Stellar Motion and Metallicity
The Gaia space probe has captured the spectrum and distance with position of about a billion stars in the Milky Way.
The probe takes multiple images so a distance can be calculated by the parallax method.
Each spectrum is analyzed for its metallicity and relative motion.
Attached are examples of the use of metallicity. Samples from Deneb, Sirius, Vega are provided.
Each star type is assumed to have a metallicity signature with absorption lines from specific elements in the star's photosphere.
These samples identify lines for Calcium, Hydrogen, Iron, Magnesium, Oxygen, Sodium, Strontium, Titanium, and water, and the chalcogen family of elements which includes Sulfur,Tellurium and others.
I suspect there are several possible problems using these absorption lines in this Gaia application.
1) these atoms are in the photosphere and are technically not part of the star's surface and below.
2) Metallicity assumes all elements are from the star not its environment.
on October 19 I posted about concerns the use of metallicity can be corrupted if atoms can be captured by the star from the interstellar medium.
3) These absorption lines are being analyzed to determine the star's relative motion to Earth.
This is not valid. These atoms are free to move randomly above the star's surface so it is impossible to distinguish the star's motion from the atom's motion.
Actually I expect no star motion can be detected. The star generates a broad spectrum of wavelengths, from UV to infrared. I expect with the star in motion its entire spectrum is NOT shifted by its motion so red becomes becomes either infrared or orange, or violet becomes blue or ultraviolet, and so on. The Sun and stars are always in motion around their galaxy. Their entire spectrum does not shift during this cyclical motion.
Stars are observed in tight chaotic orbits around the galactic center and I believe they never transition to either radio or X-ray during these rapid orbits.
We measure motion in the universe by atoms not by broad spectrum sources.
For example we measure rotation of a spiral galaxy by the hydrogen atom emission line shift (in the hydrogen gas clouds) not by checking whether the entire spectrum of an individual star has shifted.
Spectrum shifts are detected in moving atoms not in the broad spectrum of a star.
I suspect the Gaia data analysis will conclude incorrect velocities of the stars.
4) I suspect there is a small chance the metallicity analysis could be affected by elements which were captured by the star from its interstellar medium.
These billion stars are in a huge volume and I expect the interstellar medium has variations in the random atoms. It is reasonable to conclude stars in the vicinity of a supernova could capture random elements.
On October 24 I posted about the correct use of absorption lines. That post was intended for galaxies and quasars. Those simple rules must be considered for stars also.
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