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What does the abbreviation YP stand for?

  1. Apr 14, 2015 #1
    I have been looking up information about the promordial abundances of isotopes on the internet, and I find many of the papers discussing this use the notation YP (where P is usually a subscript). Can anyone tell me what this YP stands for? It appently means the primoridal abundance of the particular isotope being discussed, but what do the letters Y and P stand for?
  2. jcsd
  3. Apr 14, 2015 #2


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    Mass fraction of Helium 4
  4. Apr 14, 2015 #3
    Thank you for responding bapowell, but you misunderstood what I was asking. The context value told me that it was for He 4 , but what do the letters "Y" and "P" stand for?
  5. Apr 14, 2015 #4


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    ==quote page 1 of lecture 25 of an astro course by Prof. Coleman Miller at University of Maryland==
    Observations of Light Element Abundances

    Last time we discussed the expectations of Big Bang Nucleosynthesis (BBN to its friends). Here we talk about the observations. For a bit of change of pace, and to give some appreciation of the difficulties and the care needed, we’re going to go the sausage-making route here: lots of gory details. In some cases, we will use the standard nomenclature that X is the mass fraction of hydrogen, Y is the mass fraction of helium, and Z is the combined mass fraction of everything else.
    The notation seems to be traditional, somebody (Peebles1966? earlier?) found it convenient to divide stuff up into three categories (H, He, Other) and to study the probable division into 3 mass fractions.
    The subscript "p" as you guessed stands for "primordial".

    It sometimes happens that a notation goes back to early papers on the topic. Somebody made a (possibly arbitrary) choice and it stuck---becoming traditional.
    Here is some historical background:
    But I don't know which authors first made that division into X + Y + Z
    Last edited: Apr 14, 2015
  6. Apr 15, 2015 #5
    Hi marcus:

    Thanks very much for the history. It was exactly what I was looking for.
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