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Tom Mazanec
2014-Nov-22, 08:19 PM
IIRC Kepler had some trouble getting faint signals from small planets because of variability in the star's light.
If you look closely enough, are all stars variable (11 year cycle of Sol, for example)? Should they all have variable star names?

Shaula
2014-Nov-22, 08:52 PM
That would rather defeat the object of having a variable star/star division in your classification system. Generally a variable star is one for which the changes in light output or spectral content changes significantly more than the average for its base type.

Otherwise it is rather like saying "All people are tall to some degree so we should call everyone a tall person no matter their height"

Tom Mazanec
2014-Nov-22, 11:33 PM
Which is why I kinda feel it is a little silly to give special names to stars that are "more variable than average".

Jeff Root
2014-Nov-23, 02:30 AM
The distinction is whether variation has been detected, not
whether it is more than average.

-- Jeff, in Minneapolis

DaveC426913
2014-Nov-23, 02:35 AM
Do you mean like Cepheid variables?

"The strong direct relationship between a Cepheid variable's luminosity and pulsation period secures for Cepheids their status as important distance indicators for establishing the galactic and extragalactic distance scales"
- wiki

IOW, while virtually all stars may pulsate, they do not all do so in a way that has proven to be extremely useful in our understanding the universe. Cepheids were, arguably, the very foundation upon which we started being able to measure distances and sizes of other galaxies.

korjik
2014-Nov-23, 06:04 AM
Which is why I kinda feel it is a little silly to give special names to stars that are "more variable than average".

Why possibly would you think that? Do you think that someone who needs a booster seat to drive a car is just as tall as someone who has to take the front seat out to drive the same car?

Non-variable stars change a little. Wiki says about .1% in luminosity over the 11 year cycle for the Sun. Variable stars change more. Some Mira-type variables change by 3 to 4 orders of magnitude. Supernova reach 8 orders of magnitude. The distinction is clear.

Shaula
2014-Nov-23, 07:05 AM
Which is why I kinda feel it is a little silly to give special names to stars that are "more variable than average".
As Korjik says, why? Do you feel it is silly to describe a disease like (to take a topic example) Ebola as lethal? A cold can kill you, so surely we should call every disease lethal? Or none of them?

There are rarely binary choices in classification schemes, they all contain arbitrary breakpoints that have proven useful in understanding the things classified. Variable stars generally have some extra mechanism of variability (Cephids, Mira type etc). Thus it is useful to categorise them in this way.


The distinction is whether variation has been detected, not whether it is more than average.
I guess it is worth defining how people are using the classification Variable here. Jeff is correct in that if variability is detected a star seems to get a V after its designation in the catalogues. So just about every star is variable in that scheme. The trouble is that this basically means it tells you very little (point Corot at star, add a V). When most amateur astronomers study variable stars (as opposed to doing astroseismology) they are looking at stars with some relatively large variability.