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Spectroscopic Parallax

Spectroscopic parallax has nothing to do with parallax. But since this expression is customary among astronomers, the parallax method we discussed before is often referred to as trigonometric parallax.

The spectroscopic parallax technique requires that a star's apparent magnitude and its spectrum have been observed. Information obtained from the spectrum is used to find the star's position on the Hertzsprung-Russell Diagram (HR-diagram).




We discussed the HR-diagram  in our EBooks Stellar Radiation and Stellar Evolution.


In the HR-diagram Absolute Magnitude is sometimes listed along the vertical axis as an alternative to Luminosity. (see diagram).


Spectral type
Astronomers have classified stars according to their spectra into spectral classes (also called spectral type). The major spectral classes are type O, B, A, F, G, K, M where the O-class is the most luminous (and hottest) and the M-class is the faintest (and coolest). Each type is subdivided into 10 finer divisions (0-9), as A8 or F0.
This picture shows standard spectra for each class along the horizontal axis.

Therefore the Spectral type as determined from the observed spectrum, defines the horizontal position of the celstial object on the HR-diagram.


Luminosity class
Luminosity class is a classification of the luminosity of a star. In our EBook Stellar Radiation we explained that as the Morgan-Keenan luminosity and it can also be obtained from the observed stellar spectrum.
The luminosity class of the celestial object defines its the vertical position on the HR-diagram.


Once the star's position on the HR Diagram is identified by the intersection of its spectral class and luminosity class, one can read off its absolute magnitude.

The absolute magnitude combined with the already observed apparent magnitude and the distance modulus (see above) then provides the distance to the star.


SP calculator
Use this calculator of the University of Nebraska-Lincoln that very well illustrates the method of spectroscopic parallax.
In the animation chose Spectral type (top left), Luminosity Class (bottom right) and Apparent Magnitude (bottom right).
This produces the distance (in pc) from the Distance Modulus.



This method is generally not very accurate for individual stars. In particular the luminosity class is often not sharply defined on the HR-diagram.

But if we can process a large number of stars that are about at the same distance and we limit these to stars of luminosity class V - Main Sequence stars - we have the generally more accurate method of Main Sequence Fitting to determine distance.






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