Gliese 581 c

Erik Max Francis kirjutas:
wrote:

The Sun is a variable star - the total luminosity of the Sun changes
by roughly 0,1% with 11 year period. There are slight irregularities -
not all sunspot maxima are equals, and there has been a Maunder
minimum with hardly any sunspots.

The Sun wouldn't be classified as a variable star by astronomers by this
criterion, so you seem to be playing some sort of word game.

How exactly is a "variable star" defined? What is the minimum
amplitude of variable stars?

wrote:

How exactly is a "variable star" defined? What is the minimum
amplitude of variable stars?

Like most terms used in astronomy, it has no official, quantitative
definition by the IAU. However, it's a star that significantly changes
in luminosity or spectrum over time. _No_ star has a completely
constant luminosity and spectrum over its main sequence lifetime; after
all, _all_ stars steadily increase in brightness on the main sequence as
they consume the hydrogen in their interiors.

However, natural changes of a mere 0.1% certainly wouldn't qualify. If
a star with similar properties were somewhere in our skies, we wouldn't
classify it as a variable star.

--
Erik Max Francis && && http://www.alcyone.com/max/
San Jose, CA, USA && 37 20 N 121 53 W && AIM, Y!M erikmaxfrancis
The entire Earth is but a point, and the place of our own habitation
but a minute corner of it. -- Marcus Aurelius, ca. 170