Milky Way Density

Question

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!

Answer 1

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.)}