novae, present interesting considerations
The so-called new stars, otherwise known as temporary stars or
novae, present interesting considerations. These are stars
which suddenly flash out at points where previously no star was
known to exist; or, in a few cases, where a faint existing star
has in a few days become immensely brighter. Twenty-nine new
stars have been observed from the year 1572 to date; 19 of them
since 1886, when the photographic dry plate was applied
systematically to the mapping of the heavens, and 15 of the 19
stand to the credit of the Harvard observers. This is an
average of one new star in two years; and as some novae must
come and go unseen it is evident that they are by no means rare
objects. Novae pass through a series of evolutions which have
many points in common; in fact, the ones which have been
extensively studied by photometer and spectrograph have had
histories with so many identities that we are coming to look
upon them as standard products of evolutionary processes. These
stars usually rise to maximum brilliancy in a few days: some of
the most noted ones increased in brightness ten-thousand-fold
in two or three days. All of them fluctuate in brightness
irregularly, and usually in short periods of time. Several
novae have become invisible to the naked eye at the end of a
few weeks. With two or three exceptions, all have become
invisible in moderate-sized telescopes, or have become very
faint, within a few months. Two novae, found very early in
their development, had at first dark line spectra, a night
later bright lines appeared, and a night or two later the
spectra contained the broad radiation and absorption bands
characteristic of all recent novae. After the novae become
fairly faint, the bright lines of the gaseous nebula spectrum
are seen for the first time. These lines increase in relative
brilliancy until the spectra are essentially the same as those
of well-known nebulae, except that the novae lines are broad
whereas the lines of the nebulae are narrow. In a few months or
years the nebular lines diminish in brightness, and the
continuous spectrum develops. Hartmann at Potsdam, and Adams
and Pease with the 60-inch Mount Wilson reflector, have shown
that the spectra of the faint remnants of four originally
brilliant novae now contain some of the bright lines which are
characteristic of Wolf-Rayet stars.[2]
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