galaxy with dust line
what is the reason for galaxies to have dust-line?
Spiral (disk) galaxies are composed of stars (duh!), gas (actually mostly ionized, so properly called plasma), dark matter (a rather mysterious form of matter completely unlike anything else), and ... (wait for it) ... dust.
The disk of a spiral galaxy is pretty flat; when you look at one edge-on, it's almost like a line (with a blob in the middle, where the bulge is). It seems, however, that the stars in the disk are 'thicker' than the gas and dust in the disk; they are found further from the plane of the disk. Another way to say is that the dust (and gas) are more concentrated in the disk plane than the stars.
When you look into an edge-on spiral, you'll "see" more dust than stars, compared with a line of sight just above the plane; the dust will block more starlight (relatively speaking) in the mid-plane. And we call this ... a dust lane! 😃