The year of the Period-Luminosity is given as 1908 in the introduction, and appears again as 1912 in Use as a "standard candle". Does anyone have a more reliable source? --Chalom 21:13, 13 July 2006 (UTC)
I originally made the change from 1912 to 1908 at least in the introduction, but it got reverted back (forward? :-)) to 1912. Anyhow, I have put it back to 1908 in both places and explained (in the article - perhaps unnecessarily at this point) about the 1912 date. I wish to adduce the specific information testifying to the date claims. Quoting from her 1908 article "1777 Variables in the Magellanic Clouds" By Henrietta S. Leavitt, H.A. 60, No. 4, on p.107 she says "It is worthy of notice in Table VI the brighter variables have the longer periods". This is a statement of a period-luminosity relationship. In the 1912 article, "Periods of 25 variable stars in the Small Magellanic Clouds" on pp.1-2 she says "A remarkable relation between the brightness of these variables and the length of their periods will be noticed. In H.A. 60, No. 4, attention was called to the fact that the brighter variables have the longer periods, but at that time it was felt that the number was too small to warrant the drawing of general conclusions. The periods of 8 additional variables which have been determined since that time, however, conform to the same law.... The logarithm of the period increases by about 0.48 for each increase of one magnitude in brightness". BTW, the "8 additional" is on top of 17 from the 1908 article. So she observes and states the relationship in 1908, was unsure about its generality, gathers further results which really gave her no new information, and re-affirms the 1908 observation along with a mathematical expression of the relationship.--Netrapt 05:52, 10 February 2007 (UTC)
I feel this article might benefit from more discussion of the actual values of the luminosity/period relationship. Possibly also a graph showing the shape of it. Is it linear, exponential?? This small addition would add a lot to the article, I think. The section above this seems to give some info to that effect. 22.214.171.124 (talk) 03:14, 11 May 2009 (UTC)
I reviewed the history and realized that someone had deleted a large portion of the page, so I reverted it. That explains the relationship, but it could still use a graph. Now that the info is there, maybe I'll try to create a graphic 126.96.36.199 (talk) 21:13, 11 May 2009 (UTC)
V is visual and not violet in all sources I have seen, including the hardcopy Norton's Star Atlas 1973, see page 84. V is in fact somewhat green-yellow in colour, Norton places it at 550 nm, but real instruments can use differing filter values (see Photometric system for list) from 500 to 580 (and Hubble's Advanced Camera for Surveys uses 606 nm). None could be thought of as violet. See also UBV photometric system, B-V colour and Interstellar reddening. As my correction has been reverted please could someone else either correct this or add a citation for V=violet.
Effect of red-shift on the use of variable cepheids
Does anyone know whether the measurements of both the apparent period and the apparent luminosity of Chepeids are corrected for the effect of any red-shift?
Even if the effect of such a neglect on the distance calculated with these measurements might be small in comparison to the already greatly different value of the Hubble constant, it might be of special importance, because - in case it were overlooked - this would provide a systematic error, and the red-shift might not have a linear relationship with the distance.
I trust every astronomer understands that redshift causes the apparent period to be longer, and the apparent luminosity to be smaller. I expect therefore that the measurements are corrected for this effect, but as I cannot find any article mentioning it, I would very much like to know for sure. Velzen5 (talk) 17:27, 15 October 2010 (UTC)
The following discussion is an archived discussion of a requested move. Please do not modify it. Subsequent comments should be made in a new section on the talk page. No further edits should be made to this section.
The result of the move request was: no move per discussion Ronhjones (Talk) 01:41, 31 December 2010 (UTC)
Object you recently rewrote the article. It should go to full WP:RM to determine whether to split the article with a new one for classical, or move the article, and create a new article at cepheid. 188.8.131.52 (talk) 06:13, 22 November 2010 (UTC)
Object because the rationale is misleading. Anonymous IP 184.108.40.206 has rewritten some of this article, including the introduction. Cephid variable has been changed to "Classical cephids" in the introduction. This looks like some form of POV pushing. If the anonymous IP wishes to have an article on "Classical cephids" then the anonymous IP is encouraged to write one. ---- Steve Quinn (talk) 05:44, 26 November 2010 (UTC)
Comment I removed the classical cephid variable over writes of this article by restoring to the last edit by User:Kwamikagami. I then restored the small, non-trivial, edits of other editors that followed. This move request is probably no longer appropriate. Please propose changes on talk page before attempting to re-orient an established article to some other topic. ---- Steve Quinn (talk) 04:13, 28 November 2010 (UTC)
Is there some reason not to rename the article simply to "Cepheid" (which redirects to it)? (Although I see "Cepheids" is a disambiguation page - something's not quite right here.) --Kotniski (talk) 08:41, 29 November 2010 (UTC)
"Cephid variable" is the topic of this article, and the correct name of this type of star. I am not sure how much usage the name "Cepheid" has in our lexicon to mean this type of star. However, with "Cephid variable" it is clear what is meant. Also, I fixed the "Cepheid" redirect to go to the disambiguation page. Thanks for the heads up about that. I also added more links to the disambiguation page. ---- Steve Quinn (talk) 17:16, 29 November 2010 (UTC)
I still don't really see why we need a disambiguation page at all though. AIUI, "Cepheid" has only one meaning, namely the same as "Cepheid variable", which is a type of variable star, and has two subtypes called "classical Cepheid variables" and "type II Cepheid variables". Right? So in that case, shouldn't "Cepheid" and "Cepheid(s)" redirect straight to this page, which is the article about Cepheid (variable (star))s, and a pointer be included somewhere in this article (where it talks about type II Cepheids) that more information can be found in the separate article on that type?--Kotniski (talk) 17:52, 29 November 2010 (UTC)
Actually, I was thinking the same thing. I don't think the disambiguation page is needed - just redurect "Cepheid" and "Cepheids" to the "Cepheid variable" article. I actually almost did that, instead of adding links. If you want to go ahead and make the necessary corrections per WP:BOLD, I don't think it will be a problem. In fact, I think there is some guideline that reccomends not having a disambiguation page with only two links of similar names. That is what the disambiguation page originally had, before my editing. :>) ---- Steve Quinn (talk) 18:53, 29 November 2010 (UTC)
"Cepheid" is the demonym for the Cepheus constellation, that is a different meaning from (1) this class of variable star (2) the classical cepheid subclass of cepheid. 220.127.116.11 (talk) 20:35, 29 November 2010 (UTC)
Thank you for the update in information, and the improvement of the disambiguation page. The disambiguation page now has a valuable purpose (or function). ---- Steve Quinn (talk) 20:53, 29 November 2010 (UTC)
Hi Steve and Kotniski, the article as presently written refers primarily to Classical Cepheid variables. For example, the masses given in the 'introduction' (5-20 Msun) or the period-Mv and period-distance relations under the 'Period-luminosity relation section': are all related to Classical Cepheids. All the stars in the 'Example' section are Classical Cepheids. There is more than one type of Cepheid variable. There are two main groups: Classical Cepheids and Type II Cepheids, but there is also a class of Cepheid variables called "anomalous Cepheids". All these 'Cepheids' exhibit different masses, evolutionary histories, and period-Mv relations. 'Cepheid variables' is ambiguous, and at one point we must begin to make the distinction here on Wikipedia. I believe we should join efforts to ensure that is clear. I spent much time editing the article, please do not disregard that so easily, particularly since several pertinent references were added to support the information posted. I shall in the coming weeks create an 'anomalous Cepheids' section on Wikipedia, time pending. But in the meanwhile, we should honestly begin the task of getting this correct and being clear what we're speaking about. There should be a classical Cepheids section, a Type II Cepheids section, and an Anomalous Cepheids section, at the very least. There should be a main Cepheids page, a disambiguation page, that allows one to go to each of the classes of Cepheids. Whoever began http://en.wikipedia.org/wiki/Cepheid is on a good start. Hopefully this cumbersome task will be facilitated by all of us helping in a small way. —Preceding unsigned comment added by 18.104.22.168 (talk) 01:12, 30 November 2010 (UTC)
Please don't change the page. I think it would be better to split off and create a new page for classical cepheids, while this page is rewritten as a cover article for all cepheids, with subarticles for types. What you've done is specializing the entire text of the article to become classical, instead of sectionalizing it. 22.214.171.124 (talk) 03:16, 30 November 2010 (UTC)
OK I reverted this person's revisions. ---- Steve Quinn (talk) 05:22, 30 November 2010 (UTC)
126.96.36.199 and I were talking in #wikipedia-en-help, and what he meant to say is the current article primarily talks about Classical Cephieds. What he meant to propose was that this article be moved to Classical Cephieds, and a more general article on Cephieds be written here. Best, --Alpha Quadranttalk 06:24, 30 November 2010 (UTC)
The anonymous editor is free to write an article on Classical Cephids. This is a general article about Cephid Variables - this is an overview article, or summary article. It is not that person's purview to hi-jack this article to suit their own tastes. If the anonymous IP wishes to pretend to not get it, or refuses to get the point, then that is up to them WP:IDONTHEARTHAT. ---- Steve Quinn (talk) 07:22, 30 November 2010 (UTC)
I have added content and sections to make it clear that this is a summary article. ---- Steve Quinn (talk) 07:32, 30 November 2010 (UTC)
"Cepheid is a demonym for the Cepheus constellation" - is this a sourced meaning? A basic Google search turned up no meaning whatever for "Cepheid" other than Cepheid variable star. And even if such a meaning does exist somewhere, the variable star is still the Primary topic, surely? I propose we dispense with the needless dab page currently at Cepheids (or Cepheid if my move request is acted on), redirect both of those terms to this article, and simply explain in this article what subtypes of Cepheid variables exist and where the name comes from.--Kotniski (talk) 08:39, 30 November 2010 (UTC)
There is the Cepheid meteor shower ... which shows the demonymic adjectival form of Cepheus. There's also the rules of construction for demonyms of constellations in Latin (from when in the Middle Ages all such names originated) 188.8.131.52 (talk) 05:49, 1 December 2010 (UTC)
Hi Steve, Kotniski, and 184.108.40.206. Prior to my editing an enormous fraction of the present page already related to classical Cepheids, particularly in the introduction where 99% of it was tied to Classical Cepheids. Much of the information was unclear owing to the ambiguity of what class of Cepheid was being discussed. Again, there are different classes of Cepheids, each with their different masses, evolutionary histories, etc. Classical Cepheids and Type II Cepheids are essentially unrelated, with the former being 10s to 100 million years old, and the latter being ~10 billion years old. The masses aren't even close either, the former has between say 4-20 solar masses, and the later ~0.6 solar masses. We should avoid mixing the two types of Cepheids as Walter Baade discovered more than 70 years ago. They have little commonality beyond the historic context. For example, Type II Cepheids have more in common with RR Lyrae variables. Steve, there continue to be mistakes on your edit of the page, such as: "The stars are 5–20 times more massive than the Sun and up to 30,000 times more luminous." What stars are you referring to(?) Type II Cepheids are not 5-20 times more massive than the Sun, they are 0.6 solar masses. You are speaking about Classical Cepheids. Let's take another example, Steve, you state: "Delta Cephei is the prototype of the Cepheid variable stars". That is not true, Delta Cephei is the prototype for Classical Cepheids, not Type II Cepheids, not anomalous Cepheids, or otherwise. Type II Cepheids have their own prototypes, such as RV Tau, or BL Her, or W Vir. There are still places in the text where the type of Cepheid is not specified, but in the majority, it relates/related to classical Cepheids. A page on Cepheid variables should be a jumping point to the different classes, rather than a page merely focusing on one class of Cepheid (in this case Classical Cepheids) where its parameters and characteristics are cited as being indicative of all Cepheids. We have a good start with whomever made the disambiguation page http://en.wikipedia.org/wiki/Cepheid . We now have a Type II Cepheid page, and the information on the Classical Cepheid page was changed so to reflect the fact that the parameters which were cited are actually tied to that variable. We are making progress in addressing this entire problem. Surely we can come to a consensus on the facts. Perhaps this is what we can do, (1) the disambiguation page remains with two or three opening sentences. (2) the classical Cepheid article gets created with what I had listed before, which is essentially Steve and everyone elses article but with padded with additional references and some edits. (3) Classical Cepheid information should be added to the Classical Cepheids section, Type II Cepheid information should be added to the Type II Cepheid section. The stars are essentially unrelated in terms of age, mass, etc. (4) Perhaps a history of Cepheids is created, which talks about the about the mixing of the Cepheids in early 20th century and how that led to big problems regarding the distance scale until Walter Baade came along. I am here to help.
Cepheid is 99% of the time referred to variable stars, and not stars in that constellation. —Preceding unsigned comment added by 220.127.116.11 (talk) 13:51, 30 November 2010 (UTC)
Agreed, this article should be on Cepheids in general. 142 pointed out that the current article is primary written about Classical Cepheids. I believe that is why a page move as proposed by 142 would be logical, to preserve the edit history. A more general, historical article could then be written here. --Alpha Quadranttalk 19:38, 30 November 2010 (UTC)
Cephid variable is a common usage title in a lot of sources. Variations on the "Cepheid" name are Type I Cepheids, Type II Cephids, either Population i, or Population II stars, W. Virginis Cepheid appears to be sometimes interchangeable with Type II stars, etc., and etc. Some sources talk about both, some emphasize one. Hence, without some definitive sources that say these cannot be viewed in some similar or contrasting context this article is appropriate as a summary article (or overview). In addition, I created a new article entitled Classical Cepheid variables. Feel free to expound on Classical Cephid variables in this article, as much as desired.
And I agree that this article needs editing, but one thing at a time works really well. ---- Steve Quinn (talk) 05:10, 1 December 2010 (UTC)
Hi Steve, you are not being very accomidating, nor trying to seek a consensus based on facts. Each subtype should have their separate article so to avoid any confusion on the part of the reader or even contributors. Dwarf Cepheids (now called Delta Scuti variables), Type II Cepheids, Anomalous Cepheids, etc., should have distinct pages united by a brief disambiguation page. It was discovered ~65 years ago by astronomer Walter Baade that mixing the Cepheid terminology was problematic, and you are continuing to propagate that here, despite clear factual evidence to the contrary. I am not stating that you shouldn't add information, your additions are most often excellent. I am suggesting that a clear and unequivocal distinction be made between the Types of Cepheids so there is absolutely no confusion, particularly since they are largely unrelated in terms of age, mass, evolutionary histories, etc. For example, as I mentioned above, you state: "Delta Cephei is the prototype of the Cepheid variable stars". That is not true, Delta Cephei is the prototype for Classical Cepheids, not Dwarf Cepheids, not Type II Cepheids, not anomalous Cepheids, or otherwise. You go on to state: "the second Cepheid variable discovered following Eta Aquilae", Eta Aquilae is a Classical Cepheid. Moreover, in the example section of the article: Beta Dor, Zeta Gem, Delta Cep, and Polaris are all cited as Cepheids but they are all Classical Cepheids. The only figure in the article relates to the discovery of Classical Cepheids in M100. As I mentioned previously, a page on Cepheid variables should be a jumping point to the different classes, rather than a page merely focusing on one class of Cepheid (in this case Classical Cepheids) where its parameters and characteristics are cited as being indicative of all Cepheids. Eventually I hope period-luminosity relations tied to Anomalous Cepheids, Type II Cepheids, and otherwise are posted. Clearly, all such information should go in their respective texts, rather than just citing the period-Mv relation for Classical Cepheids as is presently done. Most of the present article, including the one figure posted, relates to classical Cepheids. I am not claiming that you shouldn't add information, I am just stating that it should go in the appropriate places. I am also not pleased that you reverted the changes on the Cepheid disambiguation page that Alpha Quandrant and I worked on. Creating distinct pages ensures that errors regarding the type of Cepheid being discussed (and they are all very different) are mitigated. Let's make the present page Classical Cepheids after a revert and some edits, and then make a brief separate page or subsection on the historical problems that the mixing of the Cepheid terminology caused (a subsection say in the Classical Cepheids page). —Preceding unsigned comment added by 18.104.22.168 (talk) 13:26, 1 December 2010 (UTC)
First, the disambiguation page was edited incorrectly, and I fixed it. Second, Type I and Type II Cepheid variables do have commonalities. There is nothing wrong with having a page that discusses the different types of Cepheid variables. They all share the same nomenclature for a reason. This is what was being attempted with the disambiguation page, although this will have longer explanations. This is an encyclopedia, and a general article such as this is appropriate. Also, see the edit history of the disambiguation page for the reasons it was restored to the correct format. ---- Steve Quinn (talk) 15:39, 1 December 2010 (UTC)
Any errors on this page can be fixed, so there is no need to jump the gun. I have not come across any sources which state clearly describing Cepheid variables in one article is inappropriate. In fact other sources (including encyclopedias) do just that. ---- Steve Quinn (talk) 15:59, 1 December 2010 (UTC)
With the statement "Delta Cephei is the prototype..." I was reporting what the source said. There was no distinciton made between Cepheid, and Classical Cepheid. I am not saying it is incorrect, I just didn't pick up on any distinction. But that can be fixed. In fact, if you see errors like that then just fix it. There is a section on Classical Cepheids. ---- Steve Quinn (talk) 16:07, 1 December 2010 (UTC)
So having said all that - it appears you have a much deeper knowledge of this subject than I do. So I am conceding. As far as I am concerned edit this article and other articles, as you see fit. The only thing is, please do the disambiguation page in the correct format. Also, it looks like you have an idea for some sort of general article on this topic - so I leave it up to you. Good luck. ---- Steve Quinn (talk) 16:17, 1 December 2010 (UTC)
If you need help merging, or some sort of general, or technical editing tasks let me know, I'll be glad to do it. ---- Steve Quinn (talk) 16:20, 1 December 2010 (UTC)
Does anyone think we need to do a Request for Comment WP:RFC on this? I don't see anyone else against this - unless User:22.214.171.124 still has misgivings. ---- Steve Quinn (talk) 16:27, 1 December 2010 (UTC)
Hi Steve, ok, very good. I have an idea in mind to address both our concerns and which will hopefully make everyone generally content, but which will at the same time require concessions from both of us. Let me think about it some more and get back to you, as I shall need your help. —Preceding unsigned comment added by 126.96.36.199 (talk) 17:03, 1 December 2010 (UTC)
Hi again, I completed changes which will certainly require continued revisions. I made edits to both the 'Cepheid variable' page and the 'Classical Cepheid variable' page you created. In certain instances I transferred sections from one to the other (such as the Period-Mv section), but the majority of all the information has been retained. As concessions on my part, I gave up the desire to have a clean separation between the various types of Cepheids and the 'Cepheid' disambiguation page may now be redundant. The concession on your part was the creation of the 'classical Cepheid' entry, and allowing me to help organize the information. Although it is probable that neither of us are completely content, hopefully this is middle ground. I am still somewhat worried that the mixing of the various classes of Cepheids will continue, and that editors will add information to the minor subsections of the variables stars rather than their main entries. But I think, in the end, much has been accomplished. I am hoping you can make Wikipedia queries of 'Classical Cepheids' or 'Classical Cepheid' go to the 'Classical Cepheid variable' section, and 'Cepheid' or 'Cepheids' go to the 'Cepheid variable' section. There are all kinds of other loose ends, but they will hopefully be fixed in time, by all of us working in harmony. ---- 188.8.131.52 (talk) 15:51, 2 December 2010 (UTC)
I expected that you would transfer sections, if you were going to use both articles. In addition, tt is still possible to have a clean seperation between articles. I didn't realize that the disambiguation page played a signifigant role for your plans. We could fudge the disambiguation page, but other editors could come along and "fix" it. In other words, the page could become a point of contention. So instead of the disambiguation page, I think we could use an "embedded list" type of article in place. I haven't used the embedded list before, so this would be the first time for both of us, if you want to use it. Here is the link to the Manual of Style page WP:EMBED.
If you want to use the embedded list, in one form or another, I can merge the Classical Cepheid variables article into this one, or the other way around. This is what I meant about helping out with general editing tasks. When merging articles or content from one article to another it is important to do this according to WP:MERGE. I have performed this procedure sufficient amount of times to be familiar with it. However, if you want to do this and want to learn how - feel free to do so. Also, now is a good time to let you know that when you transfered content from one article to another, the Merge guideline probably applied. This is not a problem. I can fix this without undoing your edits.
Anyway, let me know what you think about my above proposals. ---- Steve Quinn (talk) 18:32, 2 December 2010 (UTC)
Hi Steve, regarding "now is a good time to let you know that when you transfered content from one article to another, the Merge guideline probably applied. This is not a problem. I can fix this without undoing your edits.", excellent thanks. Apologies on the rest, I am somewhat confused. I was merely suggesting that perhaps the Cepheid disambiguation page is presently redundant granted Cepheid variable now has that information in expanded form. In other words, I am wondering if you think the disambiguation page should now be removed? If so, could you initiate that process (I'm not sure how) and also create some forwards so that when people type 'Cepheid' or 'Cepheids' in Wikipedia they are directed to the 'Cepheid variable' entry, and if they type 'Classical Cepheids' or 'Classical Cepheid' in Wikipedia they are directed to the 'Classical Cepheid variable' entry. —Preceding unsigned comment added by 184.108.40.206 (talk) 02:42, 3 December 2010 (UTC)
OK this is not a problem. I will redirect "Cepheid", "Cepheids", and the plural of "Cepheid varible" (with an "s" to "Cepheid varible"). I will also create a hatnote to the Cepheus constellation that 220.127.116.11 was talking about, on the top of the page. In addition, I will create redirects for 'Classical Cepheids' and 'Classical Cepheid' to 'Classical Cepheid variables', and I might put the same hatnote on this page. Hence, the disambiguation page will exist as a redirect page. There are also some other redirects that can be derived from the multiple nomenclatures for these objects, mentioned in the first sentence of both articles. I will leave a note here when it is done. ---- Steve Quinn (talk) 04:05, 3 December 2010 (UTC)
I created redirects to the appropriate article for 'Cepheid', 'Cepheids', 'Classical Cepheid', 'Classical Cepheids', 'Classical Cepheid variable', and 'Cepheid variables'. I still have to do redirect pages for the other nomenclatures used for these objects, which are mentioned in the first sentence of both articles. I will do this a little later. ---- Steve Quinn (talk) 05:23, 3 December 2010 (UTC)
What's the current status on this? Is there still a need for the article to be renamed? If not, can the move request be closed?--Kotniski (talk) 11:32, 30 December 2010 (UTC)
The above discussion is preserved as an archive of a requested move. Please do not modify it. Subsequent comments should be made in a new section on this talk page. No further edits should be made to this section.
How do astronomers measure the distances to galaxies by using Cepheid variables? 18.104.22.168 (talk) 08:35, 29 November 2010 (UTC)
Cepheids are known to relate their brightness to their pulsation period. If you know the period of pulsation, then you know their brightness. As you can measure their brightness on Earth, you get the distance by the relationship between distance and brightness. (The farther something is away from you, the dimmer it appears, even though, up close, they can be very bright). It's an inverse-square law relationship between distance and brightness. So... calculating from the observed brightness and the observed pulsation period, with the period-luminosity relation and inverse-square law, you can figure out the distance. 22.214.171.124 (talk) 02:14, 30 November 2010 (UTC)
That would be helpful to include in the article. That was the question I had when I came to the article, but did not find it, despite their use in measuring distance being mentioned multiple times throughout the article. — al-Shimoni (talk) 06:50, 13 January 2011 (UTC)
126.96.36.199 (talk) 12:20, 8 May 2013 (UTC) http://cosmology.carnegiescience.edu/timeline/1912 HENRIETTA LEAVITT Her hunch paid off. Leavitt discovered 25 Cepheid variables in the cluster and created a graph showing the maximum brightness of each star and the length of its period. As she suspected, there was a clear relationship. Brighter stars had longer periods. All that was needed to find actual distances was to find the distance to just one nearby Cepheid variable. A few years later a team of astronomers did just that, making it possible to measure the distance to any Cepheid in just three steps:
a) Measure the period of the star. b) Use Leavitt’s graph to determine how bright it really is. c) Measure how bright it appears and determine its distance.
By discovering the distance key, Henrietta Swan Leavitt made possible all of the subsequent discoveries in astronomy of the 19th and 20th centuries.
She is mentioned in the article, in the history section. We also have an extensive article about her: Henrietta Swan Leavitt.--agr (talk) 12:26, 8 May 2013 (UTC)
After reading the two articles, I'm having trouble distinguishing a Cepheid variable from a pulsar. As I understand it, they both blink, but a CV frequency can be directly related to it's brightness (Pardon my terminology). Could one of you Astro-folks help me out please? Pb8bije6a7b6a3w (talk) 15:55, 14 May 2013 (UTC)
Could someone please re-phrase this sentence to make it more scientifically accurate and clear "undergo pulsations with very regular periods on the order of days to months". Kind Regards Joe — Preceding unsigned comment added by 188.8.131.52 (talk) 15:14, 22 January 2014 (UTC)
The wiki page on Harlow Shapley states that he used RR Lyrae stars to find the center of the Milky Way, this article, under History states Cepheids. Which is it? — Preceding unsigned comment added by 184.108.40.206 (talk) 23:37, 22 April 2014 (UTC)