# 8.3.1 Star density

For testing the star density we compute star counts and compare their absolute values as well as the relative differences (data-model)/data, in different magnitude bins.

Results for magnitude bins 8-9, 12-13 and 17-18 are presented in Figure 8.9. The overall picture of the sky densities are very well comparable in data and model, ensuring realistic star counts in both. There are however significant discrepancies close to the Galactic plane due to the extinction model used in GOG20 which is not optimal. It is also possible that the data show some incompleteness at latitudes near 0 towards the inner Galaxy as seen in the relative difference plot for $17. At $8 at high latitudes the small number of stars lead to a noisy map of relative difference which does not show significant discrepancies. At $12 and $17, one can see the imprint of the Magellanic clouds, not present in the model. We also start to see some discrepancies in the outer Galaxy, close to the plane, that are most probably due to the warp, the flare, and the disc scale heights, not perfectly modeled in GOG20.

The average value of star counts over the whole sky has been computed in each magnitude bin to see a signature of incompleteness. Figure 8.10 shows the results for Gaia EDR3 as well as for Gaia DR2 and GOG20. Gaia EDR3 has more faint stars than Gaia DR2, but still not as many as expected from GOG20.

Summary of the results:

• The overall picture of the sky densities are very well comparable in data and model.

• The data may show some incompleteness towards the inner Galaxy for faint stars.

• At $G>18$, Gaia EDR3 has more stars than Gaia DR2, but not as many as expected from GOG20.