One Star in Ten Belongs to No Galaxy: The Cosmic Density and Background Light of Intergalactic Stars

Adding up the intracluster and intragroup light of every system in the universe gives a cosmic density of stars that belong to no galaxy, and this paper computes it from the standard census numbers. Taking the cosmic stellar density as Omega_* = 2.7e-3, the fraction of stars residing in group and cluster environments as 0.55, and the mean diffuse share within those environments as 0.18, the intergalactic stellar density is 2.67e-04, a fraction 0.099 of all the stars in the universe. In ordinary units that is 3.55e+07 solar masses per cubic megaparsec, shining at 1.18e+07 solar luminosities per cubic megaparsec. Integrated along the line of sight with a standard accumulation factor, those stars contribute 0.93 nanowatts per square metre per steradian to the extragalactic background light, equivalent to a uniform sky surface brightness of 28.0 magnitudes per square arcsecond. Roughly one star in ten, by mass, is intergalactic; their collective glow is a percent-level part of the optical background, small enough to have hidden in its error budget and large enough to matter to its precision accounting.