hotter still as it falls on to the white dwarf, provokes the eruption: a powerful nuclear explosion is produced in the atmosphere of the star, and the white dwarf suddenly ejects the blanket of foreign matter which covers it: this is the beginning of the phenomenon known as a nova. This surface-burning stage is as brief as it is violent. The star suddenly flares up in luminosity then fades away as some of the fuel is exhausted and the remainder is blown off into space. If the event happens to be visible from Earth, we see a nova. The nova’s decline in brightness results from the expansion and cooling of the dwarf’s surface layers as they are blown into space. Studies of the details of these curves provide astronomers with a wealth of information about both the dwarf and its binary companion.

Because of the binary’s rotation, material leaving the companion does not fall directly onto the dwarf. Instead, it “misses” the compact star, loops around behind it, and goes into orbit around it, forming a swirling, flattened disk of matter called an accretion disk. Due to the effects of viscosity (that is, friction) within the gas, the orbiting matter in the disk drifts gradually inward, its temperature increasing steadily as it spirals down onto the dwarf’s surface. The inner part of the accretion disk becomes so hot that it radiates strongly in the visible, the ultraviolet, and even the X-ray portions of the electromagnetic spectrum. In many systems the disk outshines the white dwarf itself and is the main source of the light emitted between nova outbursts. X rays from the hot disk are routinely observed in many galactic novae. The point at which the infalling stream of matter strikes the accretion disk often forms a turbulent “hot spot,” causing detectable fluctuations in the light emitted by the binary system.

A white dwarf in a semidetached binary system may be close enough to its companion (in this case, a main sequence star) that its gravitational field can tear material from the companion’s surface. Notice the matter does not fall directly onto the white dwarf’s surface. Instead it forms an “accretion disk” of gas spiraling down onto the dwarf.

RECURRENT NOVA
Some are known to undergo explosions repeatedly, for example T Corona appeared as a nova in 1866 and 1946, while T Pyxids did so in 1890, 1902, 1922, and 1944. It is thought that all stars experiencing the nova phenomenon probably do so repeatedly and that is the ones under-going explosion most frequently that ten to be noted like T Corona and T Pyx.

In order to go through the explosion several times it is necessary for a white dwarf to repave what had been lost in previous explosions. It is important that there should be a binary companion, for the companion stars serves as the source of material to the white dwarf. The favorable case is where the companion serving as the source is a giant type, which is to say a star of very large radius from which material escapes rather easily. So the favored system for understanding the nova phenomenon is a binary system with components not very far apart, one component a giant star and the other a white dwarf. Material can then drain from the surface of the companion star where gravitation is weak to the surface of the small white dwarf where gravitation is strong, and where only violent explosions can serve to eventually blow the material into space.

UNUSUAL MATERIAL COMPONENTS
The circumstances in which the nuclear reactions occur, especially if a supply of protons is available for mixing with carbon and oxygen, lead to the production of some nuclides that are not synthesized by nuclear processing occurring towards the center of stars. Examples are N(15) and Al(26). It is also possible for neutrons from reactions of alpha particles with C(13) lead to a form of r-process, a process which very heavy, neutron-rich nuclei are synthesized. Condensing solid grains in the gasses expelled by nova may be expected to contain such unusual nuclides, and so would form a component of interstellar material with an unusual chemical contribution.

PRIMORDIAL MATERIAL STILL AROUND
When the Solar System formed from interstellar grains, some grains derived from nova would be present. It is an interesting and controversial question as to whether a fraction of early grains of unusual composition have been preserved to this day, for example in meteorites. There is evidence to show that this is so.

The total energy of explosions of a typical nova has been estimated at 1045 ergs, which is to say about as much energy as the Sun emits in 10,000 years, or about as much as emitted by the simultaneous explosion of 1021 (1,000,000,000,000,000,000,000) manmade nuclear weapons.