In his answer to this question, TildalWave made this remark:

I think that first, we have to properly appreciate the size of the Sombrero Galaxy. It is roughly 50,000 light years (15 kilo parsecs) in diameter. That might be only half of the diameter of our own Milky Way galaxy, but still makes each and every pixel on the photograph you're attaching in your question stretch more than 100 light years in distance.

Here's the photograph:

sombrero galaxy

Since it almost feels like I can see individual 'pieces' of dust in the lane, it would follow that the particles must be very far apart. Just how far apart are they? Are they grouped in clumps, or relatively evenly space?

  • 3
    $\begingroup$ I think your assumption is incorrect. You can't see individual pieces. You are seeing the variation in particle density, and looking at the photo - there does look to be clumping, and there are bound to be tidal effects. $\endgroup$
    – Rory Alsop
    Sep 30, 2013 at 19:56
  • $\begingroup$ Note the 'feels like' - I know it's incorrect, but the dust certainly doesn't look very dense. $\endgroup$
    – user19
    Sep 30, 2013 at 19:58
  • $\begingroup$ The "structure" you are seeing has nothing to do with the size of dust particles. You are seeing the structure of clouds of absorbing dust, which indeed can be hundred of light years across. The interstellar medium is "clumpy" on these sorts of scales. $\endgroup$
    – ProfRob
    Mar 9, 2015 at 12:25

2 Answers 2


The sizes of dust particles varies enormously, and thus the density of dust particles depend upon which sizes we are talking about. The dust size distribution can be described by a power law with a slope of roughly $-3.5$; that is, the density of 0.1 µm particles of is $10^{-3.5}$ times that of 0.01 µm particles, or roughly 3000 times smaller.

Moreover, the discussion is complicated by the fact that we don't really have a formal definition of what is dust. But one "definition" takes the minumum size as a conglomeration of a few molecules, and the maximum size as "what has time to grow in a formation process" (these dust particles can later "stick together" and grow to form pebbles, rocks, asteroids, and planets. That's why any definition will be arbitrary).

In a dust-dense region of the interstellar medium, this results in a typical mean density of roughly one dust particle per cubic centimeter.


Based on 1995A&A...303..673E,

How far apart they are?

No idea. Somebody else might help to provide a density distribution.

Are they grouped in clumps or relatively even spaced?

This is what I really wanted to answer. They are in a concentric ring-like structure around the galaxy. The maximum optical depth is 0.8 with a mean value of 0.3. By the way, optical depth gives you a nice idea of how far apart they might be, not a good density distribution but you can get the physical picture in mind.

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    $\begingroup$ this doesn't answer the question $\endgroup$
    – Rory Alsop
    Oct 1, 2013 at 12:12
  • $\begingroup$ It does answer part of the question. Partial answers are acceptible. $\endgroup$ May 10, 2015 at 17:50

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