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Pollux Spectrum

johnb

By johnb
in Radio Astronomy and Spectroscopy

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johnb

johnb

Posted
Posted

Evening so here is my attempt at Pollux

Catptured with a QHY5 on a Sky Watcher 250

Captured on 15 Jan at 21.39 GMT

Captured from South london: 51.3242° N, 0.0969° W

QGVideo was used for the capture and the resulting video run through registax

Processed with RSpec, Its instrument corrected and normalised.

Thanks for looking

regards

John B

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robin_astro

robin_astro

Posted
Posted

Hi John,

There is something odd here. Pollox is a K type star which do not normally show clear Balmer lines in low resolution spectra. Could it be Castor (A1v) ?

The non normalised spectrum should tell us as the continuum shape will be very different for the two stars. See the attached Pickles spectra for K and A sars (It is better not to normalise low resolution spectra, (particularly as you have already taken the trouble to instrument response correct) as the continuum shape has useful information about the rough spectral type.

Cheers

Robin

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  • 2 weeks later...
Ags

Ags

Posted
Posted

John,

Attached is my spectrum and profile etc of Castor from my book A Spectroscopic Atlas of bright Stars showing the type of result you should get taken through my Rainbow Optics star spectroscope.

Jack

Out of curiosity, why is the Hb spike so strong and the Ha spike so indistinct - I would have expected the exact opposite...?

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the rev

the rev

Posted
Posted

Kate,

As far as I know it is not downloadable. It will be on sale at the Springer stand at a discount this weekend at Astrofest. I will be there both days.

Regards,

Jack

Thanks for the info, but can't make it to astro fest

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robin_astro

robin_astro

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Posted

I suspect the H alpha line in this spectrum is not actually as weak is first it appears to be. Absorption line strengths are measured relative to the continuum. The continuum level in this spectrum is low in the H alpha region because this spectrum was taken on film with a very low sensitivity at this wavelength. The zero level is also very high in this spectrum giving a false origin (The zero level is approximately that between the zero order and the spectrum). Additionally to get an accurate comparison the measurement should be made using the area enclosed by the line (The Equivalent Width) rather than the peak value in the line so that variations in resolution do not effect the result. There is also the problem that film is very non linear. These quantitative measurements are so much easier now we have CCD technology :-)

Cheers

Robin

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robin_astro

robin_astro

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I suspect the H alpha line in this spectrum is not actually as weak is first it appears to be.

This is only part of the story in this case though as if we look at a high resolution spectrum of Castor, for example from the UVES spectrograph on the VLT and measure the Equivalent width of the Balmer lines, we indeed find H alpha is somewhat weaker than H beta. (about 2/3) This is at first glance unexpected, as in emission nebulae for example we find H alpha is much stronger than H beta which is stronger than h gamma etc. (The Balmer Decrement)

The shape of the absorption lines in the high resolution spectra gives a clue to what is happening. In H alpha the absorption is concentrated in the narrow core which quickly becomes saturated (Only a small percentage of the atoms which can absorb at this wavelength are needed to absorb all the light and the rest contribute nothing to the strength of the line.) In H beta the wings in the line are more intense which makes the Equivalent Width larger. This broadening in the wings is due to the pressure in the atmosphere of the star . As the atoms are brought closer together by the pressure they interact more which spreads out the energy levels of the electrons in the atom and hence the width of the line (The Stark effect). This effect is more pronounced in H beta (exactly how this affects each line takes some quantum mechanical calculations which are beyond me I am afraid) and so atoms which were unable to contribute to the absorption in H alpha because their energy levels were in the saturated core are now able to add to the absorption in the wings of H beta increasing the total strength of the line

HTH

Robin

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