Novae form a very interesting class of non-stationary objects. A considerable amount of observational data, both photometric and spectroscopic, covering different stages of nova outbursts, have been stored up by observers for many years. Processing and systematization of these data has lead to a better understanding of the physical processes taking place in the nova outbursts.

The first spectra of novae at maximum brightness proved to be similar to the spectra of supergiants; their distinctive feature was the presence of the strong lines of lines from C, N and O (McLaughlin, 1936). This fact lead to suspect that these elements must be overabundant in the nova envelopes.

More than a dozen classical novae in outburst have been observed with the International Ultraviolet Explorer since its launch in 1978. These data represent a crucial breakthrough in understanding several aspects of the classical novae phenomenon including nucleosynthesis, the energetics of the explosion, the dynamics of the mass ejection, and the processes of dust formation. Also they are important to gain insight on the nature of the white dwarf components and then on the evolutionary scenario of these binary systems.

The UV emission lines include important transitions from several species such as He, C, N and O, Ne, Mg, and Al which, whenever symultaneous optical data were available, have lead to a detailed knowledge of the physical conditions in the nebular phases and to the determination, with unprecedented accuracy, of the chemical abundances in the ejecta (see e.g.: Stickland et al. 1981; Williams et al. 1985; Snijders et al. 1987). Precise abundance determinations provide crucial constraints on nucleosinthesis during TNR as well as on the efficiency of diffusion and mixing processes. The ultraviolet range provides also effective means to determine the value of the reddening from the strength of the 2200 A dust feature or from the emission line ratios; an accurate determination of this parameter from the UV is essential, for example, to determine the energy budget of novae and the temperature of the remnants.

A nova outburst is the second most violent explosion that can occur in a galaxy. While less violent than a supernova explosion, novae are considerably more frequent. More than 250 have been observed in the last century and there were more than a dozen novae being observed in outburst when this review was written. Over the past few years they have been studied at wavelengths ranging from the radio to the X-ray and these data have provided exciting new information about the nova outburst. For example, IUE studies have shown that there are two classes of novae; those that occur on carbon-oxygen white dwarfs and those that occur on oxygen-neon-magnesium white dwarfs. EXOSAT observations have shown that an intense X-ray emitting source exists in the system for years after the initial outburst has been discovered and radio studies have found that some novae are ejecting material in jets. IR studies have found high excitation coronal lines, dust formation, and have provided accurate distances. Optical studies have provided determinations of densities and temperatures in the ejecta and have discovered a class of novae that occur on strongly magnetized white dwarfs. There have been predictions but no detections of γ-ray emission. Finally, novae are time dependant and simultaneous or contemporaneous observations are required in order to learn as much as possible about the characteristics of the outburst.

Recent infrared studies of classical novae are described. The nucleation and growth of dust in nova ejecta was confirmed by optical/infrared photometry of several novae during the past five years including QU Vul, V842 Cen, V827 Her, and QV Vul. These novae provided evidence that dust of every astrophysical chemical and mineral composition can condense in nova ejecta. Infrared forbidden fine structure lines were identified in the recent novae V1819 Cyg, QU Vul, OS And, and V827 Her. The implication of IRAS measurements for nova energetics is discussed.

The discovery of soft X-rays from Nova GQ Mus with the EXOSAT satellite has added an important channnel to the observational information on classical novae during the outburst stage (Ögelman, Beuermann, and Krautter 1984). With peak luminosities in excess of 1038 erg s−1 and total liberated energies of more than 1045 erg, novae constitute the second most energetic outburst phenomena in the galactic population exceeded only by supernovae (see Gallagher and Starrfield, 1978; Truran, 1982 for a review of the observations and theory). Regardless of the details of the outburst, objects of the size of white dwarfs (~ 109 cm) radiating close to Eddington luminosities (~ 1038 erg s−1 ) should emit soft X-rays provided that the envelope is sufficiently transparent. In addition, the ejected shell, moving at velocities around 1000 km s−1, is capable of shocking the circumstellar medium to X-ray temperatures.

New results presented include the oscillations of intervals between maxima of light oscillations, while in the case of GK Per the Orion system velocities oscillated with twice the light oscillation period.

The two classical novae of 1988 in the LMC are the first extragalactic novae, which have been studied in the UV range. IUE spectra were taken under the Nova Target of Opportunity program, and quasi-simultaneous optical spectra were obtained by the authors. The spectral development in the UV and visual ranges is discussed for both objects.

PW Vul (Nowa Vul 1984 #1) was a slow classical nova which began its outburst in July 1984 and reached optical maximum in the first week of August 1984. Eight low dispersion spectra (160 A/mm) in the blue spectral range (λλ3400-5100 A) were secured between October 1984 and July 1985, with the CCS spectrograph equipped with 3.5 arcsec aperture image slicer and mounted on 90 cm Schmidt-Cassegrain telescope at Toruń Observatory. Kodak IIa-O plates were used. In addition two moderate dispersion (40 A/mm) spectra in the Hγ-Hβ range were obtained on 28 October and 3 November 1984, using the same instrumentation. Standard stars were observed on each night to remove the influence of spectral sensitivity of the photographic emulssion and atmospheric extinction. The calibration of the absolute flux scale was performed by comparing synthetic B magnitudes calculated from our spectra with published B photometry of PW Vul (Voloshina 1985, Noskova et al. 1985, Kolotilov and Noskova 1986). The accuracy of the flux calibration is ±0.1 mag.

Nova Cygni 1975 (V1500 Cyg) was outstanding in several respects. It was the most luminous galactic nova ever observed, had the largest outburst amplitude, and had the fastest evolution with time.

The spectra, with dispersion 28 A/mm and in spectral range λλ3560-5055 A, were obtained with the CCS spectrograph on 90 cm reflector at Toruń Observatory by the research group supervised by prof. A. WoszczyK. The CCS spectrograph was equipped with the 3.5 arcsec aperture Richardson’s image slicer and stars with known spectral energy distributions were observed on each night, so we were able to derive absolute energy distributions for the V1500 Cyg spectra. The calibration of the energy scale was performed by comparing synthetic B magnitudes calculated from our spectra with numerous published B photometry of the nova. The first spectra were obtained about 3h later than spectra reported by Boyarchuk (1977) and about 6h earlier than spectra discussed by Duerbeck and Wolf (1977); thus they are among the earliest spectra of the nova available. Simultaneously, the spectra correspond to the earliest state of spectral evolution of a nova ouburst yet observed. The pre-maximum spectral evolution of Nova V1500 Cyg is extremely fast. The spectra (Figures 1 and 2) secured on JD 2442654 are characterized by a very strong continuum and broad blueshifted absorption lines of OII, NII and CII of unusual strengths, as well as the Balmer series and Hel.

LV Vulpeculae is a fast nova with a relatively smooth light curve (Raikova, 1981). On Fig. 1 the picture of the radial velocities, displayed during the outburst is shown. It is based on our measurements and the published ones (a more detailed paper will appear in C. R. Acad. Bulg. Sci.). The premaximum absorption line system (A) exhibits a rapidly decreasing velocity. It has been observed for the last time on April 17.52 (Hutchings, 1970) together with the diffuse enhanced system (B) of velocity −1260 km/s. Since the presence of the last system is an indication that the light maximum has passed, we can limit a 2h40m interval in which it had occured (on April 17.385 the brightness is still up). We assume that the light maximum has taken place at the middle – on April 17.45. On April 18 the diffuse enhanced line system, now of −1400 km/s, and the principal one (C) of −780 km/s exist. It is worth-while to emphasize that in LV Vul spectrum the diffuse enhanced system appeared before the principal one.

The very slow nova X Ser (t3 = 555d), situated at a large distance from the galactic plane (z ≈ 4kpc), reached maximum light at 9m in May 1903. It was discovered 5 years later on a Harvard plate (Leavitt 1908). No outburst spectrum is known. A photographic outburst light curve is given by Walker (1923); additional photometric data were published by Sakharov (1954) and Kinman et al. (1965). The light curve, shown in Fig. 1, presents 10-day means, based on all published data, and unpublished data taken from Emily Hughes Boyce’s notebook, Harvard College Observatory Plate Collection.

Hughes Boyce (1942) discovered a 275d periodicity during minimum light. From the fragmentary light curve in Fig. 1, including three more recent ‘oscillation’ maxima, a period of 277 days is found. If one assumes that the nova reached Mpg = −6 at maximum, the typical value for a slow nova, the mean absolute magnitude at minimum, Mpg = +2, together with the observed amplitude of about 2m and a relatively long mean period, suggests a very late-type (typically M6e) semi-regular variable.

Radio observations of novae provide direct information about masses, temperatures, and velocities of ejected material, particularly when the radio remnants are resolved or imaged. The radio images of Nova QU Vul 1984 indicate that the mass in its ejecta is 3.6·10−4M⊙, confirming the generally higher masses inferred from radio light curves. All thermally-emitting nova shells that can be observed by the VLA should be resolveable, allowing determination of angular sizes and asymmetries.

Resolved nebular remnants of classical novae are discussed in terms of size, surface brightness, and shape. A list of 26 known resolved remnants is given. The conditions necessary for visibility of a resolved remnant are discussed. The structure of remnants can be studied at optical wavelengths by direct imaging and by long-slit or otherwise spatially resolved spectroscopy of emission lines. Based on recent spatiokinematic studies, most nebular remnants are prolate in outline, with sub-structure that has been characterized as consisting of “polar blobs” and “equatorial rings”. The mechanism that shapes the remnants is not yet known with certainty. There is room for much additional work in discovering new remnants and in characterizing those that are known.

We discuss the derivation and behaviour of temperatures and densities of nova ejecta during the nebular stage. A systematic comparison is made between fairly complete sets of abundances for 3 C/O and 2 Neon novae. The abundances in the ejecta might influence the formation and composition of circumstellar dust in novae.

We obtained a narrow band Hα image of V1500 Cyg during an engineering run at the R-C focus of the KPNO 4 m telescope on UT 1987 July 19. Our detector was an 800 × 800 format TI CCD, which yielded a plate scale of 0.1013 arcsec/pixel. The exposure was 2000 sec and was made through a 75 Å wide filter centered at 6563 Å. The seeing was ~ 1.2 arcsec.

Our reductions were accomplished with the DAOPHOT photometry package (Stetson 1987) and IRAF data reduction facility. After performing standard bias level subtraction and flat field division, we used DAOPHOT to find the frame’s normalized point spread function (PSF) by summing the images of several field stars. We then scaled the PSF to calculate the instrumental magnitudes of the stars and of V1500 Cyg. This procedure overestimates the brightness of the nova, since the nebula contributes extra light to the central object and modifies the expected intensity distribution. (This is especially true in the narrow Hα bandpass.) Hence, in a frame where the PSF has been used to remove the fitted images, all the stars have satisfactorily small residuals around the mean sky level, except the nova itself. The image of V1500 Cyg shows sky level at the center, but a halo that rises from the center and then falls away.

We present 8 – 13 μm spectra (R ≃ 40) of Nova Cen 1986 (V842 Cen) on 8 occasions between days 162 and 899 after outburst, and all after the onset of dust formation. All observations were made with the UCL cooled grating array spectrometer on the 3.9m Anglo-Australian Telescope or the 2.3m ANU telescope in Australia. All but the last observation (day 899) show evidence for emission from at least two dust components; a featureless continuum (usually attributed to carbon based dust), a broad emissive component centered on 10 μm (probably "silicates"), and the 11.3 μm feature from the family of unidentified infra-red dust features (UIR’s) seen on days 271 – 687.

We have observed the old nova NQ Vul in the J = 2 → 1 rotational transition of 12CO at 230.5 GHz at the University of Texas Millimetre Wave Observatory. The spectrum shows narrow features which clearly arise in the local interstellar medium. However these local features are superimposed on a broad feature which peaks at ~ 63 mK. This feature is centred on velocity VLSR ≃ 26 ± 9 km s−1–consistent with that expected for NQ Vul–and has FWHM 80 ± 20 km s−1. The peak antenna temperature corresponds to an integrated flux of 3.2 × 10−15 erg s−1 cm−2.

Assuming LTE and a distance of 1.2 kpc the mass of CO is ~ 10−6M⊙ if the line is optically thin. The CO mass is comparable with the total mass ejected in 1976 (4) so the CO we detect at millimetre wavelengths has nothing to do with the 1976 outburst. If the CO/H ratio in the emitting material is similar to that in the interstellar medium (2), the total mass is ~ 0.6 M⊙.

The CO mass rules out an origin both in the 1976 outburst and in the post-outburst phase: the CO must have originated in material ejected by the NQ Vul system prior to the 1976 outburst. There are two possibilities. First, the CO may have formed in material accumulated following a large number of nova outbursts. Second, the CO may have been present, or formed, in material ejected by the NQ Vul system during a previous evolutionary phase. The deduced mass is comparable to the mass of CO seen around planetary nebulae (3); the outflow velocity (~ 40 km s−1) would also be in line with this interpretation.

In either case, the above mass estimate of 0.6 M⊙ (based on the interstellar CO/H ratio) is likely to be an upper limit as we would expect an enhancement of heavy elements in any ejected material. A determination of 12C/13C and other isotopic ratios would be valuable to pin down the origin of the CO.

Near IR grating spectrophotometric observations (λ / Δλ ≃ 400 ) of Nova Ophiuchi 1988 between 0.9 and 1.35 μm on September 22.2, 1988 UT (J.D. = 2447426.7) are reported. We observed the nova with the Lick Observatory 3–meter Shane telescope using the Aerospace cooled grating spectrometer. A 7.5 arc sec aperture was used. The detector was a germanium photodiode, cooled to 77 K and used in conjunction with a charge integrating amplifier. These are the first IR observations ever made of a slow nova.

Figure 1 shows the spectrum which contains Paschen emission lines β, γ δ, ε, ζ, η, He I λ1.083 μm, and a very weak continuum, all consistent with a post-maximum, early nebular phase. Figure 1 also shows a number of possible weak lines including [ Fe II ] and He I as well as an unidentified feature at 1.01 μm in the far red wing of Paδ. The line to continuum ratio for He I λ1.083 is about 300. The emission lines are resolved in the stronger features ( Paδ Paζ, Paβ, and He I λ1.083) and show a mean FWHM of 900 ± 150 km/sec.

We present results for IUE, optical and IR observations of Nova Muscae 1983, from early outburst to January 1986 obtained by the European IUE Target of Opportunity Team. A detailed description of the data will appear elsewhere (Hassall et al., 1989), but here we summarise the most important results.

The outburst lightcurve initially indicated a fast speed class for this nova, but was later characterised by a rather slow optical decline with two or more secondary outbursts with sudden doubling of the bolometric flux. In Figure 1, we show the contributions of X-ray, UV, optical and IR to the total luminosity for 1200 days following outburst, assuming a distance of 4.3kpc and an interstellar extinction E(B-V)=0.5. In the absence of dust formation, first the UV and later the X-ray flux (Ögelman et al, 1984) dominate the radiative energy late into the nebular phase. There was a plateau stage lasting about 500 days, with a bolometric luminosity of ~ 1038ergs s−1 near the Eddington limit. The secondary outbursts were thus super-Eddington. We estimate a total outburst energy (including kinetic and gravitational potential energy of the ejecta) of ~ 5·l046 ergs, corresponding to a mass of ~ 4·10−6Mº of hydrogen burnt in the thermonuclear runaway.

Element abundances, electron temperatures and densities of the shell ejected during the outburst of the classical nova PW Vulpeculae were determined using ultraviolet (IUE) and optical (ESO 1.52m + B&C + IDS) spectra obtained during the nebular phase (April - July 1985). The C, N, O abundances are enhanced by factor 10 - 100 relative to solar values according to the predictions of the TNR (thermonuclear runaway) theory. The overabundances of Ne, Si, Mg are comparatively small compared with other novae indicating that the outburst of this slow nova occurred on a CO white dwarf.