I've measured the heliocentric radial velocity of Beta Cephei using the doppler shift of the helium lines, I corrected for earth's motion using the doppler shift of the oxygen in the atmosphere. I took 14 measurements over the course of 10 hours, and I found that it fluctuates between approximately 15km/s and -15km/s, with a period of 3.9 hours.

The constituents of this star system are not orbiting at this rate, though it pulsates every 4 hours and 34 minutes, which is within the uncertainty in my period measurement, leading me to believe that this is what I've measured. Could I be measuring the radial velocity of the surface of the star as it expands or contracts?


2 Answers 2


The radial velocity variations of beta Cep were discovered by Frost (1902) using a spectrograph with photographic plates. The amplitude of the variations was approximately what you have measured.

Frost (1906) confirmed the variability to be between radial velocities of $+12$ and $-22$ km/s and with a regular period of 4 hours and 34 minutes.

The variations are due to the expansion and contraction of the star.


Yes, though I'd not expect the values to vary around 0,but around $-10km/s$. For example see https://www.aavso.org/vsots_betacep

  • 1
    $\begingroup$ The fluctuations between 15km/s and -15km/s I reported were measured with helium lines. I decided to try also measuring using hydrogen lines and found the same period but instead with fluctuations between around 7km/s and -10km/s. Is the discrepancy due to errors in the measurements or is there a reason for this? Perhaps the hydrogen is present deeper in the star and so it expands and contracts at a slower speed? $\endgroup$
    – Jackson
    Commented Dec 6, 2023 at 2:49
  • $\begingroup$ @Jackson probably mostly error. Substantially all of the light you see comes from the same "depth". $\endgroup$
    – hobbs
    Commented Dec 6, 2023 at 13:57
  • $\begingroup$ @hobbs at least same "optical depth" $\endgroup$
    – uhoh
    Commented Dec 6, 2023 at 14:42

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