Cepheid variables - in lay terms?

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bobvanx

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<blockquote><font class="small">In reply to:</font><hr /><p>we assume... and infer<p><hr /></p></p></blockquote><br /><br />Don't these words belong in a SETI forum? <img src="/images/icons/wink.gif" /><br /><br />Seriously though, this is the heart of it. Who among the posters here can explain how we can be sure that all Cepheids follow an intrinsic brightness/periodicity that is even and predictable enough that it's of any use as a standard candle?<br /><br />I.E., what is it about the star's changing brightness that allows us to say with certainty that their intrinsic brightness is predictable?
 
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newtonian

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stevehw33- Besides the fact that your last post is far from accurate, it is also way off thread theme. To respond would be to join you in a way off tangent, which I will not do. <br /><br />Nice try at a bait and switch, though. <br /><br />Your wasting time trying to bait Saiph, though. He is far too intelligent and knowledgable to fall for your methods.
 
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newtonian

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bobvanx -Hi!<br /><br />Thank you for questioning what many assume to be correct. It helps us examine the reasons why we believe certain things to be true, in this case: Cepheid variables as standard candles.<br /><br />Please do not be intimidated by stevehw33 - I don't know why he is posting the way he is, but you would probably be better off ignoring his unkind remarks to you. I'm noticing he does that to others also.<br /><br />Now, I will re-examine the reasons Cepheids are accepted - meanwhile you may want to re-examine my quote of astronomer Wendy Freedman from Scientific American.
 
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newtonian

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Saiph - Excellent post!<br /><br />However, you are being modest - you did not just condense what I and others posted, you added meaningful explanatory content, notably the early history of these stars' development (=evolution).<br /><br />Thank you.<br /><br />I have a few questions on what you posted:<br /><br />You start off describing small stars, but Type I Cepheids are either Giant or Supergiant stars!<br /><br />1. Since you are describing small stars, are you describing Type II Cepheids which are low-mass stars? These are older stars than Type I's, btw - also lower absolute magnitude as per my previous post - also pulsating faster than Type I's.<br /><br />2. Your note on larger Helium burning stars is in reference to red giants? Were you including type I Cepheid variables in this "Note?"<br /><br />3. You indicate that the pulsating is reflecting expansion and contraction of the core. Are you sure? I was under the impression, as my source quote stated, that the pulsation initiated in layers closer to the surface.<br /><br />OK, My computer is having trouble (hackers, spyware, etc.), so I'm going to post this and then continue (anticipating a freeze-up) in a later post.<br /><br />
 
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newtonian

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Saiph - This question is a biggy:<br /><br />How can core contraction and expansion effect surface contraction and expansion at such a regular rate with periods of less than one day for Type II (low-mass) Cepheids? <br /><br />Wouldn't that imply some mixing due to this type of interaction from core to surface?<br /><br />I have in mind the comparison with our sun, which pulsates (vibrates, breathes) once an hour.<br /><br />As you know, I am questioning the assumption of zero mixing between star core areas of mostly Helium and surrounding layers still containing hydrogen.<br />Thank you for noting that improved understanding of Cepheid variables will likely improve our distance calculations to other objects.<br /><br />I have noted that more distant methods are calibrated by Cepheid Variables as the standard (candle), as per my quote of astronomer Wendy Freedman, et al.
 
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newtonian

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alkalin - You noticed that factor of 2 adjustment Astronomer Freedman noted also!<br /><br />Yes, that is rather astounding, as was her questioning the value of the Hubble constant.<br /><br />It shows that what we think we know may need to be rethought so we can know more than we think! (I like puns)<br />Notice, also, that this astronomer is not afraid to question things many assume to be correct.<br /><br />Now, we may end up with an age of universe of 12 billion years instead of 13.7, or we may not - and the distance adjustments that would also be required.<br /><br />We will most certainly understand better why we conclude a certain age and distance is indeed correct.<br /><br />Who knows, 1.7 billion years from now we may indeed come to know our universe is (er, will be) 13.7 billion years old!
 
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bobvanx

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N,<br /><br />Oh, I'm not intimidated by Steve's outburst. People do that sort of thing when you hold up a mirror to the corpses' nose and see the breath condensing there.<br /><br />Steve,<br /><br />I've been trying to find the math behind the assumption about the intrinsic brightness being directly related to the period of a Cepheid variable, but I keep coming up against the same set of re-packaged assumptions. There are many (oh, what's the word, I want to get this right) <i>subjective</i> observations, but I haven't found the hard science, the objective, empirical data, behind it. No one seems to have described the <i>why</i> that the measurements and assumptions are based upon.<br /><br />It's a big blind spot. The astronomy community has confused a description of what they see with a causal relationship.<br /><br />It's the exact same thinking that gives us the crackpot science of "more babies are born on the full moon." Just because we note a correlation, doesn't mean there is a causal relationship. We might be unintentionally filtering the data.<br /><br />The continuum of observations on Cepheids is discontinuous, so how can we be sure that our conclusions are correct?
 
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newtonian

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zenith - Excellent, you are learning!<br /><br />You are correct.<br /><br />Stated simply:<br /><br />Period-luminosity law = the longer the period the greater the luminosity<br /><br />What is being studied, as per Saiph's excellent post, and astronomer Wendy Freedman's (et al = and all associates) research, is the precise ratio- trying to make the standard candle more precise and accurate.<br /><br />However, absolute magnitude is not maximum brightness:<br /><br />absolute magnitude = the actual brightness of the star from 10 parsecs out (Saiph: how bright the star is)<br /><br />apparent magnitude= how bright the star appears to be from earth, LESS BRIGHT primarily because of distance - however, other things can effect the brightness, e.g. (= for example) intervening dust or gas clouds.<br /><br />Now, for cause and effect of the period and brightness (= luminosity) of Cepheid Variables:<br /><br />Brightness does not cause the change in period, nor vice versa. Rather, the variation in brightness and length of the period have the same causes, which is why they are so reliably related.<br /><br />The simplest explanation of the cause is that the outer layers through which light travels are changing their transparency, or how much light they let through, by a cause which follows a regular cycle.<br /><br />The change in transparency is caused by variation in ionization.<br /><br />Ionization: the addition or subtraction of electrons for various reasons, addition of electrons produces a negative charge; subtraction of electrons causes a positive charge. <br /><br />In the case of Cepheid Variables, electrons are stripped off of Helium layers making Helium ions, or ionized Helium, with a positive charge (+ if one electron missing; ++ if two electrons missing). <br /><br />It is the underlying trapped radiation that strips these electrons off of Helium, but this makes the Helium less transparent (= more opaque = trapping more light and radiation).<br /><br />The trapped radiation builds up
 
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newtonian

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bobvanx- Good, I will sleep better tonight.<br /><br />Please note I attempted a cause and effect description in my above post to zenith.<br /><br />Note that I also have questions.<br /><br />The standard model for the period-luminosity law is the variation in ionization in stellar layers in Cepheids.<br /><br />See my post to Zenith.<br /><br />You all: another question:<br /><br />Since ionization can cause magnetic fields, and since our sun's corona is heated by solar magnetic fields due to ions in motion deep within the sun, can we then draw any conclusions as to Cepheid magnetic fields and/or Cepheid coronas or the lack thereof?
 
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newtonian

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bobvanx- I will try to find the math for the period-luminosity law.<br /><br />One should never ignore the math - your point is well taken. <br /><br />Note the graph I described in my post of 10/16/04 09:55 AM<br /><br />The change in radius was determined by the doppler effect - apparently observations of the doppler effect from Cepheids actually show the expansion and contraction of the outer layers from which the light is emitted.<br /><br />That would be the basis for some of the math, but I haven't dug deep enough to find the actual doppler studies and the actual math.<br /><br />Maddad- good post, and you are good at math - want to help us here?
 
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zenith

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im not sure, but i think it was Newtonian who said that our sun is a variable, but its period is only that of an hour? (dont hold me to that, i have a shonky memory on all things bar "The Simpsons")<br /><br />would this then be true for all Generation II stars? but then that poses the problem, with the more massive stars, they would have a longer period, which hasnt been observed?
 
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newtonian

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zenith - Yes, that was me- but please note our sun is far more stable than a Cepheid variable!<br /><br />And the cause is probably different. For our sun, if my memory serves me correctly, the cause is shock waves or seismic waves that actually go through the sun in all directions, including through the core!<br /><br />Ionization and magnetic fields may play a role, but I think the seismic waves are somewhat independent of those other cause and effect relationships which are considered more prominent in Cepheid Variables for the cause of the vibration period.<br /><br />We had a very hard time discovering the hourly vibration of our sun from our earth.<br /><br />I also have a problem with memory: I think the sun's hourly vibration is mostly size, not so much luminosity variation. But I could be mistaken on that detail.<br /><br />We may not be able to observe this effect even in a close star.<br /><br />We certainly could not observe this effect in a star in <br />Andromeda, for example.<br />If our sun was a Cepheid variable we would have some difficulty surviving!<br /><br />Remember, Cepheids are not on main sequence anymore while our sun is on main sequence.<br /><br />However, that is a good question- hopefully in the future we will be able to observe how different our sun is from other main sequence stars in respects to its period and degree of variability, and also as to coronas and magnetic fields.<br /><br />See my former post for the causes of our suns vibrations.<br /><br />You all - do we have updates on this?<br />
 
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zenith

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"There is no necessary attempt to understand it, but simply to point out the relationship. "<br /><br />There is no necessary attempt to understand anything, but it is human nature to question, to poke and prod untill it either bites us or we know what it does and why...
 
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zenith

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like i said.. we will poke and prod untill we are bitten or untill we understand it... the inquisitive nature of homosapiens is why we even have knowledge of the universe, is it not? we dont claim to understand it, nor will we ever, but as a whole we all want to know more, more about everything..
 
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zenith

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i know we will never know much, i have never claimed otherwise, but what i am saying, is that even though we know we cant know everything, why cant we keep going? why cant we explore everything within our reach? what is the point in giving up? its not where you get to, its how you get there.
 
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Saiph

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Sorry I've been away, busy, busy life this semester.<br /><br /><br />Reason why period and luminosity are related: Empirical observation.<br /><br />Some Cepheids are close enough that we can find the distance to them via other means (parralax, HR fitting, cluster fitting etc). Knowing the distance, we can determine the absolute magnitude.<br /><br />We also note the period of the Cepheid. Upon comparing the two, we notice, empirically with no assumptions, that the longer the period the brighter the star in a tight correlation (say average of 5% variance). That means knowing the period, we know the absolute magnitude, within 5%, by observation alone.<br /><br />It's sorta like noticing how water freezes at 0 degrees celsius. However the more salt that's added, the lower the freezing point. The more salt, the colder it's gotta be to freeze. If you know the salt content, you can then predict the freezing point.<br /><br />All this without knowing specifically <i>why</i> this occurs.<br /><br />The only assumption in the Period-Luminosity relationship is that all cephieds follow it. We've noticed that isn't quite true, there are several types of cepheids (each with their own spectroscopic characteristics which can be identified independent of the period or luminosity). But within the types the hundreds we've observed all follow the trend (with only a small variation from one individual to the next).<br /><br /><br /><br /><br />Newtonian: The core does not expand or contract. The core is put under more or less pressure, thus moderating the rate of fusion. The fluctuating fusion levels causes the energy radiated to change. This radiated energy is what's holding up the surface. Change the radiated energy (light) levels and you change the size of the surface. More light = "higher" surface.<br /><br /><br />I started by discussing small stars, since they are very similar to large stars except in the helium vs carbon flash, stardeath, and a few other minor issures. However the o <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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bobvanx

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Thank You!<br /><br />I sure wish I had a textbook on this... maybe I'll go over to the University and browse. Info on the web is not doing the job for me, and usually I'm a whiz at searches...<br /><br />Anyway, regarding that empirical observation, I recall that the correllation between brightness and period was pretty good and predictive for a set of cepheids that we can measure through parallax. But there was a gap between the ones we could compute good absolute mags from, and the ones we use for standard candles in distant galaxies, and the correllation was <i>assumed</i> to be consistent into higher energies. But we lacked confirmation.
 
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zenith

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on the text books note.. does any one have any web sites, that explain in two parts... firstly it has to be breif.. a breif overview if you wish, but then it becomes very strong? i cant find any like that, and am forced to ask around (which, being the proud person i am, i loath very much)
 
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Saiph

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1) I don't know if we have a parralax on any cepheid, though we do have other distance measures (mostly HR fitting IIRC). But that's still pretty solid.<br /><br />The ones we use for distance measrues are similar to the ones we've calibrated for (i.e. we know the distance too via the HR fitting).<br /><br />I don't know of any "high energy" dichotomy.<br /><br />I do know we had a little egg on our face (the astronomy community, geesh, I'm starting to refer to it as <i>we</i>) when the different types became apparent though. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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Saiph

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yeah, but there are other methods that are calibrated using stars you can parralax.<br /><br />It's sorta like: If we know I'm 3" taller than frank, and frank is 5" taller than bob, bob is ~8" shorter than me.<br /><br />Even though no direct comparison was there, we used frank as an intermediary.<br /><br />Also, it all rests on how accurate our original tape measure was (one that measure my height). If that tape measure was off, I may not be 3 inches taller than frank, and our estimate of how tall frank is compared to bob is likely to be colored too (afterall, we were considering the "inches" on the ruler...).<br /><br />If you want more info, look up a term called the "distance ladder" for astronomy. It's the concept that each distancing method we use, rests upon the accuracy of the ones before. That's why the last method (the Hubble Expansioni law) is the least accurate, it accrues the errors of them all. It doesn't make it invalid though, just a bit more vague than the rest. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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newtonian

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zenith - I do not know why Steve posts as he does - he seems to be against scientific progress in understanding.<br /><br />Of course, science seeks to understand the why and how- science is not mere observation, it draws conclusions based on observation.<br /><br />The conclusions I posted were scientific, and can be found in good scientific sources on the subject,<br /><br />Of course, some things are not completely known at this time - they await the joy of present and future scientific discovery.<br /><br />Steve - yes, I knew the sunspot cycle but thank you for posting it. <br /><br />It is independent of the hourly vibrations of our sun, and the causes are different.
 
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newtonian

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stevehw33- You are on a tangent - so is my response to your post:<br /><br />I agree on the complexity of the human brain, but do not know why you conclude this complexity makes understanding our universe beyond the capacity of our brain?<br /><br />Your math is way off. Please correct your post with real math instead the following fantasy you posted, and I quote:<br /><br />"The total number of kinds of chemical interactions going on in our bodies at any one time is a number of the order of about 10 exp 2,000,000."<br /><br />Where did you get 10^2,000,000 chemical reactions going on in our brain at any one time?<br /><br />Try quoting or linking a source or checking your math.<br /><br />In our entire universe since our universe began there is an upper limit of less than 10^122 chemical reactions - so please show us where you came up with that number.<br /><br />Your correct conclusion that we will never know it all is expressed this way in an ancient source:<br /><br />(Ecclesiastes 8:17) 17 And I saw all the work of the [true] God, how mankind are not able to find out the work that has been done under the sun; however much mankind keep working hard to seek, yet they do not find out. And even if they should say they are wise enough to know, they would be unable to find out.<br /><br />So, I agree, we will never know it all. <br /><br />Yet, Romans 1:20 encourages us to study the things made and learn - and this is essentially the motivation which drives scientific research into why and how.<br /><br />Why do you seek to discourage scientific inquiry into why and how?<br /><br />
 
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nexium

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Hi stevehw: I hope you have recovered from whatever you were abusing when you typed 10exp2,000,000 That must exceed all the chemical reactions of all the beings in the visable universe for the past 13.6 billion years. Did you perhaps mean that many combinations are possible in theory, rather than that many actually occur in the same instant? Neil
 
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newtonian

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Neil - Yes, a more concise way of questioning where 10^2,000,000 came from. He posted that factorial math applies - which it does not. However, I suspect that is what misled him.<br /><br />Saiph - Yes, the variable nature of Cepheid variables would be in harmony with "star differs from star in glory." <br /><br />It does not surprise me that the period-luminosity law is not really an absolute constant.<br /><br />Also, note a post I made concerning astronomer Wendy Friedman [Freedman? sp?] (from Scientific American Presents: 1998) concerning a variant estimate of 70 for the Hubble constant.<br /><br />And a variable estimate for the age of the universe = 12 billion years.<br /><br />Are you sure that the Cepheid periods are caused by Helium fusion period variation in the core? That is totally variant from the cause and effect I posted concerning periodic variable ionization of surface layers alternately trapping and releasing light.<br /><br />Can you post a link or reference where you got that model?<br /><br />Such core to surface transference is by what method of transit: shock waves (or seismic waves) ? <br /><br />zenith - I hope to check more references soon. Saiph is usually accurate and I also try to be, so more research is in order.
 
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