Cepheid variables - in lay terms?

[zenith]

Whilst doing a physics report, i have come to learn a lot about Cepeid variables, but i cant string it all together. Could anyone explain in lay terms to me both the concept of cepheid variables, and why they are so significant in Magellanic Clouds?

 

[newtonian]

zenith - Hi!<br /><br />There are many different types of variable stars. Zeroing in on Cepheid variables, here is one basic description:<br /><br />"They vary in a basic and characteristic way, increasing in brightness rather more rapidly than they decrease, and having periods of between 1 and 60 days. The radius of a typical Cepheid varies by 10-20 percent, its light output fluctuates by about one magnitude, and there are variations in its spectral class."- "The World of Science," 1991, Volume 8, page 40.<br /><br />That's the basic description, details show that "star differs from star in glory" in many other ways. For example, there are Type I and Type II Cepheid variables. I will post more details later. I will allow time for others to post answers first.

 

[zenith]

thanks alot buddy, have a tip top day

 

[nexium]

My guess is the core of a variable star, uses up the hydrogen in a small portion of the volume of it's core, allowing matter to fall into the core, which rekindles nueclear fussion in that portion of the core. This process repeats reliably at 60 day (or other) intervals. Only stars in a narrow brightness range are able to produce the constant brightness cycle interval, so they make excellent standard candles. Other variable stars change brightness at irregular intervals. Neil

 

[zenith]

So how can these stars periodically gain and lose luminance/magnitude? i understand if it were to happen sporadically, but this seems to happen in a set pattern, how is this so?

 

[newtonian]

zenith- Thank you.<br /><br />Here is more detail from same source:<br /><br />"Pulsating variables 'vibrate' at their natural frequency rather as a bell rings when struck. It is the periodic storage and release of energy in a layer some way below the star's surface that sustains these vibrations. The layer consists of partly ionized helium (some of the helium atoms have lost their electrons) and electrons. As the star contracts the ionization increases (more electrons are stripped off) and the layer becomes more effective at trapping outgoing radiation. Eventually the stored radiation builds up enough pressure to push out the outer layers of the star, which then cools down and becomes less opaque, so allowing the stored radiation to escape. The star then shrinks then shrinks and initiates another cycle of expansion and contraction. The pulsation cycle continues for as long as the right conditions exist inside the star."- Ibid., p. 40.<br /><br />Did you know our sun also vibrates at a regular intervals?<br /><br />Thank God, our sun is far more stable and constant than many variable stars, including Cepheid variables.

 

[newtonian]

nexium- Interesting guess- model. However, it takes some millions of years for the radiation to reach a star's surface from its core - at least for main sequence stars like our sun.<br /><br />Note the source I quoted: the immediate cause of the pulsation rate lies closer to the star's surface.<br /><br />This does not mean you are entirely wrong: their may be more deep underlying causes for the effects, the variations, we observe.<br /><br />I'll do more research and post again.

 

[newtonian]

zenith - Study of our own sun gives clues as to the causes of these vibrations.<br /><br />Quoting Dr. Bernard Durney, a research director at Sacramento Peak Observatory at Sunspot, near Cloudcroft, New Mexico:<br /><br />: "The sun not only rotates on its axis but moves in many other ways that can be studied by viewing its surface constantly and seeing changes that occur. From these changes, we can formulate ideas about what may be occurring inside the sun and then plan studies to confirm or disprove our ideas."<br /><br />"About 1970," he continued, "a quivering, or shaking, of the sun was predicted. It is much like the shaking, or vibration, that occurs when a large bell is rung. One can also think of the illustration of a pebble thrown into a pond and how it causes the entire surface of the pond to be affected as the rings of waves cross the pond from the point of impact. The difference is that the waves in the sun go throughout the sun in all directions."- "Awake!," 3/8/90, pp. 24,25. <br /><br />These vibrations are caused both by deep internal solar layers (compare nexium's post) and by layers closer to the solar surface. <br /><br />Our sun's vibrations have a period (frequency) of about once per hour.<br /><br />Complimenting astronomers closer to home (mine) who discovered these vibrations in 1975, Russian astronomers also discovered them in 1976 ["The Soviet Union has an impressive solar research agency based in Irkutsk, Eastern Siberia. They have the world's most powerful solar radio telescope, consisting of 256 antennae that synchronously track the sun from its rising to its setting."- Ibid., p. 25].<br />The Sunspot observatory and others confirmed these solar vibrations in 1979-1980.<br /><br />OK, that was the status of solar observations in 1990. Can anyone post updates on the details of our suns vibrations both as to causes and also as to the properties including frequency?

 

[newtonian]

zenith - In harmony with the model for causes of the variations in Cepheid variables, graphs show the following characteristics for a specific Cepheid observation (from "The World of Science, Vol.8, p. 40):<br /><br />The period: 5.5 days.<br /><br />Temperature and magnitude go up on the graph fairly sychnronized (i.e.: similar), although the magnitude rises more sharply than the temperature, falling at a similar slope on the graph.<br /><br />The radius, however, changes in the opposite (reverse) way, with a sharp drop corresponding to the sharp rise in magnitude. The radius minimum is short, while the maximum forms a more gradual bell curve.<br /><br />SMALL MAGELLANIC CLOUD<br /><br />In 1912 the first Cepheid variables were discovered by Miss Henrietta Swan Leavitt, an American astronomer, in the Small Magellanic Cloud.<br /><br />Edwin Hubble's later discovery of Cepheid variables in <br />Andromeda, for example, proved the existence of other spiral galaxies external, or separate, from our Milky Way galaxy.<br /><br />As nexium noted, they are predictable enough to be used as standard candles; i.e. they can be used to calculate distance independent from the red shift model.

 

[newtonian]

shuttle_guy - Good point about gauging distance beyond the parallax method.<br /><br />And I try to be humble - but no one??<br /><br />nexium mentioned Cepheid variables were used as standard candles in his post of 10/16/04 08:46 AM.<br /><br />And I just posted above your post:<br /><br />As nexium noted, they are predictable enough to be used as standard candles; i.e. they can be used to calculate distance independent from the red shift model.<br /><br />See what I mean?<br />

 

[bobvanx]

Yes...<br /><br />But I never fully understood how astronomers were able to demonstrate that Cepheids have the same brightness. I recall there was a "gap" in the standard candle yardstick, and that the notion that we could assert that this star <i>x</i> at this distance <i>i</i> is 10x brighter than star <i>y</i> so most be 10x closer.

 

[newtonian]

zenith - There are two varieties (variable) of Cepheid variable. <br /><br /> Type I first:<br /><br />These are supergiant stars, very bright: absolute magnitude -2 to -6.<br /><br />They are no longer on main sequence (see the H-R diagram). <br /><br />They obey a period-luminosity law. The longer the period, the greater the absolute luminosity.<br />By comparing the apparent luminosity (as we see them at this great distance) with the absolute luminosity [that they calculated using the above law] (the actual light radiation emitted by said star) scientists calculate the distance of said Cepheid variable.<br /><br />In contrast with these supergiants, Type II Cepheid variables are low-mass stars.<br /><br />They are generally older (low-mass stars have a longer life-span usually).<br /><br />Like the I's, II's are also off main sequence, burning (nuclear fusion) mostly helium. <br /><br />They have a lower absolute magnitude than Type I's, i.e.: about 0.5 absolute magnitude.<br /><br />They are similar to RR Lyrae stars.<br /><br />Their vibrations are closer to our sun's one hour 'breathing,' mostly ranging from 0.3 to 1 day.<br />Remember, type II's are from 1 to 60 days in period ( = variation, etc.).<br /><br /><br />BTW - other variable stars include the already mentioned RR Lyrae stars, plus mostly old variables which are becoming unstable:<br /><br />Novae<br />Flare stars<br />T-Tauri (young pre-main sequence stars)<br />Dwarf Cepheids<br />Long Period variables<br />Red semiregular, irregular and Mira variables<br />RV Tauri variables<br /><br />Again, that is as of 1991 - any updates?<br /><br />Have we discovered Cepheid variables more than 15 million light years (=ly) away? <br /><br />BTW #2- You might compare the much faster pulsating Pulsars - and magnetars.

 

[newtonian]

bobvanx- Yes, there are doubters.<br /><br />Astronomer Wendy Friedman did extensive research on dating our universe and came up with a somewhat lower age estimate, about 12 billion years according to a recently aired TV broadcast.<br /><br />Here is some of her published research zeroing in on Cepheid variables:<br /><br />From Scientific American Presents, 1998article entitled The Expansion Rate and Size of the Universe, by Wendy L. Freedman, pages 94,95<br /><br />One of the difficulties with the Cepheid method is that<br />dust between stars diminishes apparent luminosity. Dust<br />particles absorb, scatter and redden light from all types of<br />stars. Another complication is that it is hard to establish<br />how Cepheids of different chemical element abundances<br />differ in brightness. The effects of both dust and element<br />abundances are most severe for blue and ultraviolet light.<br />Astronomers must either observe Cepheids at infrared<br />wavelengths, where the effects are less significant, or ob-serve<br />them at many different optical wavelengths so that<br />they can assess the effects and correct for them.<br />During the 1980s, my collaborator (and husband) Barry F.<br />Madore of the California Institute of Technology and I re-measured<br />the distances to the nearest galaxies using charge-coupled<br />devices (CCDs) and the large reflecting telescopes at<br />many sites, including Mauna Kea in Hawaii, Las Campanas<br />in Chile and Mount Palomar in California. As a result, we<br />determined the distances to nearby galaxies with much great<br />er accuracy than has been done before.<br />These new CCD observations proved<br />critical to correct for the effects of dust<br />and to improve previous photographic<br />photometry. In some cases, we revised<br />distances to nearby galaxies downward<br />by a factor of two. Were it feasible, we<br />would use Cepheids directly to measure<br />distances associated with the universe?s<br />expansion. Unfortunately, so far we<br />ca

 

[zenith]

so would a cepeid variable last longer than a H fusion star, but not as long as a He fusion star, because of this change in luminosity?<br /><br />also, how do we use these stars as a "yard stick" as mentioned? wouldnt they be more difficult to measure the distance because they have an always changing apparent magnitude?

 

[newtonian]

zenith - Well, I guess I went to deep or long for you.<br /><br />Assuming the above problems do not effect the age and distance (distance=age because it takes time for light to travel the distance), then the period-luminosity law is your answer.<br /><br />That would be: the longer the period the greater the luminosity. That would be absolute luminosity. The lower luminosity we actually see is due to the distance, the inverse square law applies, if I remember correctly.<br /><br />I.e. the luminosity decreases with the square of the distance, and if we have the absolute (original- from 10 parsecs) magnitude correct, as we would if the period-luminosity law is correct, then we can calculate the distance of the Cepheid variable and therefore the galaxy within which said variable is located.<br /><br />Of course, the above problems do effect the distance estimate.<br /><br />Which is why I am asking a question - do we have updates on Cepheid dating?<br />On your question about lifespan of stars - well, Cepheids are He not H fusion stars. Main sequence is hydrogen (H) fusion; Cepheids are off main sequence because they are fusing Helium (He). <br /><br />The change in luminosity - why would you think that would cause the star to last longer?<br /><br />Are you adopting my model of mixing caused by the causes of these variations? <br /><br />Please note my model is not the standard model - feel free to consider it as an alternative, but it is far from proven.

 

[newtonian]

shuttle_guy - Thank you - of course apology accepted. <br /><br />And your comparing the parallax method complimented nexium and my posts.<br /><br />I hope you don't mind my attempting to answer zenith's question to you.<br /><br />Please continue adding input - two heads are better than one! And three......

 

[zenith]

hey just thought id say my last post was before i realised there was a second page to the string, but thanks anyway Newtonian, you seem very knowledgable.

 

[newtonian]

zenith- Thank you. I try. It helps to have good sources to quote, of course.<br /><br />Really, this question is very important. It involves many other things, notably our estimates for the age of the universe.<br /><br />And, as Wendy Friedman noted, new instruments including satellites should refine estimates.<br /><br />You all - can anyone post the new data from the new satellites Friedman cited in Scientific American? <br /><br />To repost the specific experiments anticipated in 1998 for around now:<br /><br />Scientists are excited about results in the next decade. <br />The recently installed NICMOS infrared camera on the Hubble telescope will allow us to refine the Cepheid distances measured so far.<br /><br />Large, ground-based telescope surveys will increase the number of galaxies for which we can measure relative distances beyond the reach of Cepheids.<br /><br />Promising space missions loom on the horizon, such as the National Aeronautics and Space Administration's Microwave Anisotropy Probe (MAP) and the European Space Agency's Planck Surveyor. These two experiments will permit detailed mapping of small fluctuations in the cosmic microwave back-ground." - Sciam, 1998, astronomer Wendy Freedman.

 

[newtonian]

stevehw33 - What are you intimating I missed? Did you miss shuttle_guys apology?<br /><br />Now, you clearly missed my posts' content also.<br /><br />And you also missed Bob's point, which can be summed up by the "if" in your post.<br /><br />Thank you for your alluding to recent Hubble observations. Do you have anything more specific?

 

[Saiph]

Newtonian, good info. You don't mind if I condense it do you?<br /><br /><br />Cepheid Variable stars: <br /><br />These are stars past their prime, they have moved past core hydrogen burning (they're either done, or it's in a shell around the core now) and onto the next phase, usually helium fusion.<br /><br />The trasition from hydrogen fusion is not a smooth transition. Small stars are unable to fuse the helium "ash" that is the result of this hydrogen fusion. The helium builds up and grows in the core. Under intense pressure the helium becomes degenerate (all atoms are in lowest, most compact possible state). If the star is large enough (quite likely) the pressure will mount until the helium can fuse. However the degenerate state rapidly transmits the energy of the newly kindled fusion reaction, causing a flash fusion of a large percentage of the core. This explosion of energy is called a helium flash.<br /><br />Note: Larger stars fuse helium before it becomes degenerate, thereby avoiding this flash. However they tend to have carbon flashes, which are similar.<br /><br />This explosion causes the star to swell, the outer layers growing and cooling significantly. Despite being cooler (and thus radiating less light per square meter) the star actually ends up far, far brighter than it's main sequence counterparts. This is because the surface area of the star has swelled immensely, making up for the lack of intensity by sheer amount of radiating surface.<br /><br />The rapid expansion has eased the pressure and temperature in the core, thus slowing the rate of fusion. The energy output of the core is no longer able to support the outer layers of the star, and so the star begins to contract.<br /><br />The contraction raises the pressure and temperature, causing the collapse to slow, and reverse thus expanding yet again.<br /><br />The star falls into a natural oscillation, pulsating as the mechanisms repeat over and over.<br /><br />Observation of cepheid variables show <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>

 

[newtonian]

Saiph - Thank you. Now I'm busy- off to worship and then work.<br /><br />Will more carefully examine your post later.<br /><br />Thank you again.

 

[bobvanx]

<blockquote><font class="small">In reply to:</font><hr /><p>Observation of cepheid variables shows that the period of their pulsation (how long it takes to brighten, dim and repeat) is directly related to how bright the star really is (the absolute magnitude).<p><hr /></p></p></blockquote><br /><br />Aha! <i>That's</i> the part that seems like an assumption based on hopes and hand-waving, to me. Why would a longer period make a star intrinsically brighter?<br /><br />(Or if I have it backwards, why would a shorter period make a star intrinsically brighter?)

 

[alkalin]

Other factors that should be considered in Cepheid stars: At what distance can they still be seen in distant galaxies due to intergalactic dust or gas, combined perhaps with dimness due to distance, or just too many other stars drowning out or overwhelming their presents in more distant galaxies. I noted Friedman’s correction to distance as a factor of two. Does this mean Andromeda is closer to us by a factor of two? Maybe this factor is off by more than two? Pay attention, this is significant stuff. The whole universe is smaller than we thought by a factor of at least two????? I feel claustrophobic already.<br />Just kidding.<br />alkalin<br />

 

[zenith]

well.. im not a hundred percent certain of what im about to say, but this is my interpretation on the period/magniture..<br /><br />observations have been made that variables that have longer periods, ALSO have a higher absolute magnitude, or maximum luminocity, and that this ISNT completely because of the period, infact, wouldnt the period length be detirmined by the absolute magnitude?,<br /><br />because the longer the star takes to go from maximum to minimum to maximum luminocity again would be indicative of the change in luminocity?<br />i.e. the longer it takes a cepheid variable to complete a full cycle, the brighter the star must be.

 

[Maddad]

zenith<br />We have many methods of measuring distances to stars. Each method has its own effective range. At the shortest scale, we measure the distance to objects within our solar system by radar - timing how long it takes to bounce a signal back to us. This works out to about 100 astronomical units. We use radar to measure the distance to other planets such as Venus or Mars, and from there determine their absolute distance to the Sun, and ours as well.<br /><br />We then use this numerical value for our own distance to the Sun to get the base of a triangle across the orbit of Earth. The nearby stars will appear to shift their position slightly between a measurement taken today and another one taken six months from now. Measuring this shift, its parallax, and using trigonometry gives us the distance to these nearby stars.<br /><br />We then look at these nearby stars and notice that there is a correlation between their color and how massive they are. (We can know how massive they are by observing how long it takes another body to orbit them.) When we look further than the parallax will let us measure and infer that more distant stars of a certain color must have a corresponding mass.<br /><br />Cephid variables do not randomly change their brightness. They scale up and down in a predictable pattern. The more massive they are, the slower or faster they cycle. We can take an average brightness and use it to infer how massive the Cephid must be. To do this we assume that a distant Cephid behaves in the same manner as a closer one, a star that we can measure by its color. In this way we can know how massive the Cephid star is even though it is too far away to measure its mass by just its color.<br /><br />Cephids were instrumental in deciding that Andromeda was not just another structure within our Milky Way galaxy, but instead was a galaxy unto itself. We measured the average brightness of some Cephids in Andromeda and decided that they must be 900,000 lightyear