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Stargazing How do we know how bright a cepheid variable star is?

  1. Oct 30, 2016 #1
    Most of us know what that we can judge the distance of an object such as a car headlight by how bright that headlight is. The things is we know how bright car headligjts are from 1 foot away so that seems pretty obvious.

    A Cepheid variables brightness is commonly used to measure the distance to the galaxy that they are within. What i dont understand is how that works becuase how do we know original brightness of the cepheid?

    We cant stand next to a supernova and measure how bright it is so how can we determine its distance from how bright or dim it seems.

    Do all these stars go supernova at a certain mass, and is that how we "estimate" how bright a cepheid variable is?

    (I have asked this before but cant get an answer or find the thread sorry)
  2. jcsd
  3. Oct 30, 2016 #2

    Jonathan Scott

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    Some Cepheids are close enough for their distance to be measured accurately by other means (for example Delta Cephei, after which the class is named, which is close enough to show parallax). It was found that for local Cepheids there is a predictable relationship between the brightness and period (although later other factors were also found to be relevant, making it a bit more complicated). It is therefore possible to use the period to determine the intrinsic brightness and then to use the observed brightness to determine the distance.

    For more information, see the Wikipedia page: Cepheid variable
  4. Oct 30, 2016 #3


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    The keyword here is cosmic distance ladder. There are different overlapping methods to estimate distances for various length scales, on short scales they are based on the astronomical unit and parallax measurements. The Gaia telescope should increase the accuracy there a lot in the next years, and extend the method to larger distances.
  5. Oct 30, 2016 #4


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    Cephid variable stars are big ones. Bigger stars have a higher rate of reaction; they run hotter. A Cephid is 'not quite' a Nova. Whereas a Nova just blows up, Cephid collapses at its maximum output and cools down again. The process repeats and repeats. The period is related to the mass. Bigger is faster and ( so the theory goes) all Cephid of the same period have the same brightness range (absolute magnitude). So you assess a Cephid's visible magnitude and measure it's period. This (from the inverse square law) tells you how far away it is. Nearby Cephids were used to calibrate the method because their distances could be found by the parallax method. (Parsecs).
    An inspired method in my opinion.
  6. Oct 31, 2016 #5
    Determining cosmic distances is still the "Holy Grail" of astronomy. There are several methods used, some more accurate than others.

    Parallax is by far the most accurate measurement of distance. However, it is limited to approximately one thousand parsecs. Beyond that we are not able to make accurate angular measurements, therefore we require another means for measuring distance.

    As Jonathan Scott correctly mentioned, we discovered a relationship between the pulsation of certain stars and their luminosity. Delta Cephei was the first Cepheid variable star used, and it was close enough to measure its distance using parallax. Therefore, by finding other Cepheid variable stars with the exact same pulsation rate they can determine the absolute magnitude of the star. Then by measuring its apparent magnitude we can determine its distance. However, one has to be very careful since there are several different types of Cepheid variable stars, each with different rates of pulsation and different absolute magnitudes. Cepheid variable stars can be used to measure cosmological distances out to approximately one million parsecs. Beyond that distance you need to use supernova.

    The very first Type 1a Supernova (SNIa) was measured using a Cepheid variable star, and because it was (incorrectly) assumed that all SNIa have the exact same mass (1.44 solar masses as determined by Chandrasekhar's Limit) when they go supernova, that gave them the absolute magnitude of -19.3 at peak brightness. Therefore, by measuring the apparent magnitude one can determine the supernova's distance. As a result many have called SNIa a "standard candle." However, since then we have discovered both super-luminous, a.k.a. super-Chandrasekhar, SNIa and super-dim (with an absolute magnitude ranging between -14.2 and -18.9), or sub-Chandrasekhar, SNIa (which has been recently [2013] been given a new classification as SNIax). There have been different theories suggested for the cause of super-luminous SN1a. There are also ways of distinguishing between super-luminous and super-dim SNIa, but more data needs to be captured before that can be done. For example, the ejecta of all SNIax travels at a rate that is less than or equal to 8,000 km/s, and all SN1a have ejecta rates that exceed 10,000 km/s. But unless we specifically measure the blue-shift of the ejecta, we are not able to distinguish between the two. It is estimated that from 14% > 31% < 44% of all SNIa have been misclassified and should actually be SNIax. That would have a significant impact on not only the age of the universe, but also the rate of its expansion, since all that information was deduced primarily using what was erroneously presumed to be a SNIa "standard candle."

    The last, and least accurate, means of determining cosmological distance is by using red-shift. This method is used only for galactic size objects that are far beyond even the range of visible supernova.

    Type Iax Supernovae: A New Class of Stellar Explosion - The Astrophysical Journal, March 25, 2013, Volume 767, Number 1 (free edition)
    A possible mechanism for over luminous type Ia supernovae explosions inspired by dark matter - ArXiv 1610.05578v1
    Type Ia supernovae within dense carbon-oxygen rich envelopes: a model for 'Super-Chandrasekhar' explosions? - Oxford Journals, Monthly Notices of the Royal Astronomical Society, May 12, 2016, Volume 463, Issue 3 (arXiv free reprint)
    A Luminous Peculiar Type Ia Supernova SN 2011hr: More Like SN 1991T or SN 2007if? - The Astrophysical Journal, January 26, 2016, Volume 817, Number 2 (arXiv free reprint)
  7. Oct 31, 2016 #6


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    Nice one. I hope my Noddy version will give some people a way into your more complete version. :smile:
    It all shows how ingenious Astronomers have to be when they want to glean information about the Universe from such limited measurements as Earth (and even Hubble)- bound observers have available to them.
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