Now that the JWST is peering further and further back into the cosmos, I came to a few questions that I would love to see answered. We know that models of the big bang nucleosynthesis result in specific predictions on the ratios between hydrogen, deuterium, isotopes of helium, and isotopes of lithium, which we can observe within our immediate galaxy group.

  1. Do we expect the ratios mentioned above to vary wrt time since the big bang (or rather wrt time since BBN)?
  2. If the answer to question #1 is yes, then is there anyone using the JWST to track how the element abundancies change wrt time?
  • $\begingroup$ Where are these elements? In galaxies or outside galaxies? The abundances are changed by processes that occur in galaxies. $\endgroup$
    – ProfRob
    Commented Aug 10, 2022 at 12:58
  • $\begingroup$ @ProfRob I was thinking about elements inside the galaxies. That's a good distinction to point out. $\endgroup$ Commented Aug 10, 2022 at 13:13
  • $\begingroup$ 1. Yes 2. Yes. This is a major subfield of astronomy. Much has been done with using old stars in the galaxy. Much has been done by looking at the farthest galaxies. One can not really begin to summarize it here. But, here are beginning points to learn about it. annualreviews.org/doi/10.1146/annurev-nucl-101917-021141 and iopscience.iop.org/article/10.3847/1538-4357/abf4c1 $\endgroup$
    – eshaya
    Commented Aug 10, 2022 at 18:49

1 Answer 1


Obviously since all stars by definition changes its chemical composition by the process of fusion the chemical composition is expected to change over time. Elements heavier than iron are produced in a somewhat different manner.

Regarding your second question there is a FAQ from NASA regarding James Webb. Here is one citation of what Webb is capable of in the near-infrared:

"Webb will conduct deep-wide surveys of galaxies in the rest-frame optical and near infrared over the redshift range 1 < z < 6 with both imaging and spectroscopy. The microshutter array, which provides multi-object near-infrared spectroscopy of ~100 targets simultaneously, will allow large samples of galaxies to be divided into bins of redshift, metallicity and morphological structure. "

The instrument for detecting radiation with a wavelength of between 5 and 28 micrometers has another spectrometer that can only focus on four regions simultaneously:

"MIRI uses an image slicer and dichroics to provide imaging spectroscopy over four simultaneous concentric fields of view ranging from 3 to 7 arcsec on a side."

All data collected from James Webb is released to the public one year after it is collected.

Answer: The instrument for collecting radiation in the near infrared region on James Webb up to a wavelength of about 5 micrometers will be used to do more or less automatized categorisation of up to 100 galaxies at the time based on spectrographic redshift, (which is a measure of time) and metallicity (which is basically the chemical composition) and this data is released to the public after at most one year.

  • $\begingroup$ I've learned (to some extent) what pupil slicer is. Now I'm wondering; is an "image slicer" the same or similar, or different? If you think it deserves a new question I can ask it separately. $\endgroup$
    – uhoh
    Commented Sep 29, 2022 at 12:59
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    $\begingroup$ @uhoh that is beyond me. $\endgroup$
    – Agerhell
    Commented Sep 29, 2022 at 23:41
  • $\begingroup$ that makes (at least) two of us :-) $\endgroup$
    – uhoh
    Commented Sep 29, 2022 at 23:47

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