4 ms·
On the mass scales we talk about, even small rocks would add up and lead to an observable dimming. Nebulae usually are usually even less dense, just a few mole
by cedilla 5y ago
On the mass scales we talk about, even small rocks would add up and lead to an observable dimming.
Nebulae usually are usually even less dense, just a few molecules per cubic metre, but they are still easily observable.
- magicalhippo 5y agoI ran some numbers, for the Milky Way[1], the visible matter is roughly 1e11 solar masses, while the dark matter halo contains[2] about 1-2e12 solar masses (depends on your cut-off), I'll use 1e12. The moon is about 3.7e-8 solar masses[3], so you' need about 2.7e19 of these "rocks" to make up the Milky Way dark matter halo. For comparison the Milky Way has about 250e9 stars[4]. So we're talking over 100 million "cold rocks" per star in our galaxy. [1]: https://en.wikipedia.org/wiki/Dark_matter_halo#Milky_Way_dark_matter_halo https://en.wikipedia.org/wiki/Dark_matter_halo#Milky_Way_dar... [2]: https://arxiv.org/abs/1510.06409 https://arxiv.org/abs/1510.06409 [3]: https://www.google.com/search?q=mass+of+the+moon+in+solar+mass https://www.google.com/search?q=mass+of+the+moon+in+solar+ma... [4]: https://public.nrao.edu/ask/how-many-stars-do-the-andromeda-and-milky-way-galaxies-have/ https://public.nrao.edu/ask/how-many-stars-do-the-andromeda-...
- geokon 5y agoNebulae are an argument in favor. They are observable b/c they're not dense. They have a very high surface area to mass ratio. Light passes through them and they have absorption spectrum. They also emit a lot of black body radiation. By contrast a moon size object has a very low surface area to mass ration. It has no absorption spectrum b/c light can't pass through - and any black body radiation would be negligible b/c of relatively small surface area