Cepheid variables - in lay terms?

newtonian · Oct 25, 2004

bobvanx, zenith, Saiph, you all- Here is a starting link: http://imagine.gsfc.nasa.gov/docs/science/mysteries_l1/cepheid.html Saiph - Note how variable the other end of the yardstick from Cepheid varibles is, according to this NASA link: "Over the past few decades, astronomers, using different data sets and methods, have reported values for the Hubble constant which range between 50 km/s/Mpc and 100 km/s/Mpc. Resolving this discrepancy is one of the most important outstanding problems in observational cosmology."

Saiph · Oct 25, 2004

Newt: yeah, and dozens of other things fit the "star differs from star in glory". My guess: It applies to, and ends with, the observation that stars have different brightness in the sky. Cepheid and other variable stars are likely not included in that very general statement. Anyway, we're likely both correct. The shift in energy output will cause various changes in the atmosphere, like ionization and excitation, which can cause more or less absorption, causing the atmosphere to swell or deflate. The key is, the energy output has to vary for these changes to occur. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"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" -- The Tenth Doctor, "Blink" </div>

zenith · Oct 25, 2004

see i found that site, but i also found it a bit difficult... cos see i find pictures work very well for me..

Maddad · Oct 25, 2004

nexium "Hi stevehw: I hope you have recovered from whatever you were abusing when you typed 10exp2,000,000 Big numbers can be a little bit daunting. I'm not familiar with the one that Steve quoted about the total number of chemical reactions going on in the body, but this number does not surprise me. I've worked with bigger numbers, such as the amount of information encoded in a single human DNA molecule. It's so large that steve's number is only about the 950th root of this one - 101,900,000,000!

newtonian · Oct 26, 2004

stevehw33- Glad to hear you don't use drugs. However, you also don't listen well. Factorial math is very useful in determining the probability of a specific chemical reaction product among all the other possible chemical reaction products. This is why the probability of a statistical protein is so very low, lees than one in 10^112th. However, as nexium accuarately noted, this is not the actual number of chemical reactions occurring in the human body. So, why do you continue to misapply factorial math to the actual number of chemical reactions which occur in the human body at any one time? And why have you ignored my request to cite some sort of reference or link to your wildly high estimate for the number of chemical reactions occurring in the human body? You are so far off in your estimate that I find it unbelievable you would actually post such foolishness!

newtonian · Oct 26, 2004

maddad- How are you mathematically quantifying the information contained in DNA? What units of information are you using? I am not disagreeing with you, btw. You need to specify units so I can check your numbers out. However, Steve's error is that he is confusing factorial math with the actual number of chemical reactions occurring in the human body per second. Factorial math applies to the number of different combinations that could be tried, with no attempt at considering how long it would take to try said combinations. For example, calculating how many ways 100 different amino acids can be combined, and the relative probability of specific 20 amino acid combinations used in life, does involve factorial math. However, such calculations ignore the time factor. BTW- this is way off thread theme. Do you and Steve want to start a different thread concerning factorial math applications?

newtonian · Oct 26, 2004

Saiph - Well, for me, the statement "star differs from star in glory" gives reason to research said differences, including Cepheid variables. The context of the statment also considers differences in animals, plants, etc. And the deeper we study biology the more differences we find. The same can be expected in astronomy - and I find all of these discoveries beyond fascinating - awesome fits well. Well, we could both be right. Less likely, we could both be wrong! Certainly, these models are worth studying in comparison with further observations. I would expect the ionization model I quoted from the Andromeda Oxford Ltd. Science Encyclopedia (1991) "The World of Science" would amplify the variations that underly those outer Cephied star layers. If things get a little mixed up, that is fine with me - as I consider our sun may be more mixed up than we generally thought!

newtonian · Oct 26, 2004

zenith- Yes, a good library might be in order. The science encyclopedia I was quoting is well illustrated, but I have not advanced to scanning and posting pictures and diagrams. Meanwhile, I hope to try other links and references.

Maddad · Oct 26, 2004

Newtonian "How are you mathematically quantifying the information contained in DNA? DNA is series of steps like a ladder that can have one of four configurations at each rung. The molecule has 3,200,000,000 of these steps. We might label these rungs A, T, C, and G if we use the first letter of the name of the chemical constructing them. If I had just one rung to the ladder, I could specify one of four pieces of information. With a second rung I could specify 16 pieces of information by using any one of these 16 combinations: GG, GC, GT, GA CG, CC, CT, CA TG, TC, TT, TA AG, AC, AT, AA If I were to add a third rung to the ladder I could specify any one of 64 pieces of information ranging from GGG to AAA. The formula to determine how many pieces of information I can specify would be the number of different configurations of any one rung of the ladder, 4, raised to the power of the number of rungs in the system. For DNA we would raise 4 to the 3,200,000,000th power. Since most calculators freak out at numbers this large, we're allowed to cheat a little to get an estimate of the number. We multiple the log of 4, which is 0.6, by 3,200,000,000 to get 1,900,000,000. This represents the number of zeros in the answer, not the significant digits. This number is so huge that it could be 50 million times bigger, or only a 50 millionth as large, before it is no longer the same size.

zavvy · Oct 31, 2004

Cepheids and the Cosmic Distance Ladder Taking advantage of the very high spatial resolution provided by the Very Large Telescope Interferometer, a team of French and Swiss astronomers [1] has measured directly the change in angular diameter of four southern Cepheid variable stars over their pulsation cycle. When combined with spectroscopic radial velocity measurements, this allowed the astronomers to measure very accurately the distances of these stars in a quasi-geometrical way, and to calibrate the zero-point of the Cepheid Period-Luminosity empirical law. These observations constitute a fundamental step towards an independent verification of the extragalactic distance scale by interferometry. (more at link..)

bobvanx · Nov 1, 2004

Let me see if I understand the uncertanties correctly. While we've got a well established relationship for the intrinsic brightness for a Cepheid based on its period, the absolute magnitude is still a guess because we don't have a measure that's super reliable to the Cepheids in SMC. There's a circular logic loop in here somewhere. Even this latest set of measurements has to assume the star's diameter. It measures the change in diameter, and uses inferences to come up with the diameter, and then uses trigonometry to work out a possible distance to the star. So we dtill don't know the real true birghtness, and we're still guessing the diameter, and we're still using these guesses when we make calculations in which a few percent matters. It's one big guess-and-check table.

Saiph · Nov 1, 2004

Bobvanx, there is no circular logic loop here. BTW, intrinsic brightness, and absolute brightness are the same thing. We have a well established relationship between the period of a star and it's absolute magnitude (a.k.a intrinsic brightness). This is found using various other means to find the absolute magnitude (parrallax, HR diagrams, or the diameter thing). Due to inherent error in these methods, and idividual variation between cepheids, the relationship is not a pure line. It's sort of a range, but it's a pretty tight range (e.g. stars with a certain period are 5th magnitude, give or take .1 ) It's much along the line of: I give you two sets of numbers, about two individual people. One person is 5ft 9 inches tall, and weights 180 lbs. The other is 5ft 6 inches and weights 130 lbs. Which one is male? This is the same sort of deal. Sure, there's variation and things that can mess it up. However the relationship between period and luminosity for a cepheid is much tighter than the relationship between height and weight vs gender. So while we'll end up with some inherent error, it's relatively small. And in both cases, you don't have to know why it works, just that it does work (and statistically it works almost always, which is good enough). <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"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" -- The Tenth Doctor, "Blink" </div>

bobvanx · Nov 2, 2004

<blockquote>In reply to:<hr />Which one is male?<hr /></blockquote> Statistically, the 5'9" 180 lb person would be the male. That's good enough for me. Statistics works great for a large enough population, and Cepheids seems large enough. But I beg to differ: there is a loop. A 100 year old loop. When I read the literature really carefully, I see we don't actually have a measure for these stars' absolute magnitude. We've never actually measured the parallax to one. We don't have a measurement of their diameters, either. We don't really know how bright a Cepheid with a period of n days is, only that it is brighter than a Cepheid of n-1 days. So we make a guess about the diameter (an informed guess, yes, based on our understanding of main sequence stars we have measured) and use that to make a guess about these stars' absolute magnitude. Then we use this twice removed guess to work out a distance to the first set of Cepheids. Even this newest data doesn't measure the diameter, it measures the change in diameter, which we then tie back into our assumptions, which gives us an angular measure, which we match up to our derived absolute magnitudes, which gives us the distance to the stars. I'm just noticing that similar guessing, when it involves microbes on Mars or where the Universe came from, is roundly shouted down.

Maddad · Nov 2, 2004

We do not have to measure the parallax for cephids. We use parallax to check nearby stars to determine their color and mass. This lets us know how massive their are based on their color. Since we can determine color from a much greater distance then we can measure a parallax, we can determine the average color of a cephid from a much greater distance.

bobvanx · Nov 2, 2004

Yeh, but don't you see? We still don't really know, we have to guess that a Cepheid's average color can give us this information, even though it's a very different type of star.

bobvanx · Nov 2, 2004

<blockquote>In reply to:<hr />values given by Hipparchos are not accurate enough<hr /></blockquote> All in all, I'm heartened. Within the n-dimensional space that bounds the inquiry, the global max is certainly emerging. Demonstrating that sometimes, even in astronomy, plugging in a guess and solving for your other variables, them refining and refining, works out okay.

Saiph · Nov 2, 2004

bobvanx: Sure, we haven't used parralax, but we have used many other systems, like maddad describes. Here's one: Using the HR diagram (look one up), and the color of the star, we can plot the stars position on the x-axis. The y-axis of the HR diagram is luminosity (absolute magnitude). If we can find a way, by looking at the star, to get the y axis, without assuming luminosity (what we're looking for) we're good to go. Now, there are a handfull of trends on the HR diagram (statistics yay!) each with thier own characteristics. If you head off the main sequence, the star gets brighter, but because it gets bigger and less dense. This drops the pressure in the stellar atmosphere, altering the tell tail characteristics of the spectral lines (they become thinner). This thinning, or thickening of the spectral lines can tell us which trend line (that are at different luminosity lines, and run parrallel to the x axis, for the most part). This gives us an independent method for determining the Y-axis. Once this is done, we now have a luminosity. No assumptions of diameter, just observations about spectral line widths, and how that interacts with size, temperature and luminosity. And since we have the luminosity, we can find a distance. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"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" -- The Tenth Doctor, "Blink" </div>

bobvanx · Nov 3, 2004

<blockquote>In reply to:<hr />This drops the pressure in the stellar atmosphere, altering the tell tail characteristics of the spectral lines (they become thinner).<hr /></blockquote> Really? Cool. Or... Hot? Anyway, I didn't know that we can use spectral emission/absorbtion lines to tell us information about a star's density. Trés nifty! What I like about that, is it gives us a tool to get to the luminosity at that instant, so that even a variable star gives up its information about its absolute magnitude, at whatever that moment is. No averaging, no assumptions. Thank you!

Saiph · Nov 3, 2004

no problem bob. If you look up keywords like "astronomy distance ladder" and "spectral parallax" you'll run into a slew of indirect methods we use to determine distance. BTW, the only direct method we have is geometric parallax. All the others are based upon parallax, or upon a method thats based on parallax or etc... I.e. the further out you go, the more methods between the one used and parallax exist, thus reducing overall accuracy. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"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" -- The Tenth Doctor, "Blink" </div>

bobvanx · Nov 6, 2004

Yeh, That's what I did. But I could see a gap between the direct measurements, and the indirect ones. And there really are many indirect methods which make assumptions, arrive at a value, and then that value is plugged back into the assumption, and so on... Which as long as we're honest about it, that we're building towards an accurate (mathematic) global maximum solution as we circle around the peak, that's fine. But many, many of the sources I found use words (possibly for brevity's sake?) like "this proves such and so." Temperature/luminosity is great, but a small blue star is going to throw off less light than a giant blue star. So you calibrate for that. And your ladder gets more accurate. I thought we were still infering the size of Cepheids. By measuring the radial velocity during expansion/contraction, and the change in diameter, you can verify each of those against the other, but you still have to assume something about the star's atmosphere to derive a result about its size. Since we can be pretty sure that the star doesn't expand at non-sensical velocities, we can constrain the result, but to use it to claim we've "proven" our atmospheric models are accurate is circular.

Saiph · Nov 8, 2004

most of the discourses open to the layman leave out some important, but very involved and complicated, details. For instance, in my stellar models class right now, we whipped through about 3 pages of "back of the envelope" calculations that involved energy conservation, hydrostatic equilibrium, and thermodynamics. The result: We ended up with a model that was within 1% of all obseved values for the sun. Heck, we're now working on better predictions by studying how a certain type of equation behaves (let alone what the values do). The accuracy and flow of logic is often obscured to a certain extent. The logic isn't nearly as circular as you seem to see (mostly cause there's a factor they don't tell you about). Take that cephied example you mentioned. Doppler shifts let us know the rate of expansion, and observation of luminosity tells us the proportion of expansion and the time required. Say it's luminosity jumps by a factor of 4 (four times brighter). That means the surface area increased by a factor of four, and the radius jumped by a factor of 2 (surface area is 4*pi*r^2). We also know it took 2 days for that to occur, since we watched it go from minimum brightness to this new maximum brightness. With good doppler techniques, we'll notice the lines are shifted. And we know they are shifted according to speed. By measuring the magnitude of the shift, we get an expansion velocity, of say 1km/s. Okay, well there are 86400 seconds in a day, for 172800 seconds in two days. If the evelope was expanding at 1km/s, for two days, we now that the star swelled by: 172800 km (assuming constant rate...but this process can be easily addjusted for non-constant expansion). Good, great, we know the radius increased by 172800 km. Whoop, de do! But, we also know the radius increased by a factor of 2 over the original (due to the luminosity boost!). If the change was 172800 km, <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"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" -- The Tenth Doctor, "Blink" </div>

Cepheid variables - in lay terms?

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