Novae
Novae. Close binary systems with orbital periods from 0.05 to 230
days. One of the components of these systems is a hot dwarf star
that suddenly, during a time interval from one to several dozen or
several hundred days, increases its brightness by 7-19 mag in V,
then returns gradually to its former brightness over several
months, years, or decades. Small changes at minimum light may be
present. Cool components may be giants, subgiants, or dwarfs of
K-M type. The spectra of novae near maximum light resemble A-F
absorption spectra of luminous stars at first. Then broad
emission lines (bands) of hydrogen, helium, and other elements
with absorption components indicating the presence of a rapidly
expanding envelope appear in the spectrum. As the light
decreases, the composite spectrum begins to show forbidden lines
characteristic of the spectra of gas nebulae excited by hot
stars. At minimum light, the spectra of novae are generally
continuous or resemble the spectra of Wolf-Rayet stars. Only
spectra of the most massive systems show traces of cool
components.
Some novae reveal pulsations of hot components with periods of
approximately 100 s and amplitudes of about 0.05 mag in V after an
outburst. Some novae eventually turn out to be eclipsing
systems. According to the features of their light variations,
novae are subdivided into
fast (NA)
slow (NB)
very slow (NC)
and recurrent (NR) categories.
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