Question about galaxy brightness

wrote:
On Thu, 3 Nov 2005 12:08:21 +0000, GSV Three Minds in a Can
wrote:

Bitstring , from the
wonderful person said
Ok, I'm reading the Coldfire trilogy. In this series, the planet was
settled by a colony ship that set out from Earth, and kept traveling
until it was just about to run out of planets and head off in to deep
space. Here's some descriptions of the night sky as seen on the
planet:

"The cloudless sky was still half-filled with stars, a thousand
brilliant point of light that twinkled in the cobalt heavens like
diamonds on jeweler's velvet. Toward the west there were so many of
them that their light ran together, pooling like molten gold along the
horizon, crowning the sea with fire. Soon Erna's second sun would set
- a false sun, made up of a million stars - but until then the Ernan
colonists need have no fear of darkness. Only the creatures who
feared true sunlight would call this time night."

It's explicitly mentioned that these people set out from Earth, so
this is the Milky Way galaxy that we are talking about. Given that
the Milky Way appears as a relatively faint band of stars here on
Earth, which is closer to the center of the galaxy than Erna is, is
this description of what they call "the Core" as unrealistic as I
think it is?

You have to understand that any 'science' in the Coldfire trilogy is
there purely by accident. This is a Fantasy series.

However if I have to retcon this back so somewhere near believable,
maybe if you set off at right angles to the plane of the galaxy you
could achieve this effect (especially of a planet with little/no light
pollution) - the only reason the Milky Way doesn't look like nearly
solid star-stuff from here is that there is immense amounts of gas/dust
between us and the core.

So if you were above/below the plane of the Galaxy (where there are
relatively few stars to be sure) you might get a view somewhat like
'Andromeda galaxy, face on', but rather bigger/brighter. Of course I
can't imagine that anyone leaving Earth would head in that direction..

Well, I can picture it being there. What I am having the teensiest
bit of difficulty with is that it is apparently bright enough to
provide visible light nearly equal to the local sun. Iirc, there is
even mention of shadows cast by Corelight.

Not "even". The Corelight is said to cast shadows here on Earth!

The Moon is much dimmer than the Sun, but moonlight casts shadows
without problems. Which is no wonder. At -12,76 or so, the Moon is over
8 magnitudes brighter than the next compact object, Venus. Which means
about 2000 times brighter. There is a huge difference between ground
illuminated by Moon and ground where the light of Moon is cut off.

Now, how bright in absolute terms are the major diffuse light sources
here on Earth? The magnitudes of point lights like Venus, Mars, Sirius
et cetera are easy to find. But just how big is the total illumination
from the Milky Way or the zodiacal light compared to Venus, or Jupiter,
or Sirius, or all distinct stars in total?

The astronomers claim that in the dark, BOTH the zodiacal light AND the
core of Milky Way around Sagittarius will cast diffuse shadows. Can
anyone confirm?

Also, just how bright is the core as far as we are, but for the dark
clouds? We are within the plane of the clouds. Suppose that we were to
go at about the same distance from centre (8 kpc) to about 1 kpc or so
from the Galactic plane. How much brighter would the Milky Way look? We
would still see the stars near edge-on, concentrated in a band, but the
dust extinction should be much less.

Or maybe they headed out to somewhere near a globular cluster? Anyway,
don't worry about it. If minor stuff like that upsets you, the rest of
the book will freak you our. Get a larger hawser to suspend your
disbelief from ...

Oh, it doesn't upset me, I just wanted to be sure that I was entirely
crazy for saying "huh?" about this. I will say that it took me a bit
in the first book to realize that they were out on the edge of the
galaxy, since my assumption when they started talking about the Core
and how bright it was was that they were close to the center. But no,
I'm not reading the series for the science. :-)

Rebecca

wrote:
wrote:
On Thu, 3 Nov 2005 12:08:21 +0000, GSV Three Minds in a Can
wrote:

Bitstring , from the
wonderful person said
Ok, I'm reading the Coldfire trilogy. In this series, the planet was
settled by a colony ship that set out from Earth, and kept traveling
until it was just about to run out of planets and head off in to deep
space. Here's some descriptions of the night sky as seen on the
planet:

"The cloudless sky was still half-filled with stars, a thousand
brilliant point of light that twinkled in the cobalt heavens like
diamonds on jeweler's velvet. Toward the west there were so many of
them that their light ran together, pooling like molten gold along the
horizon, crowning the sea with fire. Soon Erna's second sun would set
- a false sun, made up of a million stars - but until then the Ernan
colonists need have no fear of darkness. Only the creatures who
feared true sunlight would call this time night."

It's explicitly mentioned that these people set out from Earth, so
this is the Milky Way galaxy that we are talking about. Given that
the Milky Way appears as a relatively faint band of stars here on
Earth, which is closer to the center of the galaxy than Erna is, is
this description of what they call "the Core" as unrealistic as I
think it is?

You have to understand that any 'science' in the Coldfire trilogy is
there purely by accident. This is a Fantasy series.

However if I have to retcon this back so somewhere near believable,
maybe if you set off at right angles to the plane of the galaxy you
could achieve this effect (especially of a planet with little/no light
pollution) - the only reason the Milky Way doesn't look like nearly
solid star-stuff from here is that there is immense amounts of gas/dust
between us and the core.

So if you were above/below the plane of the Galaxy (where there are
relatively few stars to be sure) you might get a view somewhat like
'Andromeda galaxy, face on', but rather bigger/brighter. Of course I
can't imagine that anyone leaving Earth would head in that direction..

Well, I can picture it being there. What I am having the teensiest
bit of difficulty with is that it is apparently bright enough to
provide visible light nearly equal to the local sun. Iirc, there is
even mention of shadows cast by Corelight.

Not "even". The Corelight is said to cast shadows here on Earth!

The Moon is much dimmer than the Sun, but moonlight casts shadows
without problems. Which is no wonder. At -12,76 or so, the Moon is over
8 magnitudes brighter than the next compact object, Venus. Which means
about 2000 times brighter. There is a huge difference between ground
illuminated by Moon and ground where the light of Moon is cut off.

Now, how bright in absolute terms are the major diffuse light sources
here on Earth? The magnitudes of point lights like Venus, Mars, Sirius
et cetera are easy to find. But just how big is the total illumination
from the Milky Way or the zodiacal light compared to Venus, or Jupiter,
or Sirius, or all distinct stars in total?

The astronomers claim that in the dark, BOTH the zodiacal light AND the
core of Milky Way around Sagittarius will cast diffuse shadows. Can
anyone confirm?

Also, just how bright is the core as far as we are, but for the dark
clouds? We are within the plane of the clouds. Suppose that we were to
go at about the same distance from centre (8 kpc) to about 1 kpc or so
from the Galactic plane. How much brighter would the Milky Way look? We
would still see the stars near edge-on, concentrated in a band, but the
dust extinction should be much less.

Let's see, the Andromeda Galaxy has a magnitude of 6.3 and it's
2,000,000 light years away, and our Galactic core is 30,000 light years
away. The square of (2,000,000/30.000)
is about 4,444, so assuming that most of the light from Andromeda comes
from the galactic core, the Andromeda Galaxy from 30,000 light years
away would be 4,000 as bright as a magnitude 6.3 object. The log of
4,444 is 3.6478. Divide that by 0.4 and we get an increase of 9.187
magnitudes. 6.3 minus 9.187 gives a magnitude of minus 2.88.
I don't know how much dimmer our galaxy is than Andromeda, but I'd
guess the magnitude should be somewhere between minus 2 and minus 2.88-
A. McIntire


Or maybe they headed out to somewhere near a globular cluster? Anyway,
don't worry about it. If minor stuff like that upsets you, the rest of
the book will freak you our. Get a larger hawser to suspend your
disbelief from ...
Oh, it doesn't upset me, I just wanted to be sure that I was entirely
crazy for saying "huh?" about this. I will say that it took me a bit
in the first book to realize that they were out on the edge of the
galaxy, since my assumption when they started talking about the Core
and how bright it was was that they were close to the center. But no,
I'm not reading the series for the science. :-)

Rebecca