Mira variables
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Mira variables are named after the famous long
period variable Mira. Miras are old stars that have evolved into
so-called asymptotic
giant branch stars (AGB). Miras are cooler and much larger
than
the sun. Typically at a late stage of the stars evolution the outer
atmosphere can be blown away so that this phase of the stellar life is
relatively short (tens of thousands of years).
Molecules such as SiO
form as the gas condenses. Typically the infrared signature is the sum
of many Planck spectra. In many cases the dust cloud is not thick
enough to hide the star. This
combination of a visible star and sum of many Planck distributions
rules out the possibility that such a star is a pure Dyson sphere.
For one explanation of a Miras star see the Harvard-Smithsonian
Miras site.
The figure on the left shows the low resolution spectrograph
data from IRAS for Mira (HD14386, IRAS 02168-0312). The blue diamonds
are the flux (/k), the red dots are the filter readings, the dashed red
line is a Planck fit to the filters (including the 60 micron filter),
the green dots are the 2MASS values, the small red and blue bars with
the associated interpolating lines are the LRS values, and the solid
blue line is a Plank "fit" to the LRS values. Clearly a single Planck
spectrum does not fit. Note that the filters alone would be quite
deceptive in this case.
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