http://www.aavso.org/variables-what-are-they-and-why-observe-them?page=10Research on variable stars is important because it provides information about stellar properties, such as mass, radius, luminosity, temperature, internal and external structure, composition, and evolution. . . .
. . . Mira variables give us a glimpse into the future evolution of our own star, the Sun.
http://www.aavso.org/stellar-evolutionThere are parts of the H-R diagram where we find lots of variable stars. One of these is called the instability strip, which runs from upper right (luminous and cool) to lower left (faint and hot) in the H-R diagram. When a star lies within the instability strip, it may begin to pulsate. In all stars, certain layers within the star can become more opaque to radiation if they become hotter or cooler. When this happens, energy from inside the star can become trapped in that layer, increasing its temperature and pressure. If this layer is located at just the right depth within a star, the layer can act like a piston that drives the outer layers of the star up and down in a periodic fashion, making the star pulsate. We now know that only stars within the instability strip have this layer at just the right depth. We also know based on stellar modeling that stars can lie within this strip at certain parts of their lives depending upon how massive they are. Stars more than a few times the mass of the Sun cross the instability strip after the main sequence. These are the Cepheid variables, named after the class prototype delta Cephei. One of the very important things about Cepheids is that the time it takes them to complete one pulsation cycle (the period) is proportional to the luminosity or absolute brightness of the star. If we can measure the period of the star, then we know its luminosity. This is known as the period-luminosity or P-L relation, and also by the name Leavitt Law, after its discoverer Henrietta Swan Leavitt.
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Perhaps this will helpAidyan said:Yes, but I don't think this answers the question. Can we say for sure that, for instance, 10.000 years ago the sun had the same luminosity of today? I tend to say that it wasn't. If we look at the HR diagram we don't see thin evoultionary paths but thick lines, like a statistical fluctuation, as if stars could variate in luminosity and color at a specific fixed evolutionary state. Of course this is because the statistics is made of different stars, but we can't say for sure that this is the only cause. And we have not sufficient time record to exclude this eventaulity, for our as for other stars, since our meausrements are at best only 3-4 centuries old.
One can look at the loss of solar mass of 100 years, 1000 years, 10 ky, and calculate the impact of nominal luminosity.We show that over the course of its ~7 year chromospheric activity cycle, 18 Sco’s brightness varies in the same manner as the Sun’s, and with a likely total brightness variation of 0.09%, similar to the 0.1% decadal variation in the total solar irradiance.
Aidyan said:Thanks for the effort, it directed me to: http://cc.oulu.fi/~usoskin/personal/aa15843-10.pdf
According to the above the sun's irradiance changed only about 2/1365 over the past 10.000 years. Hmmm... I'm skeptic... :uhh: ... Anyhow, thanks so far.
Drakkith said:Skeptic of what?
Yes, variable stars can stop being variable. This can happen for a variety of reasons, such as a decrease in the star’s brightness due to a decrease in its activity or a change in its physical properties.
Yes, non-variable stars can become variable. This can happen if the star’s physical properties change, causing fluctuations in its brightness or activity.
The speed at which a variable star becomes non-variable can vary. It may happen gradually over a period of time as the star’s physical properties change, or it may happen suddenly due to a catastrophic event such as a supernova.
The speed at which a non-variable star becomes variable can also vary. It may happen gradually over time as the star’s physical properties change, or it may happen suddenly due to a sudden increase in the star’s activity.
Yes, variable stars can switch between different types of variability. This can happen if the star’s physical properties change, causing it to exhibit different patterns of variability. For example, a star may transition from being a pulsating variable to an eclipsing variable as its physical properties change.