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Eclipsing binary U Cephei


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About this observation
Mike Harlow
Time of observation
25/02/2019 - 19:30
Eclipsing binary U Cephei
Observing location
Ipswich, UK
12 inch F3.6 astrograph
Starlight Xpress H16 CCD
Astrodon luminance filter
8 inch 12 degree objective prism
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Spectroscopic observation of U Cephei

U Cephei is a rather nice Algol type eclipsing binary. A giant G-type star eclipses a much brighter but smaller B-type star with a period between eclipses of 2 days 11 hours and 50 minutes. At 82 degrees north it is an easy target all year round.

Observing back in February I was curious to see how the spectrum changed from total eclipse through to the partial phase.  At totality (spectrum A) only the giant G-type star is visible at mag. 9.2.  The spectrum is quite faint but remarkably shows strong H alpha (and beta) emission lines.  These are unlikely to come from the G star and the hot B-type star is hidden in eclipse, so most likely the emission comes from gas swirling between the stars.

About an hour into the partial phase (spectrum B) light from the B star starts to dominate.  Interestingly the H alpha absorption line is 'filled in' by the with the spectrum of A-type star HD6006 in the same field of view where the full set of H Balmer lines are visible.

Very long period eclipsing binaries get a lot of attention from spectroscopists, e.g. epsilon Aurigae, VV Cephei, but even 'ordinary' short period binaries can show some unusual features!

Seeing the spectrum change 'in real time' as the images came down from the telescope was quite remarkable!!!


allenhugh10's picture

Hi Mike,

I really enjoy your Star Analyser spectra where a whole field of stars can be captured and compared. Just to note that it is conventional to put the blue end of the spectrum always on the left, to avoid confusion in a b&w image.

I just picked up on the U Cep spectrum which is very interesting. I captured the same transition from B-type to G-type in my Alpy spectroscope in 2016, but I saw no emission lines in either spectrum. There must have been some interesting changes in this star since then. Worth a follow-up observation, but not easy to be ready for the brief primary eclipse phase!



Hi Hugh,

Thanks for your comments.  I would be interested to see your results from a few years ago and yes any changes in the H emission over time would be worth following up on.

Like you I have a chemistry background so seeing atomic spectroscopy in action is rather satisfying!

I have mentioned the non-conventional orientation in previous I know it's all wrong! At least the annotation of the image avoids ambiguity.  And the system I use is an objective prism not a grating; the simplicity appealed to me.  My home-made mount isn't up to guiding on a slit so capturing spectra of everything in the field is the easiest approach!


allenhugh10's picture

Hi again Mike,

I took a spectrum of U Cep on Friday evening (about 16 hours after primary eclipse) and found that the H-alpha absorption was significantly more faint than the spectrum I captured outside of primary eclipse back in August 2016 (about 24 hours before primary eclipse). I'd send you the plotted spectra if I could attach them to this note but it doesn't seem possible. Anyway, one of the 2.5 day timings of the primary eclipse was 06:30UT on 13/9/19, stepping back by about 20 minutes every 5 days (so 06:10 on 18/9 etc etc.). See So the primary eclipse is moving into darkness for good observation during the winter. Definitely worth a follow-up during the winter to see if the emission lines are still visible



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