by Chris Peterson » Thu Jul 07, 2022 1:46 pm
Ann wrote: ↑Thu Jul 07, 2022 6:19 am
VictorBorun wrote: ↑Thu Jul 07, 2022 5:53 am
I wonder if the dark patches of cold dust are all in the foreground somewhere nearer then 6000 ly or are they more like a pair of jets from NGC 6914?
The dark dust is at the same distance as the blue reflection nebulas and the stars illuminating the nebulas. Dust is a prerequisite for reflection nebulas, as dust is scattering light from stars and spreading their light into larger patches. The mechanism is similar to how light from car lights is being spread out in a fog.
To be more precise, there are two mechanisms we need to consider,
reflection and
scattering. These are different processes (with some overlap). When we see brown dust, we're mainly seeing reflection. The apparent color is largely dependent on the color of the dust (modified slightly by the color of the star, but that can largely be taken as white). When we see blue, we're seeing the product of scattered light. Because short wavelengths are preferentially scattered by particles the typical size of interstellar dust, we see blue enhanced. And that also means that, if the dust isn't too thick, we might see things from behind reddened (because of the loss of that blue light to scattering).
Often, of course, we're seeing a combination of all of these things in dusty areas, leading to a wide range of colors.
[quote=Ann post_id=323993 time=1657174771 user_id=129702]
[quote=VictorBorun post_id=323992 time=1657173213 user_id=145500]
I wonder if the dark patches of cold dust are all in the foreground somewhere nearer then 6000 ly or are they more like a pair of jets from NGC 6914?
[/quote]
The dark dust is at the same distance as the blue reflection nebulas and the stars illuminating the nebulas. Dust is a prerequisite for reflection nebulas, as dust is scattering light from stars and spreading their light into larger patches. The mechanism is similar to how light from car lights is being spread out in a fog.
[/quote]
To be more precise, there are two mechanisms we need to consider, [i]reflection [/i]and [i]scattering[/i]. These are different processes (with some overlap). When we see brown dust, we're mainly seeing reflection. The apparent color is largely dependent on the color of the dust (modified slightly by the color of the star, but that can largely be taken as white). When we see blue, we're seeing the product of scattered light. Because short wavelengths are preferentially scattered by particles the typical size of interstellar dust, we see blue enhanced. And that also means that, if the dust isn't too thick, we might see things from behind reddened (because of the loss of that blue light to scattering).
Often, of course, we're seeing a combination of all of these things in dusty areas, leading to a wide range of colors.