<p>I searched for a Cepheid Variable on this database (<a href="https://cepheids.konkoly.hu/NGC4548.php?gal_id=40" rel="nofollow noreferrer">https://cepheids.konkoly.hu/NGC4548.php?gal_id=40</a>) and selected, at random, the C01 Cepheid in the NGC 4548 / M 91 galaxy. C01 has a period of 33.2 days, according to this original database.</p>
<p>I then searched by its coordinates in Vizier to find raw observation data from the Hubble catalogue: (<a href="https://vizier.cds.unistra.fr/viz-bin/VizieR-4?-c=188.84892%2b14.47583&amp;-c.rs=5&amp;-out.add=_r&amp;-sort=_r&amp;-out.max=$4" rel="nofollow noreferrer">https://vizier.cds.unistra.fr/viz-bin/VizieR-4?-c=188.84892%2b14.47583&amp;-c.rs=5&amp;-out.add=_r&amp;-sort=_r&amp;-out.max=$4</a>)</p>
<p>I expected that, if I plotted MJD on the x-axis with apparent magnitude on the y-axis, I would get a typical Cepheid light curve and would be able to read-off the period of pulsation and compare it with the 33.2 days originally quoted.</p>
<p>However, when I make this plot, I get something more like this:
<a href="https://i.sstatic.net/RPPFz.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/RPPFz.png" alt="Apparent Magnitude over time" /></a></p>
<p>So then I thought I should perhaps average the given apparent magnitudes for the observations on the same day and the characteristic light curve would emerge.</p>
<p>However, when I tried this, I also produced an underwhelming result which doesn't seem to me to align with a Cepheid of period 33.2 days.
<a href="https://i.sstatic.net/idB8v.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/idB8v.png" alt="enter image description here" /></a></p>
<p>My question is really - what am I missing here? What reasons would there be for the raw data from a star classified as a Cepheid with a period of 33.2 days not showing a pulsating apparent magnitude with that period?</p>
<p>Thanks for your help!</p>

CC BY-SA 4.0
2023-12-24T18:17:44.537Z
Which filter are you plotting (F555W or F814W)?
Peter Erwin
CC BY-SA 4.0
2023-12-24T21:09:07.223Z
I think the fundamental question to answer is "know your data".  I've not looked into this, but there are a lot of data points for the same day... with long gaps between.  And you are looking at a star that is in a nearby galaxy, so there will be lots of other stars nearby.  So,  how do you know that you are actually plotting data from the same object, and not data from lots of different objects in lots of different filters?  What actually are you plotting?
James K
CC BY-SA 4.0
2023-12-28T00:17:39.143Z
The Vizier page you link to indicates a search radius of 5 arcsec, so I think James K is right: you're probably plotting measurements of several different stars (in two different filters), not just the one Cepheid you're interested in. You'd probably be better off tracking down the original paper that presented the classification and measurements and getting the data for a particular star that way.
Peter Erwin
<cepheids>
Cepheid Variables - why doesn't the raw data show pulsation?
Basil Letts
# Cepheid Variables - why doesn't the raw data show pulsation?

(Asked by: username_0 on 2023-12-24)

I searched for a Cepheid Variable on this database (<https://cepheids.konkoly.hu/NGC4548.php?gal_id=40>) and selected, at random, the C01 Cepheid in the NGC 4548 / M 91 galaxy. C01 has a period of 33.2 days, according to this original database.

I then searched by its coordinates in Vizier to find raw observation data from the Hubble catalogue: (<https://vizier.cds.unistra.fr/viz-bin/VizieR-4?-c=188.84892%2b14.47583&-c.rs=5&-out.add=_r&-sort=_r&-out.max=$4>)

I expected that, if I plotted MJD on the x-axis with apparent magnitude on the y-axis, I would get a typical Cepheid light curve and would be able to read-off the period of pulsation and compare it with the 33.2 days originally quoted.

However, when I make this plot, I get something more like this:
[![Apparent Magnitude over time](https://i.sstatic.net/RPPFz.png)](https://i.sstatic.net/RPPFz.png)

So then I thought I should perhaps average the given apparent magnitudes for the observations on the same day and the characteristic light curve would emerge.

However, when I tried this, I also produced an underwhelming result which doesn't seem to me to align with a Cepheid of period 33.2 days.
[![enter image description here](https://i.sstatic.net/idB8v.png)](https://i.sstatic.net/idB8v.png)

My question is really - what am I missing here? What reasons would there be for the raw data from a star classified as a Cepheid with a period of 33.2 days not showing a pulsating apparent magnitude with that period?

Thanks for your help!

**Tags:** cepheids
> **Comments:**
> **username_1**: Which filter are you plotting (F555W or F814W)?  
> **username_2**: I think the fundamental question to answer is "know your data".  I've not looked into this, but there are a lot of data points for the same day... with long gaps between.  And you are looking at a star that is in a nearby galaxy, so there will be lots of other stars nearby.  So,  how do you know that you are actually plotting data from the same object, and not data from lots of different objects in lots of different filters?  What actually are you plotting?  
> **username_1**: The Vizier page you link to indicates a search radius of 5 arcsec, so I think James K is right: you're probably plotting measurements of several different stars (in two different filters), not just the one Cepheid you're interested in. You'd probably be better off tracking down the original paper that presented the classification and measurements and getting the data for a particular star that way.  

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## 0 answers

