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It seems to be a simple question, but I wasn't really able to find an appropriate answer: How dense is the Milky Way? I am certain that there are reliable statistic, maybe even new ones from the GAIA catalogue?

I am very aware of the fact that the tremendously varies between the core, the bulge, the spiral arms and the inter-arm regions, but I'd like to have a single number for the whole galaxy, something like "a solar system per pc$^3$". Is there anything around?

Thank you very much in advance!

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  • $\begingroup$ You want an average density? Just take the mass over volume ratio. Mass of stars is, if I remember correctly $10^11$ solar masses and volume I don't know but you can easily calculate it using the radius and the height of the disk. If you want to consider dark matter too you have to add the extra volume of the dark matter halo Which you can consider a sphere in first approximation $\endgroup$ – Run like hell Nov 26 '18 at 10:57
  • $\begingroup$ Someone asked about the density profile a while ago, but it's been left unanswered :( $\endgroup$ – Kyle Kanos Nov 26 '18 at 11:02
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It's possible to give a rough estimate with the data on Wikipedia:

  • Diameter: 46–61 kpc
  • Thickness of thin stellar disk: 0.6 kpc
  • Number of stars: 1–4 × 1011

If we take the average of the range, the volume of the disc is $0.6 \times \frac{\pi}4 53.5^2=1348.8 \,\text{kpc}^3$ (the volume of the core is negligible, given the inaccuracy of the number of stars). So the average number of stars per $\text{pc}^3$ is $\frac{2.5 \cdot 10^{11}}{1348.8 \cdot 10^9} ≈ 0.2$, but given the inaccuracies in the estimates it could be three times higher or lower.

(Also, not every star has its own solar system.)

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  • $\begingroup$ Thanks a lot. I took the same numbers and came to a range of 0.029 and 0.201 stars per pc$^3$. $\endgroup$ – kalle Nov 26 '18 at 11:32

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