How would you estimate the period or range of periods for two stars, please? I know the right ascension, declination, and distance. I could estimate the masses and assume that the stars are in apocentrum. Which other parameters do I need to know?

I cannot use Kepler law, right?

$$P = 2\pi\sqrt{\frac{a^3}{G(M_1+M_2)}}$$

  • $\begingroup$ I don't really understand what you are asking. The only way to calculate the period is from Kepler's third law, or something like it. RA, and Dec are pretty useless. You can measure the period by observing a light curve or looking at Doppler shifting of the stars' spectra. But essentially, unless you know M1 M2 and the semi major axis (or can estimate these) you can't calculate the period. $\endgroup$
    – James K
    Jan 4, 2023 at 16:13
  • $\begingroup$ If it was a visual binary (or you had an interferometer) then measuring the RA, Dec positions (or offsets from the primary) over time would give the orbital period. Would likely take a very long time to measure a period though... $\endgroup$ Jan 4, 2023 at 17:41
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    $\begingroup$ After the assumption of both components being in apocentrum, we can calculate from RA and DEC their separation, and it will be equal to 2a, right? $\endgroup$
    – Elena Greg
    Jan 5, 2023 at 7:42
  • $\begingroup$ Your question is unclear. What kind of period are your referring to (orbital or variability)? In case of orbital period, how do you know whether these stars are in a binary orbit? Do they have similar/identical proper motions and/or radial velocities? $\endgroup$
    – Walter
    Jan 5, 2023 at 9:26
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    $\begingroup$ I am referring to the orbital period. I do not know whether these stars are binary, but I would like to estimate the period if they were. I have proper motion at my disposal. $\endgroup$
    – Elena Greg
    Jan 6, 2023 at 5:54


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