Inside a nebulae.

Dear members of sci.astro.research,

From our point of view inside the milky way, the sky is mostly
dark(ish) (at least in the visible range) punctuated by a few
thousdands stars (unaided vision).

On the other hand, a galaxy like M31, seen from outside,
i.e. from our planet, glows and looks like a single object
even though we know is very similar to our galaxy, made by
many billions and billions of stars.

So, my question is: if we were inside the Orion Nebula,
would the previous analogy hold? Would we see a mostly dark
sky with a few "streaks" of matter where the gas/dust
concentration is higher? Or would we see the whole sky
glowing like a permanent and possibly blinding daylight?

Thanks for your help.

Manu

In article ,
(Emanuele D'Arrigo) writes:
So, my question is: if we were inside the Orion Nebula,
would the previous analogy hold? Would we see a mostly dark
sky with a few "streaks" of matter where the gas/dust
concentration is higher? Or would we see the whole sky
glowing like a permanent and possibly blinding daylight?

Two facts will help you here. First, surface brightness is
independent of distance, so if we were just outside the Orion Nebula,
its surface would look just as bright as it does from Earth, but of
course it would fill half the sky.

The second fact is a little less obvious: at visible wavelengths the
Nebula is optically thin. This means, more or less, that its surface
brightness is proportional to the amount of nebula in the line of
sight. (Actually, surface brightness at most wavelengths is
proportional to "emission measure," which is the integral of density
squared along the path. The "forbidden lines" are an exception.)
Thus if you were in the middle of the Nebula, and if the Nebula were
symmetric, the surface brightness of the whole sky would be half what
we see from the outside.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

In article ,
(Emanuele D'Arrigo) writes:
So, my question is: if we were inside the Orion Nebula,
would the previous analogy hold? Would we see a mostly dark
sky with a few "streaks" of matter where the gas/dust
concentration is higher? Or would we see the whole sky
glowing like a permanent and possibly blinding daylight?

Two facts will help you here. First, surface brightness is
independent of distance, so if we were just outside the Orion Nebula,
its surface would look just as bright as it does from Earth, but of
course it would fill half the sky.

The second fact is a little less obvious: at visible wavelengths the
Nebula is optically thin. This means, more or less, that its surface
brightness is proportional to the amount of nebula in the line of
sight. (Actually, surface brightness at most wavelengths is
proportional to "emission measure," which is the integral of density
squared along the path. The "forbidden lines" are an exception.)
Thus if you were in the middle of the Nebula, and if the Nebula were
symmetric, the surface brightness of the whole sky would be half what
we see from the outside.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)