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We know from big bang nucleosynthesis that baryonic matter accounts for about 5% of the universe's total mass-energy density. What is the current best estimate of how much of this is in the form of stars? I'm guessing that this would be known only very roughly. It seems like you would just have to survey large volumes of space for stars and larger structures made out of stars. Although galaxies may pretty efficiently cycle their hydrogen and helium through stars, I would assume that there is a lot of hydrogen out there in the spaces between superclusters that has never had a chance to form a star and never will.

related: How do we estimate $10^{23}$ stars in the observable universe?

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"Only about 10 percent of the total baryonic matter is sufficiently condensed by gravity to form stars and galaxies. More than 90 percent was left between the galaxies."

http://hubblesite.org/hubble_discoveries/science_year_in_review/pdf/2008/searching_for_baryonic_matter_in_intergalactic_space.pdf

6% of baryonic matter is within stars according to the following lecture:

https://www.sron.nl/files/HEA/XRAY2010/talks/9/bregman.pdf

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    $\begingroup$ Do you think you could expand this a little, maybe including some information about how we know? The ideal is for SE answers to be self-contained. This is not much more than a link-only answer, which is discouraged. $\endgroup$ – Ben Crowell Nov 3 '14 at 22:24
  • $\begingroup$ this link says 1/4: caltech.edu/content/… $\endgroup$ – Ben Crowell Nov 3 '14 at 22:26
  • $\begingroup$ @BenCrowell The 6% number is coming from Fukugita & Peebles, "The Cosmic Energy Inventory" (2004) iopscience.iop.org/0004-637X/616/2/643/fulltext/60693.text.html The based their number on optical luminosity and models of mass-luminosity ratios. $\endgroup$ – DavePhD Nov 4 '14 at 19:21
  • $\begingroup$ @BenCrowell According to a new study, as much as half of all stars might reside outside of galaxies. So it seems there's still a lot of uncertainty. $\endgroup$ – Pulsar Nov 7 '14 at 20:29

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