Average Seperation

[Dust Raven]

I was reading through Star Hero and came across the Drake Equation. An excellent tool I must say, but when running the numbers it says the number of technological civilazations have an "average seperation" of X light years. How is this average seperation determined? It doesn't really go into that...

 

If it makes it easier for anyone (I love math, but anything to save time) I've established my universes stats as follows:

 

The galaxy is if similar size and composition as the Milky Way. I'm only taking into account stars in the Galactic Habitably Zone (approx 15,000 ly to 35,000 ly from the galactic center).

 

N* = 100 billion (stars in the GHZ)

fp = .5 (stars of proper age/type)

Np = 10 (average number of planets per star)

fh = .07 (planets suitable for life)

fl = .7 (lifebearing planets)

fc = .1 (lifebearing planets that develop technology)

Lc/Lg = .000001 (ratio of the average age of a technological civilization to the age of the galaxy)

 

This gives me approximately 2,450 technological civilizations (I haven't figured out how many of these have actually developed FTL technology yet though). Technically I'm fudging it a bit, and calling fc the proportion of lifebearing planets the develop intelligent life (which theoretically will develop technology, but hasn't necessarily done so). This allows for a number of pre-industrial worlds. But that's not really important right now...

 

All I need to know is how far apart these civilizations are on average. I need to know this so I can decide how fast to allow FTL travel.

 

Thanks in advance!

[Basil]

Re: Average Seperation

 

I was reading through Star Hero and came across the Drake Equation. An excellent tool I must say' date=' but when running the numbers it says the number of technological civilazations have an "average seperation" of X light years. How is this average seperation determined? It doesn't really go into that...[/quote']

 

What you have to do is figure the volume of the Galactic Habitable Zone, divide by the number of civilizations, treat each species' volume as a cube, and find the distance from center to center; this is the length of a side of that cube.

 

A cylinder 37,000 ly in radius, 1000 ly thick, with a 14,500 ly radius hole through it, is about 3.64 * 10^12 cubic ly. 125 civilizations "have" about 2.91 * 10^10 cubic ly each, for a distance of ~3080 ly.

180,000 civilizations "have" about 2.02 * 10^7 cubic ly each, for a distance of ~272 ly. (The figures in Terran Empire are significantly off.)

 

If it makes it easier for anyone (I love math, but anything to save time) I've established my universes stats as follows:

 

The galaxy is if similar size and composition as the Milky Way. I'm only taking into account stars in the Galactic Habitably Zone (approx 15,000 ly to 35,000 ly from the galactic center).

 

N* = 100 billion (stars in the GHZ)

fp = .5 (stars of proper age/type)

Np = 10 (average number of planets per star)

fh = .07 (planets suitable for life)

fl = .7 (lifebearing planets)

fc = .1 (lifebearing planets that develop technology)

Lc/Lg = .000001 (ratio of the average age of a technological civilization to the age of the galaxy)

 

This gives me approximately 2,450 technological civilizations (I haven't figured out how many of these have actually developed FTL technology yet though). Technically I'm fudging it a bit, and calling fc the proportion of lifebearing planets the develop intelligent life (which theoretically will develop technology, but hasn't necessarily done so). This allows for a number of pre-industrial worlds. But that's not really important right now...

 

All I need to know is how far apart these civilizations are on average. I need to know this so I can decide how fast to allow FTL travel.

 

Thanks in advance!

 

With 2450 civilizations, each "has" about 1.49 * 10^9 cubic ly, for a distance of ~1140 ly.

 

Hope that helps!

[DrTemp]

Re: Average Seperation

 

I was reading through Star Hero and came across the Drake Equation. An excellent tool I must say, but when running the numbers it says the number of technological civilazations have an "average seperation" of X light years. How is this average seperation determined? It doesn't really go into that...

 

That's because you, as the universe-builder, are to determine by sheer force of will what the "average separation" shall be. ;-)

[Nyrath]

Re: Average Seperation

 

The method I found on some website or other is to use the following simplistic equation:

dist = cube_root( galactic_volume / num_civs)

that is, take the volume of the galactic habitable zone, divide it by the number of technological civilizations, find the cube root, and the result is the average distance between civilizations, in the same units as the volume. If your calculator does not have a cube root button, use the X to the Y power button, and use 0.33333333333 for "Y" (i.e., divide galactic volume by num civs, press the x to the y button, enter 0.33333333, and press the "=" button).

 

So taking Basil's galactic volume as 3.64e12 cubic light years divided by 2450 civilizations, take the cube root, and the result is an average separation of 1140 light years. (3.62e12 is a more compact way of saying 3.62 x 10^12)

 

There is more at these websites:

http://www.projectrho.com/smap05d.html

http://www.projectrho.com/rocket/rocket3ab.html (scroll down to "Interstellar Empires")

 

I read somewhere that the entire galaxy has a volume of 5.65e12 cubic light years, counting both the galactic habitable zone and the low-rent districts of the core and the rim.

[Dust Raven]

Re: Average Seperation

 

Supercool! Thanks guys!

 

1140l lr seperation... that's a lot. FTL needs to be fast. At 40 points of FTL, a ship would reach the next civilization in about 12 days, and cross the entire GHZ in about 2 years (if flying through the core). 12 days could be a long time for reinforcements/supplies and such, but could let PCs have a bit more autonomy amung the stars regardless of their vocation. At 50 points of FTL, it's a mere 9 hours between civilizations and takes a short month to span the GHZ. On top of that, a trip to the nearest galaxies would only take a few years. This would allow PCs to get anywhere fast, and allow them to lend backup to others and have the chance to get reinforcements. But it doesn't leave much left to explore. Either say it could be fun.

 

Thanks again!

[Basil]

Re: Average Seperation

 

The method I found on some website or other is to use the following simplistic equation:

dist = cube_root( galactic_volume / num_civs)

that is, take the volume of the galactic habitable zone, divide it by the number of technological civilizations, find the cube root, and the result is the average distance between civilizations, in the same units as the volume. If your calculator does not have a cube root button, use the X to the Y power button, and use 0.33333333333 for "Y" (i.e., divide galactic volume by num civs, press the x to the y button, enter 0.33333333, and press the "=" button).

Which is what I said, I just also explained why.

 

So taking Basil's galactic volume as 3.64e12 cubic light years divided by 2450 civilizations' date=' take the cube root, and the result is an average separation of 1140 light years. [i'](3.62e12 is a more compact way of saying 3.62 x 10^12)[/i]

Again, just what I said.

 

BTW, I prefer "3.62 * 10^12" because it's clear to anyone who's done enough arithmetic to recognize "^" as the "to the power of" symbol. I'd use superscripts if I knew how to do them on these boards. OTOH, "3.62c12" is only known to those who've used some specific computer programs.

 

There is more at these websites:

http://www.projectrho.com/smap05d.html

http://www.projectrho.com/rocket/rocket3ab.html (scroll down to "Interstellar Empires")

 

I read somewhere that the entire galaxy has a volume of 5.65e12 cubic light years, counting both the galactic habitable zone and the low-rent districts of the core and the rim.

 

Using "The Cambridge Atlas of Astronomy", we get the following dimensions for the galaxy (all are approximate):

Central Bulge:

Diameter: 6 kiloparsecs

Thickness: 1 kiloparsec

Disk:

Diameter: 30 kiloparsecs

Thickness: 300 parsecs

Halo:

Diameter: 30 kiloparsecs

 

Converting to lightyears (and giving radii as appropriate):

Central Bulge:

Radius (actually semi-major axis): 10,000 ly

Thickness (actually minor axis): 3000 ly

Disk:

Radius: 50,000 ly

Thickness: 1000 ly

Halo:

Radius: 50,000 ly

 

(NB: the radius of the Halo may actually be 60,000 ly or more, but I'd rather use the more conservative value)

 

The Central Bulge is an oblate sphereoid, so the formula is (pi)*(4/3)*(semi-major axis squared)*(semi-minor axis), which is about 6.28 * 10^11 ly. The whole disk is a cylindar with a hole through it: (pi)*(50,000^2 - 10,000^2)*(1000) = about 7.54 * 10^12.

 

So the overall total is about 8.17 * 10^12 for the disk and bulge. (This is about 45% higher than the figure you gave).

 

The halo + disk + bulge (or grand total) volume is (pi)*(4/3)*(50,000^3), or about 5.24 * 10^14. Which is about 64 times as big as the disk+bulge. However, the density of stars is around 1/100,000 or less.

[Dr. Anomaly]

Re: Average Seperation

 

So the overall total is about 8.17 * 10^12 for the disk and bulge. (This is about 45% higher than the figure you gave).

Well, the Star Hero approach deals with a "habitable zone" for the galaxy, similar to the "green zone" around a star -- the area in which a habitable planet can exist. For example, in the core of the galaxy, the radiation level is too high, and the frequent close passes of stars are unlikely to leave stable planetary systems intact.

 

If you rule out those parts of the galaxy unlikely to host planets where "life as we know it" could exist, then you get quite a bit less than the actual volume of the physical galaxy.

[Basil]

Re: Average Seperation

 

So the overall total is about 8.17 * 10^12 for the disk and bulge. (This is about 45% higher than the figure you gave).

Well, the Star Hero approach deals with a "habitable zone" for the galaxy, similar to the "green zone" around a star -- the area in which a habitable planet can exist. For example, in the core of the galaxy, the radiation level is too high, and the frequent close passes of stars are unlikely to leave stable planetary systems intact.

 

If you rule out those parts of the galaxy unlikely to host planets where "life as we know it" could exist, then you get quite a bit less than the actual volume of the physical galaxy.

 

If you look at my first post, you will see I determined the volume of the Galactic Habitable Zone from the figures given in Star Hero. Thus, its size is a different matter from the size for the galaxy as a whole.

 

 

BTW, Dust Raven: I just realized I didn't use the numbers (15,000 ly and 35,000 ly) you gave in your original post. Using those, the volume of the GHZ is about 3.14 * 10^12 cubic ly. Also, there's only about 86 billion stars in that volume, so there's only about 2100 civilizations. These two changes lead to an average separation of --- 1140 ly. The two factors balance out, after all.

[Dust Raven]

Re: Average Seperation

 

BTW' date=' Dust Raven: I just realized I didn't use the numbers (15,000 ly and 35,000 ly) you gave in your original post. Using those, the volume of the GHZ is about 3.14 * 10^12 cubic ly. Also, there's only about 86 billion stars in that volume, so there's only about 2100 civilizations. These two changes lead to an average separation of --- 1140 ly. The two factors balance out, after all.[/quote']

I figured they would. I'm not actually using the Milky Way for my campaign, just a galaxy similar to it in size and age. I picked the numbers I did to make them easier to remember and figured I might as well keep the number of stars at 100 billion.

[Dust Raven]

Re: Average Seperation

 

I'm reaching a frustrating time populating my galaxy.

 

I've readjusted the number a little, and now have 17,500 races with an average seperation of about 600 light years.

 

The thing is, I want them grouped close together (say, within 100 lys), but I don't want several million races flooding the galaxy. I'm thinking of just taking the numbers I have and bunching them all up in groups, which are then spread across the galaxy. The races within each group may be 100 or less lys apart from each other, but each group might be thousands of lys from the next. This would enable me to have slower FTL while still enabling quick transport from world to world, and get to leave much of the galaxy unexplored to allow for those "how did we get here?" adventures when the hyperdrive malfunctions.

 

So a new question... is there any basis (one way or the other) for having populated worlds be grouped together like this? If I have to alter the type of galaxy I suppose I could, but I'd like to stick with something familiar to the players. If push comes to shove, I'm running a Space Opera campaign and can just say "this is how it is" and leave it at that. I would prefer some spiffy realism though. My goal is to run an epic space opera with fantastic events, but with each nifty thing actually having a speculative basis in hard science.

[DrTemp]

Re: Average Seperation

 

So a new question... is there any basis (one way or the other) for having populated worlds be grouped together like this?

 

I believe there is.

 

First, the galactic habitable zone is an approximation that does not take into account the destructive effects of nearby supernovae, which can happen just anywhere in the galaxy. If a supernoa strikes, a few centuries later the surrounding life-bearing planets will have lost much of their higher life forms. Thus, it is quite probable that only in regions that haven't had supernovae for a long time there cann be an civilizations.

 

Second, of course, the average separation is just that, an average. It is quite possible that simply by chance, several intelligent species are closer to each other in the galaxy. There could still be those farther away- but nobody would know them.

 

Third, there is always the option of the "progenitor race", which sed life on a few clusters in the galxy. The progenitors were the first race (or a federation of races), and only on their planets did intelligent life evolve naturally. A variant of this might be an extinct genocidal race that was successful in some regions of space, but failed to exterminate intelligent life in others.

[Nyrath]

Re: Average Seperation

 

Which is what I said' date=' I just also explained why. [/quote']

I know, I know. It is just that you didn't explicitly explain how to find the distance from center to center. I was trying to explain the method in insulting detail for the benefit of readers who were less than mathematically inclined.

[Nyrath]

Re: Average Seperation

 

Which is what I said, I just also explained why.

BTW, I prefer "3.62 * 10^12" because it's clear to anyone who's done enough arithmetic to recognize "^" as the "to the power of" symbol. I'd use superscripts if I knew how to do them on these boards. OTOH, "3.62c12" is only known to those who've used some specific computer programs.

 

So the overall total is about 8.17 * 10^12 for the disk and bulge. (This is about 45% higher than the figure you gave).

Well, the 3.62e12 is clear to anyone who's done enough computer programming with floating point arithmatic. The notation is almost universal among all computer programming languages that support floating point.

 

Your figure for the volume of the galactic disk and bulge seem convincing enough for me. I'll update my website with the new figure. I got the original 5.65e12 cubic light years figure from http://www.astronomynotes.com/lifezone/s5.htm

[Nyrath]

Re: Average Seperation

 

The thing is, I want them grouped close together (say, within 100 lys), but I don't want several million races flooding the galaxy. I'm thinking of just taking the numbers I have and bunching them all up in groups, which are then spread across the galaxy. The races within each group may be 100 or less lys apart from each other, but each group might be thousands of lys from the next. This would enable me to have slower FTL while still enabling quick transport from world to world, and get to leave much of the galaxy unexplored to allow for those "how did we get here?" adventures when the hyperdrive malfunctions.

So a new question... is there any basis (one way or the other) for having populated worlds be grouped together like this?

There is always the Beserker Hypothesis, one of the standard answers to the Fermi Paradox. It says the reason that the entire galaxy (including Earth) was not colonized billions of years ago by the first intelligent specie to evolve is because Something Awful is prowling the galaxy, annihilating intelligent species as it goes. Actually probably more like a swarm of Something Awfuls.

 

Imagine that the planet killers move slower than light in million year long patrol routes. If they are sparce enough, this could lead to wide swaths of stars without civilizations. This means the surviving civilizations would be more closely clustered than one would expect (i.e., clustered into corridores in between the barren swaths).

 

Or as an alternate, you might argue that the civilizations were clustered due to Dr. Geoffrey Landis' Percolation Hypothesis

[Dust Raven]

Re: Average Seperation

 

There is always the Beserker Hypothesis' date=' one of the standard answers to the [b']Fermi Paradox[/b]. It says the reason that the entire galaxy (including Earth) was not colonized billions of years ago by the first intelligent specie to evolve is because Something Awful is prowling the galaxy, annihilating intelligent species as it goes. Actually probably more like a swarm of Something Awfuls.

 

Imagine that the planet killers move slower than light in million year long patrol routes. If they are sparce enough, this could lead to wide swaths of stars without civilizations. This means the surviving civilizations would be more closely clustered than one would expect (i.e., clustered into corridores in between the barren swaths).

 

Or as an alternate, you might argue that the civilizations were clustered due to Dr. Geoffrey Landis' Percolation Hypothesis

Interesting reading! Thanks!

 

I'm not sure I like the Berserker Hypothisis. It would seem to be just another X factor in the Drake Equasion (fraction of civilizations delf destructed or destroyed by outside threats). The crazy thing is, the theory is supported by the fact that Earth is visited at regularly intervals by something that causes mass extinction every 50-60 million years. It would help group the aliens together nicely, leave large empty bits of space with mysterious ruins and an unknown alien menace lurking somewhere. Maybe I do like it...

[Basil]

Re: Average Seperation

 

There is always the Beserker Hypothesis' date=' one of the standard answers to the [b']Fermi Paradox[/b]. It says the reason that the entire galaxy (including Earth) was not colonized billions of years ago by the first intelligent specie to evolve is because Something Awful is prowling the galaxy, annihilating intelligent species as it goes. Actually probably more like a swarm of Something Awfuls.

 

Imagine that the planet killers move slower than light in million year long patrol routes. If they are sparce enough, this could lead to wide swaths of stars without civilizations. This means the surviving civilizations would be more closely clustered than one would expect (i.e., clustered into corridores in between the barren swaths).

 

Or as an alternate, you might argue that the civilizations were clustered due to Dr. Geoffrey Landis' Percolation Hypothesis

 

Great reading there! Thanks a lot!

The Percolation Hypothesis is a fascinating idea. Just one problem from the Science Fiction POV: it requires that there be no FTL (or, at least, that FTL be not a whole lot faster than c, and that the FTL drive can't go faster than some specific number).

 

There is, BTW, another 'solution' to the Fermi Paradox. If it took the galaxy about 5 billion years to build up a fraction of trans-helium elements sufficient for life-bearing planets to form, and if it takes at least, say, 4 billion years for technological sapients to evolve, then Homo sapiens sapiens is among the first, and we haven't been visited because our 'predecessors' haven't gotten this far.

One thing about this speculation: in the Drake Equation it is no longer Lc/Lg, but Lc/Le, where Le is the time during which technological sapients could have evolved. If that is 1 billion years, the Drake Equation's result is mutiplied by 10!.

A counter-intuitive result to put it mildly.

[Dr. Anomaly]

Re: Average Seperation

 

The thing is' date=' I want them grouped close together (say, within 100 lys), but I don't want several million races flooding the galaxy.[/quote']

Well, there is another possible approach, but it's a lot less "dramatic" or "romantic" than things like the Berserker speculation.

 

There's been a theory for a long time that the reason life appeared so quickly/early on Earth is that the first living things (microbes) arrived in one or more of the comets that brought the water that makes up Earth's oceans. We also know that rocks from one planet, cast off by a large impact event, may make their way to another planet in the same solar system...Mars rocks have been found on Earth, for example.

 

Put these two together, and consider this:

 

Perhaps life arose in a couple of dozen places scatter around the galaxy. Impacts on those first life-bearing worlds scattered rocks (containing microbes) from those planets out into space. Some of those got sling-shotted out of their native solar system by a too-close encounter with another planet. Drifting through space for a few billion years, they eventually fall into the gravity well of other planets with the potential for life, and "seed" them.

 

In this way, you could have a couple of dozen "bubbles" of life-bearing worlds in the galaxy, with the older/more advanced worlds near the center and the newer, less advanced worlds that had been "colonized" by the ultimate Slow Boat To China spread out from there. In fact, once life got a foothold on a new world, impact events on that world might end up spreading further "lifeboats" even farther afield. Each "bubble" would be a bit like an oasis in a desert, with large streches of empty/non-life-bearing systems in between.

 

That would let you build clusters of worlds reasonably close together without resorting to just "statistical clustering", and you wouldn't have to drastically increase the number of civilizations, either.

 

It might well also have the effect of making "good" real estate (those worlds already 'seeded' by microorganisms which went on to convert the primordial atmosphere, etc. into something desirable as living space for higher life forms) a rather scarce commodity, worth fighting over. And if all the available "seeded" worlds in a particular bubble have been taken/used, a civilization looking for more breathing room might well take the extreme measure of crossing a large expanse of empty "star desert" and start trying to colonize/conquer worlds in the next "oasis".

 

Thoughts?

[Basil]

Re: Average Seperation

 

Well, there is another possible approach, but it's a lot less "dramatic" or "romantic" than things like the Berserker speculation.

 

There's been a theory for a long time that the reason life appeared so quickly/early on Earth is that the first living things (microbes) arrived in one or more of the comets that brought the water that makes up Earth's oceans. We also know that rocks from one planet, cast off by a large impact event, may make their way to another planet in the same solar system...Mars rocks have been found on Earth, for example.

 

Put these two together, and consider this:

 

Perhaps life arose in a couple of dozen places scatter around the galaxy. Impacts on those first life-bearing worlds scattered rocks (containing microbes) from those planets out into space. Some of those got sling-shotted out of their native solar system by a too-close encounter with another planet. Drifting through space for a few billion years, they eventually fall into the gravity well of other planets with the potential for life, and "seed" them.

 

In this way, you could have a couple of dozen "bubbles" of life-bearing worlds in the galaxy, with the older/more advanced worlds near the center and the newer, less advanced worlds that had been "colonized" by the ultimate Slow Boat To China spread out from there. In fact, once life got a foothold on a new world, impact events on that world might end up spreading further "lifeboats" even farther afield. Each "bubble" would be a bit like an oasis in a desert, with large streches of empty/non-life-bearing systems in between.

 

That would let you build clusters of worlds reasonably close together without resorting to just "statistical clustering", and you wouldn't have to drastically increase the number of civilizations, either.

 

It might well also have the effect of making "good" real estate (those worlds already 'seeded' by microorganisms which went on to convert the primordial atmosphere, etc. into something desirable as living space for higher life forms) a rather scarce commodity, worth fighting over. And if all the available "seeded" worlds in a particular bubble have been taken/used, a civilization looking for more breathing room might well take the extreme measure of crossing a large expanse of empty "star desert" and start trying to colonize/conquer worlds in the next "oasis".

 

Thoughts?

 

An interesting varient on Hoyle's theory.

One thing: the life within each "bubble" would be based on similar chemistry, but that in another bubble might be based on very, very different chemistry.

 

However, it should be reasonably easy to deliberately "seed" new planets (at least, those that *could* support life). While some sapients might "cross the desert," I would think most would put their efforts into "vitaforming" (altering to support/have life) planets that got missed.

[Nyrath]

Re: Average Seperation

 

The Percolation Hypothesis is a fascinating idea. Just one problem from the Science Fiction POV: it requires that there be no FTL (or, at least, that FTL be not a whole lot faster than c, and that the FTL drive can't go faster than some specific number).

Well, it seems to me that there is another problem: it assumes that a civilization's sphere of influence is a barrier to FTL travel.

 

The Percolation Hypothesis has that the expansion minded species are "bottled up" by being surrounded by the overlapping spheres of influence of non-expansion minded species. If the FTL drive can allow the colony ships to "jump over" the blocking spheres (by a jump drive with a loooong range or by travelling for loooong distances in hyperspace before breakout) then the blocking spheres don't block anymore.

 

Either the FTL is a non-jumping type, or the jump is too short to jump over the blocking sphere, or the civilizations inside the spheres can somehow interdict the jumping ships (by patrols in hyperspace or titanic anti-jump force fields)

[Nyrath]

Re: Average Seperation

 

There is' date=' BTW, another 'solution' to the Fermi Paradox. If it took the galaxy about 5 billion years to build up a fraction of trans-helium elements sufficient for life-bearing planets to form, and if it takes at least, say, 4 billion years for technological sapients to evolve, then [i']Homo sapiens sapiens[/i] is among the first, and we haven't been visited because our 'predecessors' haven't gotten this far.

Well, yes, if you look at the next paragraph of the link I posted you will find that covered. The paragraphs above cover the other possibilities: they exist but they are hiding, and they exist and are inhabiting all those UFOs and flying saucers buzzing around.

[Dust Raven]

Re: Average Seperation

 

Okay, I think I've got it (kinda)...

 

I read in one of those articles (or one they linked to... I don't know... I just kept going until I realized it was the next day) that Earth's solar system is on the edge of one of the spiral arms, moving at the same speed as shock waves (or something like that) radiating out from the arm. This keeps it safe, stable and relatively unaffected by many outside influences, allowing for life to not only develop, but survive, thrive and have enough time to evolve into sapience. Typically, the arms are filled with all sorts of gasses, radiation, supernovae and all sorts of things that would likely hamper the development of life, or almost certainly prevent it from evolving into anything that would wonder where it came from. So, technically, this would restrict lifebearing worlds to the stars between the arms (which happen to be small and dim, like our sun, go figure).

 

This would make things kinda easy for me. All those races I can group into the space between the arms, and poof, I've got my groups with lots of space (the arms) seperating large sections of populated areas. Unfortunately, this will mean I'll have to redo the Drake Equasion with some new numbers. Technically I can just leave it be, and say that's how they're seperated, but I figure if I can just increase the number of potential worlds, but reduce the number of stars, I can at least figure out how many alien races there are in one particular area. Which leads to the question... how many stars are there between the arms? Or, more specifically, how many stars are there (on average) between any given two arms?

 

On a related note, is there any specific term for the "space between the arms" of a galaxy?

[Dr. Anomaly]

Re: Average Seperation

 

On a related note' date=' is there any specific term for the "space between the arms" of a galaxy?[/quote']

They're usually just called "lanes".

 

Let me correct a possible misconception, though. The dark lanes between the arms are not empty of stars, nor are they choked with dust (making them darker). They have a normal population of stars. The reason they look darker is that the arms are brighter than "normal", due to high rates of star formation. (The current best theory is that compression waves traveling through the interstellar medium can compress gas & dust clouds sufficiently to trigger star formation. The "arms" aren't actually spinning around the galaxy [even though of course the stars, etc. that make up the arms are orbiting the galaxy's center] but what moves, and gives the "arms" the illusion of moving, are the shock/pressure waves. It rather like watching the whitecaps on ocean waves roll up the beach...the individual water molecules don't move in toward the beach, just the wave energy comes in...but watching the white foam on the crests of the waves gives the impression that the water that makes up the waves is moving toward the beach. The wave is a transient phenomena that passes through the water on its way to the beach. In the same way, the "arms" of a galaxy are a transient phenomena that circle the galaxy as the pressure waves circle the galaxy, triggering star formation.)

 

Apologies if this is "old news", but you'd be surprised at the number of people who think the "gaps" between the "arms" are gaps in the sense of having few or no stars.

[Dust Raven]

Re: Average Seperation

 

They're usually just called "lanes".

 

Let me correct a possible misconception, though. The dark lanes between the arms are not empty of stars, nor are they choked with dust (making them darker). They have a normal population of stars. The reason they look darker is that the arms are brighter than "normal", due to high rates of star formation. (The current best theory is that compression waves traveling through the interstellar medium can compress gas & dust clouds sufficiently to trigger star formation. The "arms" aren't actually spinning around the galaxy [even though of course the stars, etc. that make up the arms are orbiting the galaxy's center] but what moves, and gives the "arms" the illusion of moving, are the shock/pressure waves. It rather like watching the whitecaps on ocean waves roll up the beach...the individual water molecules don't move in toward the beach, just the wave energy comes in...but watching the white foam on the crests of the waves gives the impression that the water that makes up the waves is moving toward the beach. The wave is a transient phenomena that passes through the water on its way to the beach. In the same way, the "arms" of a galaxy are a transient phenomena that circle the galaxy as the pressure waves circle the galaxy, triggering star formation.)

 

Apologies if this is "old news", but you'd be surprised at the number of people who think the "gaps" between the "arms" are gaps in the sense of having few or no stars.

I wouldn't be too surprised actually. Up until a few years ago when I started studying astronomy more in depth I was one of them. Recently I've been reading about what the arms actually are, but you've put it into the terms I couldn't remember. Thanks.

 

So the gaps are called "lanes" huh? Interesting. So all I need to do if create a number of additional populated worlds that, like Earth, escape the compression waves. I'm figuring this won't be too accurate, scientifically, but it'll work for a game and seem at least remotely plausable. At least I think it will. I hope so anyway. One of my players knows a LOT more about this stuff than I do, and another's actually taking classes.

[Basil]

Re: Average Seperation

 

Well' date=' yes, if you look at the [b']next paragraph[/b] of the link I posted you will find that covered. The paragraphs above cover the other possibilities: they exist but they are hiding, and they exist and are inhabiting all those UFOs and flying saucers buzzing around.

 

The first line from that paragraph:

"Others argue that the conditions for life, or at least complex life, are rare."

I'm suggesting not that they're rare per se, but that they have only recently (from an astronomic POV) come into being. Thus, there's a growing number of sentients out there, but not yet so many we ought to have been visited.

 

BTW, after re-reading that page, I notice there's a highly important assumption to the Fermi Paradox: sentient species continue indefinately, or at least for a very, very long time. Yet the Drake Equation leaves the question of "how long" up to the user to decide.

 

If FTL is impossible, and a sentient species dies off (for whatever reason) in a few thousand years, there is no paradox; we haven't been visited because nobody's run into us before they died off, and odds are we won't ever be.

 

Yeah, the Fermi Paradox is a very interesting thing. Makes me think about all sorts of things!

[Basil]

Re: Average Seperation

 

I wouldn't be too surprised actually. Up until a few years ago when I started studying astronomy more in depth I was one of them. Recently I've been reading about what the arms actually are, but you've put it into the terms I couldn't remember. Thanks.

 

So the gaps are called "lanes" huh? Interesting. So all I need to do if create a number of additional populated worlds that, like Earth, escape the compression waves. I'm figuring this won't be too accurate, scientifically, but it'll work for a game and seem at least remotely plausable. At least I think it will. I hope so anyway. One of my players knows a LOT more about this stuff than I do, and another's actually taking classes.

 

Why do you need to "escape the compression waves"? Yes, they are the site of star formation, and thus of the death of short-lived stars (like O's and B's), and thus supernovas, and thus have slightly higher background radiation levels. However, the difference is too slight to be an overall concern.

 

Now, if a star system is within a few lightyears of a supernova, any life on it is pretty well kaput. However, even though supernovas are commoner in the arms, that doesn't mean they're common. Just a little bit less rare.

 

BTW, could you tell me where you read that Sol was moving at the same speed as the spiral arm? I think the shock-wave is faster, and in fact Sol has been in and out of spiral arms a number of times in its 4.5 billion years of existance.

 

. The dark lanes between the arms are not empty of stars' date=' nor are they choked with dust (making them darker).[/quote']

True, they have the same overall level of dust and gas as the bright arms. However, the dust and gas they have is dark---well, darker. That's because it takes an energetic star to cause the gas to fluoresce, or the dust to reflect enought light to be noteworthy. Indeed, the shortlived O's and the fluorescing gas, account for much of the brightness of the spiral arms.