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# Determining distance of dark/absorption nebulae

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In recent topic in imaging, we had brief discussion about dark nebulae, or rather naming of these objects, and in fact ways to distinguish them from regular "empty" space (in context of imaging, nothing fancy).

That got me thinking - what would be ways to determine distance to object that does not emit light, but rather blocks it?

So far, I've come up with some fairly basic ideas:

1. Star count

If we assume that starlight is blocked by such objects in visible region of spectrum, one can take image of such object, calculate angular surface and count visible stars in front of it (if there are any). Then from data available on average density of stars in our surrounding, we can calculate expected number of stars for a given volume of space (angular size of object + distance to it). By comparing expected number of stars visible (in front of the nebula) to that of what we see - we can figure out the likely distance.

This of course depends on transparency of such object - some of the bright stars behind could shine thru with certain level of attenuation. More complex model involving expected stellar magnitude could be devised to account for transparency (based on density / size relations).

2. Spectral analysis

I'm not sure about this one, but we could maybe determine thickness of such object by examining extinction in visible spectrum vs that of infra red (or rather magnitudes). From IR part of spectrum we can deduce stellar class, and associated absolute magnitude and then figure out based on distance in IR - attenuation. Applying some sort of density formula we can approximate thickness of object. Rest is up to morphology of such cloud and gravity simulations to see likely shape. If we can find shape / dimensions - we can find distance

Anyone has any insight in this topic? Or maybe some other ideas worth discussing?

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Posted (edited)

Dark nebula don't block all light. so Stars behind the nebula are visible in near IR. So you can get distance to the stars in front of the nebula and distances to stars within and beyond the nebula, which surely gives us the distance to the nebula? I don't know if GAIA measures distances in Near IR however.

Edited by Ags

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48 minutes ago, Ags said:

Dark nebula don't block all light. so Stars behind the nebula are visible in near IR. So you can get distance to the stars in front of the nebula and distances to stars within and beyond the nebula, which surely gives us the distance to the nebula? I don't know if GAIA measures distances in Near IR however.

I think this is a real possibility. They may also share proper motions with local cluster stars giving an estimate though this might be difficult to pln down.

Regards Andrew

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4 hours ago, Ags said:

Dark nebula don't block all light. so Stars behind the nebula are visible in near IR. So you can get distance to the stars in front of the nebula and distances to stars within and beyond the nebula, which surely gives us the distance to the nebula? I don't know if GAIA measures distances in Near IR however.

Yes very interesting idea - close to what I thought about examining both visible and NIR spectrum - one can easily say if star is in front or behind.

We can use IR part of spectrum to determine distances of those behind, and also in the process estimate how much of the light is attenuated - that will tell us a bit of density / depth of dark nebula.

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Actually it might not work so well as the structures can be quite small:

These are only light months across - not many stars in front or behind?

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