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Bart Bok gave his name to dust clouds of 2-50 solar masses, which by their darkness may be 100 times more frequent than telescopes can detect.

If the solar system traversed a Bok Globule, how dark would the sun become towards the surface of the Earth, as visible by humans?

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Not noticeably darker.

Assuming such a globule has a mass of 50 solar masses and a diameter of 1 light year, that would make it's average density about $2.2\times 10^{-16}\,kg\,m^{-3}$ which is fairly close to not being there at all in human terms.

An imaginary tube of this stuff 1 AU tall and of area 1 meter squared would contain about $3\times 10^{-5}\,kg$ of mass. That is an imaginary tube from Earth to the Sun.

That much matter won't block much light.

For comparison the density of air at ground level is about $1.2\,kg\,m^{-3}$. So less than a millimeter of air at ground level blocks more sunlight than the dust cloud would.

Also

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  • $\begingroup$ Question calls them "dust clouds" and Wikipedia says about 1% dust. I think this won't change "Not noticeably darker." but the dust particles may be more important to the answer than the gaseous components of the cloud. $\endgroup$ – uhoh Oct 3 '18 at 2:42
  • $\begingroup$ Presumably, though, gravitational effects would make the cloud concentrate in the inner solar system? $\endgroup$ – adrianmcmenamin Oct 3 '18 at 12:00
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    $\begingroup$ @StephenG I don't think that's how astronomers handle dust, just saying it's the same as an equivalent amount of gas. In this particular case with such a short path length and low density, doing this wrong will still give a reasonable answer, but it's still not correct to suggest dust can be treated as gas. $\endgroup$ – uhoh Oct 3 '18 at 16:52
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    $\begingroup$ @adrianmcmenamin I'm not at all clear on how the cloud would interact with the Sun's solar wind (and the "bow shock" where the Sun's solar wind meets the interstellar medium). This might reduce the extent to which the cloud can penetrate, although we're talking about very low densities anyway in human terms. $\endgroup$ – StephenG Oct 3 '18 at 16:55
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    $\begingroup$ @uhoh I didn't make any attempt to treat the cloud in any detailed way, and you should view my estimate as strictly "back of the envelop" stuff. If I'm within a couple of orders of magnitude I'd be happy. $\endgroup$ – StephenG Oct 3 '18 at 16:57

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