Academy, Saturday 7th February 1874
Mr. Birmingham, of Tuam, has lately been examining the red
stars comprised in the list formed by Schjellerup some years ago, and has
already communicated some interesting results to the Astronomische Nachrichten
and Monthly Notices. He finds curious changes of brightness in some stars,
whilst others have disappeared altogether; and his results are confirmed by
observations made at the Royal Observatory, Greenwich.
The connection between colour and variability in stars is
very curious. It would appear that the red stars are in the condition of a fire
dying out, and that the embers are sometimes raised to a white heat by some
unknown causes only to die down again in an equally mysterious, manner. We are
much in the dark as to the how and the when of many of these changes, but this
much we do know, that solid bodies become brighter as they become hotter, and
in doing so pass from red through yellow to bluish white. But what is the
behaviour of gases under such conditions, is a problem of the immediate future,
and it must be remembered that the changes in stars are probably due to gases;
so that we are hardly yet in a position to speculate.
The interesting
question remains, whether we can justify a division of variable stars into two
classes, the one of short period (a few days), showing the phenomena of
rotation or of the interposition of a dark satellite; the other of periods ranging
from a month to many years, in which the change has a physical origin. The
stars of the latter class are generally red.
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