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Hi all,

Attached is an image of alpha Centaurus taken during the day. Note what appears to be blue shifted spectral lines in the stellar spectrum compared to the background solar spectrum. I'm curious to know if anyone has seen this before, and whether this is a genuine RV shift or an optical artefact. Although very faint, the background sky spectrum appears 'behind' the stellar spectrum, which would indicated that we might be looking at the star's actual spectrum, which conveniently is also a G2V.

Apologies for only having a JPEG (it was only a curiosity noted during day time set up).

Comments welcome,

Paul

Hi Paul, I have never seen before the Alpha Centaurus spectrum, congratulations for the day time observation!

Just a calculation attempt...

Between the two indicated Fe I lines in your spectrum, 52.6A distance, there are 258 pixels, so the dispersion is 0.2A/pix.



Doppler shift of any line is about 4 pixels, then 0,8A.

If you observed recently, the Earth moved toward the target with about 21 Km/s. Radial velocity of Alp Cen from Simbad is -22.3 Km/s. The total radial velocity is about -43 Km/s (blueshifted spectrum). The corresponding Doppler shift at 540 nm is 0.77A, a value very close to the measured one.

I hope it is correct... I think you showed very well the Doppler RV shift of the star.

Clear skies, also in daytime!
Paolo

Hi Paolo,

Thank you for your reply and analysis. I may try and repeat this exercise now that I have seen that it offers some potential for basic RV practice. It also appears to offer an interesting representation of doppler shift, especially for students, by using an actual spectral image.

Much appreciated,

Paul