Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A Change in average atomic mass in universe through time?

  1. Aug 25, 2016 #1
    My understanding is that matter tends to converge and form stars and in those stars fusion creates heavier atoms from lighter atoms, and this process repeats continuously. if this is true wouldn't it imply that throughout these star cycles the average atomic mass will just increase, and as the universe gets older there will be less light atoms and more heavier atoms? or does fission occur in novas producing light atoms from heavier atoms and thus creating some sort of steady state spread of atomic mass in these cycles?
  2. jcsd
  3. Aug 25, 2016 #2
    Nope. Good question, but when fusion occurs in stars it often takes more than one atom to create the next. So if you were to take 100 atoms (say hydrogen) and put them through the process of fusion, you'll end up with 50 heavier atoms (Helium). But the heavier atom doesn't actually have the same mass as the sum of the masses of the two lighter atoms! This means there is a net decrease in mass by fusion. In the process of fusion, you have light released (hence why stars shine), and Einstein told us energy and mass are equivalent. When light is released by fusion it essentially takes away a little mass in the form of energy. So, the mass in the universe can fluctuate up and down through various processes, but energy is constant. (This is under the assumption that the universe as a whole is a closed system which to the best of my knowledge is the accepted scientific stance.)
  4. Aug 25, 2016 #3


    User Avatar
    Science Advisor
    Gold Member

    Yes. The relative abundance of heavy nuclei in the universe provides one way to estimate its age, since the initial abundances can be reasonably well calculated.
    It's also a marker of which generation a given star belongs to (high 'metallicity' = recently born; low metallicity = old geezers; where 'metals' are all elements heavier than Helium).

    @Physicist97 if you have a 100 hydrogen atoms, and you combine them to form 50 deuterium atoms, then your average atomic mass nearly doubled. Their total mass is lower than before, but that's not what the OP was asking about.
  5. Aug 25, 2016 #4


    User Avatar

    Staff: Mentor

    The average atomic mass will increase as the percentage of heavier elements increases, but you are correct that a heavier element has less mass than the sum of the masses of the lighter elements used to create it.

    That's right.

    Various processes in stars, mainly supernovas, create high-mass elements which are radioactive and will decay through fission until they reach stability. However the amount of high-mass radioactive elements is far outweighed by the amount of stable mid-weight elements such as iron, nickel, etc. The overall trend is for the percentage of hydrogen, helium, and other light elements to decrease and for the percentage of mid-mass elements to increase.
  6. Aug 25, 2016 #5
    thanks for the feedback all, does anyone know of any specific theories that deal with this? maybe something that relates stars initial composition and mass to final composition?
  7. Aug 25, 2016 #6

    Ken G

    User Avatar
    Gold Member

    It should also be pointed out that you will end up with 25 helium, not 50.
  8. Aug 25, 2016 #7


    User Avatar

    Staff: Mentor

    Sure. Here you go: https://en.wikipedia.org/wiki/Stellar_nucleosynthesis
    That article and the others it links to should give you a good overview of the processes involved.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted