..... Click the link for more information. ; variable star variable star, star that varies, either periodically or irregularly, in the intensity of the light it emits. Other physical changes are usually correlated with the fluctuations in brightness, such as pulsations in size, ejection of matter, and changes in spectral
..... Click the link for more information. .
nova
Any of a class of stars whose luminosity temporarily increases by several thousand up to a million times normal. Most appear to be close binary stars, one of which is a white dwarf star drawing in matter from the other until it becomes unstable, causing an outburst in which the outer layer of material is shed. A nova reaches maximum luminosity within hours after its outburst and may shine intensely for several days or even a few weeks; it then slowly returns to its former level. The process can repeat at intervals ranging from a few dozen to hundreds of thousands of years. Stars that become novas are usually too faint to see with the unaided eye until their sudden increase in luminosity, sometimes great enough to make them readily visible in the night sky. To observers, such objects may appear to be new stars; hence their name (Latin for “new”). See also supernova.
nova
nova [′nō·və]
| (processor) | Nova - A minicomputer(?) introduced by Data General
in 1969, with four 16-bit accumulators, AC0 to AC3, and a
15-bit program counter. A later model also had a 15-bit
stack pointer and frame pointer. AC2 and AC3 could be
used for indexed addressing and AC3 was used to store the
return address on a subroutine call. Apart from the small
register set, the NOVA was an ordinary CPU design. Memory could be accessed indirectly through addresses stored in other memory locations. If locations 0 to 3 were used for this purpose, they were auto-incremented after being used. If locations 4 to 7 were used, they were auto-decremented. Memory could be addressed in 16-bit words up to a maximum of 32K words (64K bytes). The instruction cycle time was 500 nanoseconds(?). The Nova originally used core memory, then later dynamic RAM. Like the PDP-8, the Data General Nova was also copied, not just in one, but two implementations - the Data General MN601 and Fairchild 9440. Luckily, the NOVA was a more mature design than the PDP-8. Another CPU, the PACE, was based on the NOVA design, but featured 16-bit addresses (instead of the Nova's 15), more addressing modes, and a 10-level stack (like the Intel 8008). |
Nova
a star whose luminosity abruptly increases thousands or even millions of times (104 times on the average) and then slowly decreases. The greatest luminosity is observed for periods ranging from one to two hours (fast novae) to several days (slow novae). It takes several years for the luminosity to decline to the initial value.
The term “nova” originated in antiquity, when stars that became visible in the sky as a result of increase in brightness were thought to have been newly formed stars. Photographic studies have refuted this view: by the early 20th century it was proved that these stars exist before their outburst but have a considerably lower luminosity, to which they return after the outburst. The light curves of various novae resemble one another (Figure 1). During the period of maximum brightness, some novae have a magnitude of 1 or 2, sometimes higher. Such novae were observed in 1901 in the constellation Perseus, in 1918 in the constellation Aquila, in 1925 in the constellation Pictor, in 1934 in the constellation Hercules, and in 1942 in the constellation Pup-pis. As of the 1970’s, a total of more than 180 nova outbursts have been observed in the Milky Way Galaxy. According to statistical calculations, there are about 100 nova outbursts in the Galaxy each year, but only one or two are observable from the earth. Novae have also occurred in neighboring galaxies: 230 in the Andromeda Galaxy and 15 in the Magellanic Clouds.
The rise to maximum brightness is rapid, and consequently the light curve at this stage has been poorly studied. It is known that when the brightness reaches a value two magnitudes below maximum, the increase in brightness temporarily stops for several hours to several days.
The light curves of novae exhibit the greatest diversity in the transition stage, where three basic types of brightness variation are noted: (1) a gradual decrease in brightness, (2) large periodic oscillations, and (3) a deep minimum of several weeks’ duration followed by a partial restoration of brightness.
The changes in brightness are accompanied by large shifts in the spectra. Before their outburst, novae are hot stars of spectral class O or B. However, there are few observations of the spectra of novae before flareup.
As a nova approaches maximum brightness, its spectrum acquires features characteristic of high-luminosity stars of spectral class A or F, with narrow absorption lines displaced toward the short-wavelength end. This indicates that the outer layers of the nova’s atmosphere are expanding at a rate of about 1,000 km/sec; for slow novae, the rate is somewhat less. Immediately after maximum, emission lines, mainly of hydrogen and ionized metals, appear in the spectrum. The decline in brightness is accompanied by intensification of the emission lines, as well as the appearance of new absorption lines. This is associated with an additional ejection of material. When the star’s brightness decreases five magnitudes, the nova’s nebular stage begins, and its spectrum at this time strongly resembles that of a planetary nebula. The nebular stage lasts for several years. Many years after the outbursts, the spectra of novae resemble those of white dwarfs.
Nova outbursts are connected with a loss of stability in a star’s outer layers and the ejection of material. However, the explosions do not affect the star as a whole. The fraction of the star’s mass ejected during the outburst averages about 10–5 of the star’s mass, or about 1028g. The total energy of a nova outburst is equal to about 1045 ergs, or 1038 joules. The star’s envelope is ejected either at the very start of the outburst, that is, when the brightness begins rising, or, according to the Soviet astronomer E. R. Mustel’, at maximum brightness. In the latter case, the increase in brightness is related to the expansion of the star itself, which begins to contract after the maximum. The appearance of bright emission lines and other features in a nova’s spectrum after maximum is caused by processes originating in the ejected envelope. The spectral emission lines arise as a result of both the envelope’s absorption of light from the very hot exposed layers of the star and the interaction of atoms in the envelope with high-speed particles emitted by the star for some time after maximum brightness. As the envelope expands, its density decreases and it becomes more ionized. At a density of about 10-19 g/cm3, spectral lines characteristic of a highly rarefied gas appear, indicating the onset of the nebular stage.
Several years after the outburst, the envelopes ejected by many novae expand great distances from the stars and become visible from the earth. As a rule, they are inhomogeneous and form two large clumps, called polar condensations, in opposite directions from the star. The star’s magnetic field may play an important role in the formation of the envelope’s shape: if this field, as is proposed, has a dipole character, then the ejection of material occurs primarily along the axis connecting the star’s magnetic poles. We can determine the distance to a nova by means of data on the angular velocity of expansion of nova envelopes and the velocity of expansion obtained from an analysis of the envelope’s spectrum.
It was discovered in the 1950’s that novae are members of close binary-star systems, in which the distance between the components is of the order of the radii of the stars themselves. The second components of the pairs are cooler stars. The study of binary systems with novae has made possible the first reliable estimates of the masses of novae. It turns out that, on the average, nova masses do not differ significantly from the mass of the sun.
The luminosities of novae in the Galaxy cannot be determined with great accuracy. One of the principal methods of estimating the luminosities at maximum brightness is provided by an empirical relation between the absolute stellar magnitude at maximum and the rate of decrease after maximum: the higher the maximum, the faster the brightness diminishes; novae are classified as fast or slow on the basis of the rate of decrease in brightness. This relation has the form
Mv, max = –11.5 + 2.5 log t3
where Mv, max is the nova’s absolute visual magnitude at maximum and t3 is the time (in days) it takes for the star’s brightness to decrease three magnitudes. This relation is satisfied not only by novae in our Galaxy but also by those in the Andromeda Galaxy and the Magellanic Clouds. The average absolute visual magnitude of novae at maximum is
Mv = –7.3 magnitude
Thus, novae are the brightest objects in our Galaxy, after super-novae. Because of their high luminosity, novae serve as indicators of distances to nearby galaxies. At minimum brightness, the absolute magnitude of a nova is relatively faint; on the average Mv, min = +3.5 magnitude. In some stars, the light at minimum is determined by the cooler component, which at this stage is brighter than the nova itself. By all parameters—mass, luminosity, and size—novae in the stable state are dwarfs.
Recurrent novae do not significantly differ from regular novae, except for the speed with which the star returns to its prenova state. This time is usually about one year. As of the 1970’s, 11 recurrent novae are known. Of these, the star T Pyxi-dis experienced the greatest number of outbursts (five) in the period 1890 to 1967.
In the late 1960’s it was discovered that novae emit intense infrared radiation, which increases in intensity as the brightness diminishes. In the novae observed during this time, maximum infrared radiation was recorded about 100 days after maximum brightness in the visible region of the spectrum. It is possible that this infrared radiation is caused by heated dust particles ejected by the nova or formed in the ejected envelope.
The causes of nova outbursts are still not very clear. However, there is no doubt that the outbursts are the result of increased instability within dwarfs of small mass. Most contemporary hypotheses view a nova outburst as a thermal explosion occurring as a result of a disruption of the thermal equilibrium in the deep inner layers. The shock wave from the explosion travels to the star’s surface with velocities of the order of 1,000 km/sec and tears off the outer layers of the photosphere. Similar hypotheses were developed by the Soviet astronomers A. I. Lebedinskii and L. E. Gurevich, the French astronomer E. Schatzmann, and others. According to Schatzmann, the outburst is caused by the accumulation in the star’s interior of the isotope 3He, which leads to a nuclear explosion within the star; the isotope is destroyed during the explosion but then accumulates anew, which may explain the recurrence of the outbursts. After novae were discovered to be binaries, hypotheses were advanced linking outbursts to the structure of close binary stars. According to the hypothesis proposed by Schatzmann (1958), the coincidence of the orbital period with the period of natural oscillation of one of the binary components may lead to an explosion with an ejection of material both in the direction of the perturbing companion and in the opposite direction; this is how the observed shapes of nova envelopes are explained.
The place of novae in the general scheme of stellar evolution has not been established with certainty. However, there is no doubt that nova outbursts occur in the late evolutionary stages of stars, probably binaries. The outbursts may precede a star’s transformation into a white dwarf.
REFERENCES
Vorontsov-Vel’iaminov, B. A. Gazovye tumannosti i novye zvezdy. Moscow-Leningrad, 1948.Zvezdnye atmosfery. Edited by J. Greenstein. Moscow, 1963. Chapter 17. (Translated from English.)
Eruptivnye zvezdy. Moscow, 1970. Chapter 1.
Payne-Gaposchkin, C. The Galactic Novae. Amsterdam, 1957.
V. P. ARKHIPOVA
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