Sam wrote:This discussion has raised a question in my mind: just how dusty is this nebula? If it can hide a supernova :shock:
it must be pretty dusty!
It probably didn't hide the supernova. What's hiding is a supernova remnant, or a bubble of shocked gas and dust produced by an earlier supernova. That is many, many orders of magnitude dimmer than the supernova itself was during its brief flare.
In general, how much dust is needed to completely block that much visible light?
That depends not just on the density of the dust, but on how much of it you're looking through. Dusty nebulas are billions of times less dense than the Earth's atmosphere. But we are typically looking through them for many light years.
Consider typical window glass. It seems pretty transparent. Indeed, for many people, glass is the very definition of transparency. But a meter thick piece of window glass is about as opaque as a rock. You can't see any light through it at all. One of the early challenges of fiber optics was to develop glasses a million times more transparent than ordinary glass. That's why light travels for kilometers practically unattenuated in a fiber optic cable.
Or consider our atmosphere. When you look straight up at the stars, you are only looking through a few kilometers of dense air, and it hardly attentuates those stars at all (stars from the ground don't look much dimmer than they would from space). But when you look towards the horizon, you are looking through hundreds of kilometers of air- enough that even the Sun is attenuated enough to be viewed directly. Same atmosphere, more of it.
A dusty nebula represents a hard vacuum. If you were on a planet circling a star in such a nebula, you could look around you and see nearby stars easily, and you wouldn't even know you were in a dust cloud based on your view of other planets in the system, or of the star itself. Over distances of a few AUs, such a nebula seems completely transparent.
