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a rec designates a recurrent nova;Z And is a type of symbiotic variable star—a complex interacting binary star system probably involving a hot dwarf star encircled by a disk fed by a gas stream produced by the companion red giant star.

a rec designates a recurrent nova;Z And is a type of symbiotic variable star—a complex interacting binary star system probably involving a hot dwarf star encircled by a disk fed by a gas stream produced by the companion red giant star.

white dwarf is slower because of a mass difference between the pair. The more massive the object, the faster the evolution. As we have mentioned already, the process of stellar evolution causes a star, sooner or later, depending on its mass, to expand into a red giant. When it does so in a binary star system, the red giant envelope material facing the other star may be as equally attracted to that other star as to its own, and with the outward pressure, it begins to stream or spiral down onto the other star. The star that is currently the white dwarf would have had to undergo this process, losing mass both to its companion and to the outside universe. The mass gained by the companion helped it to evolve faster, and eventually, to return mass to the (now) white dwarf. There is a big difference, however, in the way this mass is received by the white dwarf, and how the receiver deals with it. As the material, mostly in the form of hydrogen gas, spirals downward in the gravitational field of the white dwarf, it heats up and overlays the surface matter of the white dwarf. Mixing with still hotter material in the interior of the white dwarf ignites a runaway thermonuclear explosion, burning the fresh hydrogen. The resulting fireball greatly exceeds the size of the area of the white dwarf and may increase the brightness of the system by 10,000 times or more. It is likely that all novae are members of binary star systems. If it is always the case, then all novae are recurrent to some extent, but the interval cannot be predicted with much precision, because it depends strongly on the transfer rate. The rate of mass transfer varies from system to system, and material may be "stored" in an accretion disk around the white dwarf component for some time before a cataclysmic eruption again takes place. It may take years or centuries or even longer, with some systems erupting much more frequently than others. Novae reach peak brightness in a few days or less and may slowly decline for hundreds of days or so before reaching the former level of faintness. One of the brightest recent novae was V1500 Cygni (Nova Cygni 1975) that, at peak brightness was visible in the twilight sky along with the stars of the summer triangle.39 It declined

39 Altair, Deneb, and Vega.

below the naked-eye limit (~6th magnitude) within about three months. The rates of decline of novae are tied to their brightness—the greater the peak brightness, the quicker the initial decline. As a consequence, novae are often classified as "fast" or "slow." About 10 novae per year are estimated to occur in the galaxy.

Table 5.10 lists some of the brighter novae and novae-like objects of recent times. The positions are to the equinox 2000. Note the frequencies of recurrence documented for T Pyxidis and RS Ophiuchi. Recurrence is also suspected among the eruptive variables classified as "novae," which were recorded by ancient astronomers as well.

A selected sample of the historic eruptive variables is provided in Table 5.11, where "rec." means the star has been observed to be a recurrent nova, and Z And is a type of symbiotic star—a system in which there is evidence of a hot blue object and a cool giant star that interact.

In the Chinese chronicles from the middle of the Han dynasty onward, novae are listed as kho hsing, "guest stars," presumably because they are not seen to have taken up permanent residence in the sky once they appear. These stars are distributed on the sky in the same way as modern novae, verifying that in the main, these records do in fact refer to galactic objects and are not astrological inventions. The oldest record of a nova may be the inscription on an oracle bone (from an ox), dating from 1300 b.c. According to the translation provided by Needham/Ronan (1981, pp. 205-206), the inscription states, in part, that "On the 7th day of the month a chi-ssu day, a great new star appeared in company with Antares." Another inscription from the same period reads that "On a hsin-wei day, the new star dwindled" (or "disappeared"). The day referred to, hsin-wei, is two days after the day, chi-ssu, but the names repeat at 60-day intervals (see §10.1.2.1 for a discussion of the Chinese sexagenary cycles), and we do not know if the inscriptions refer to the same event. Among the many other guest-stars recorded are four we today would consider supernovae.

Most spectacular of all the eruptive variables are the supernovae. A true cosmic catastrophe, a supernova explosion means the destruction of a star. The increase in brightness may be up to hundreds of millions of times. From the

Table 5.11. A sample of ancient :

novae.

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