<p>Yes, though I'd not expect the values to vary around 0,but around <span class="math-container">$-10km/s$</span>. For example see <a href="https://www.aavso.org/vsots_betacep" rel="noreferrer">https://www.aavso.org/vsots_betacep</a></p>

CC BY-SA 4.0
2023-12-06T02:49:22.153Z
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?
Jackson
CC BY-SA 4.0
2023-12-06T13:57:43.583Z
@Jackson probably mostly error. Substantially all of the light you see comes from the same "depth".
hobbs
CC BY-SA 4.0
2023-12-06T14:42:00.870Z
@hobbs at least same "[optical depth](https://astronomy.stackexchange.com/search?q=is%3Aquestion+photosphere)"
uhoh
CC BY-SA 4.0
2023-12-06T01:23:54.370Z
2023-12-06T01:23:54.370Z
planetmaker
<p>The radial velocity variations of beta Cep were discovered by <a href="https://ui.adsabs.harvard.edu/abs/1902ApJ....15..340F/abstract" rel="noreferrer">Frost (1902)</a> using a spectrograph with photographic plates. The amplitude of the variations was approximately what you have measured.</p>
<p><a href="https://ui.adsabs.harvard.edu/abs/1906ApJ....24..259F/abstract" rel="noreferrer">Frost (1906)</a> confirmed the variability to be between radial velocities of <span class="math-container">$+12$</span> and <span class="math-container">$-22$</span> km/s and with a regular period of 4 hours and 34 minutes.</p>
<p>The variations are due to the expansion and contraction of the star.</p>

CC BY-SA 4.0
2023-12-06T09:59:14.817Z
2023-12-06T09:59:14.817Z
ProfRob
<p>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.</p>
<p>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?</p>

<spectroscopy><variable-star><radial-velocity>
Does the expansion and contraction of a variable star affect the measured radial velocity?
Jackson
# Does the expansion and contraction of a variable star affect the measured radial velocity?

(Asked by: username_0 on 2023-12-06)

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?

**Tags:** spectroscopy, variable-star, radial-velocity

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## 2 answers

### Answer by username_4 on 2023-12-06. Score: 10 | Accepted answer

The radial velocity variations of beta Cep were discovered by [Frost (1902)](https://ui.adsabs.harvard.edu/abs/1902ApJ....15..340F/abstract) using a spectrograph with photographic plates. The amplitude of the variations was approximately what you have measured.

[Frost (1906)](https://ui.adsabs.harvard.edu/abs/1906ApJ....24..259F/abstract) 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.


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### Answer by username_1 on 2023-12-06. Score: 5

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>

> **Comments:**
> **username_0**: 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?  
> **username_2**: @Jackson probably mostly error. Substantially all of the light you see comes from the same "depth".  
> **username_3**: @hobbs at least same "[optical depth](https://astronomy.stackexchange.com/search?q=is%3Aquestion+photosphere)"  

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