Density of Stars Compared to Real Galaxy

[Narl]

I am sure this has been asked before, but I am having no luck searching for it. I am about to start a campaign with a new group and I know they will ask this sort of question.

How does the stellar density in the Imperium compares to the real galaxy? What I mean is, when I look at a map of the Spinward Marches, I know there is a star (or more than one in some cases) for every world on the map. How many stars are not shown on the map?

Are there twice as many? Five times as many? Maybe even a hundred times as many? Just trying to get a general idea on this. I would expect there would be loads of red dwarfs with no useful planets, or stars with no planets at all.

Any insight is appreciated.

 

[Lapthorn]

Comparing the Traveller map to real-life stellar cartography is kind of pointless because the Traveller map is in 2D, and space is in 3D.

But for what it's worth, in real life, stellar density in Sol's neighbourhood is approx one system per 7 or 8 cubic lightyears, or about 2.5 cubic parsecs.

Standard Traveller map density is a system per 2 hexes, i.e. every 2 parsecs.

 

[whulorigan]

I am sure this has been asked before, but I am having no luck searching for it. I am about to start a campaign with a new group and I know they will ask this sort of question.

How does the stellar density in the Imperium compares to the real galaxy? What I mean is, when I look at a map of the Spinward Marches, I know there is a star (or more than one in some cases) for every world on the map. How many stars are not shown on the map?

Are there twice as many? Five times as many? Maybe even a hundred times as many? Just trying to get a general idea on this. I would expect there would be loads of red dwarfs with no useful planets, or stars with no planets at all.

Any insight is appreciated.

The first problem to appreciate is that (obviously) the real galaxy has 3 dimensions, not just a flat 2-dimensional distribution as we see on maps in Traveller. The actual galactic arms of the Milky Way are somewhere between 300-600pc thick (estimates I have seen vary). Since in traveller 1 hex = 1 pc (and since Sol is very near the mean galactic plane), that means there are 150-300 "hexes" both above and below Sol in real space.

So if (for sake of argument) I consider the Traveller Map to be 1pc (1 hex) in thickness, a 1pc-thickness "slice" of the galactic plane actually (by coincidence) has a stellar density that is approximately correct (give or take, depending on local density factors). But the real galaxy has 150-300 of those "slices" both above and below that plane.

So looking down from above the "top" of the galactic disk and superimposing all of them onto a flat map, there would be so many stars that a white-paper hex-map would literally be black. (And note that this is not accounting for solitary Brown Dwarfs). Also keep in mind that there is not a "1 pc minimum distance" between independent stars in real space, so it is entirely possible that a given hex may have more than 1 star system located in it (i.e. they are NOT individual components of a larger multiple star-system but actual independent systems within the same 1pc-daimeter spherical volume).

So the real question to ask is: Presuming that the Traveller map is NOT considered to be only 1pc thick (but rather that it has some unspecified generic thickness range), how many stars would be unaccounted for? If you consider each sector to be representing an average thickness of 10pc (for example), then the maps in Traveller are only showing you about 1/8th - 1/10th of the stars that are actually there (the rest are presumably not commercially useful or desirable for colonization). And most of them would be Red Dwarfs & Brown Dwarfs.

 

[Narl]

Thanks for the helpful responses!

It sounds like I can characterize it how I prefer then. I like the idea of there being other star systems, but without a use (habitable, resources, gas giant), they do not show on the map. Having those unmarked systems also lends a bit of mystery. . . .

 

[whulorigan]

Thanks for the helpful responses!

It sounds like I can characterize it how I prefer then. I like the idea of there being other star systems, but without a use (habitable, resources, gas giant), they do not show on the map. Having those unmarked systems also lends a bit of mystery. . . .

Just be careful that you do not place "unmarked" star-systems with refueling resources in hexes that would make astrographic rifts crossable. For example, in the Spinward Marches the Abyss Rift forces interstellar traffic to "go the long way around" the center of the Lanth Subsector (partly also because of the Interdicted Worlds). If you place one or more "unmarked" Red Dwarfs or Brown Dwarfs in empty hexes in the Rift, make sure that they do not have Gas Giants or iceball worldlets that are easy to locate. Otherwise, such systems would have been marked, as they would be strategically located to provide refueling stop-over points for cross-rift traffic.

 

[Adam Dray]

Using a list of nearby stars, you can get an idea of star density around Sol. The near star page lists separate stars, even if they're binary or trinary systems (or more--Lalande 27173 has four stars). I'm combining those into single systems.

Jump-1: NONE
Jump-2: 2 = 2+0 (Alpha Centauri A/B/Proxima and Barnard's Star)
Jump-3: 7 = 5+2+0
Jump-4: 26 = 19+5+2+0
Jump-5: 48 = 22+19+5+2+0
Jump-6: 78 = 30+22+19+5+2+0

Look at a standard 2D hex map. For any given hex, there are only so many hexes within a given jump distance.

Jump-1: 6
Jump-2: 18
Jump-3: 36
Jump-4: 60
Jump-5: 90
Jump-6: 126

Number of hexes is the geometric area, which is our 2D stand-in for 3D volume. So if there are 78 systems within 6 parsecs of Sol in 3D, we can calculate that 78 of 126 hexes should be filled, or 62%.

Now, I did read somewhere that our local neighborhood is a little denser than usual, and there are definitely clusters of stars that are more densely packed, so feel free to play with the density number (62%).


Interesting note: According to that star list, only three of the 78 systems within jump-6 of Sol are known to have planets. It reports that Epsilon Eridani has one planet and Ross 780 has three. Of course, that list might not reflect new science discoveries, such as the European Southern Observatory's report that Alpha Centauri B has a planet with Earth-like mass.

Just outside the range of that star list at 22 LY, Gliese 667C has THREE potentially habitable planets.

So here's hoping that planets are way more common than we currently detect.

 

[Xerxeskingofking]

Bear in mind the majority of planets we can detect with current methods are quite large planets fairly close* to thier parent stars. It's only in the last few years that planets that could be habitable can be detected, and i'm pretty sure we still could not spot the Earth at interstellar ranges, and we definatly can't see something like the Solar System's Gas Giants.


at the moment, it's like trying to work out how many trains run on a line by watching a the exits of a train station for the rush of people getting off. you wouldn't know about a express train that bypasses the station, or a train were all the passengers getting off want to transfer to another train, or a train that nobody got off......

So, while we can only see 4 planets in 6pc of earth. that number will rise as we get better tools, have a better idea what to look for, etc.


But to be honest, the only way to really know is to go would be to go their....


*in this context "quite large" means "several times bigger than Jupiter" and "Fairly close" means "closer than Mercury, right up to nearly damm touching the star".

 

[aramis]

So, while we can only see 4 planets in 6pc of earth. that number will rise as we get better tools, have a better idea what to look for, etc.


But to be honest, the only way to really know is to go would be to go their....


*in this context "quite large" means "several times bigger than Jupiter" and "Fairly close" means "closer than Mercury, right up to nearly damm touching the star".

Actually, a surprisingly large number of worlds are being found that are subjovian; they're being found by occlusion rather than perturbation. Several have lower limits of 1 to 2 Earth Masses.

There is still hope for the TPF space telescope interferometer to get launched. But, in the meantime, the Gemini Planet Finder tool on the Gemini South telescope (in Chile) promise a new wave of information.

The Terrestrial Planet Finder Interferometer — which last I heard was close to being canceled — would make direct imaging pretty standard. Interferometry has two neat benefits: massive increases in sensitivity, and concomitant decreases in glare.

But, hey, there are several worlds detected within 8 LY. Almost every system has planets, even binaries.

 

[Adam Dray]

Just as an update, I found this gem in my astronomy research:

Real estate within 100 parsecs: M dwarf stars for exoplanet surveys and Galactic archaeology

M dwarf stars are the staple of the Galaxy. By far the dominant type of hydrogen-burning body, they also have a knack for "hiding in plain sight" due to their relatively low luminosities. As a result, the census of M dwarfs has traditionally been very incomplete, even within the neighborhood of the Sun. I will present results from my SUPERBLINK survey, which is now identifying over 90% of all M dwarfs within 100 parsecs of the Sun -- over 300,000 new and/or barely explored objects. This new census opens up huge real estate opportunities in the search for exoplanets. I will explain why M dwarfs make such attractive targets for current exoplanet surveys, particularly in the search for Earth-mass objects within a "habitable" zone. In addition, M dwarfs live such long lives, that they hardly change at all over cosmological timescales (>10 billion years). I will describe my recent identification of thousands of very old M stars (also known as "M subdwarfs") in the Sloan Digital Sky Survey, and argue that these true "stellar fossils" hold a major key to understanding the formation and evolution of our Galaxy.

--Real estate within 100 parsecs

This researcher is reporting over 300,000 M dwarf stars within 100 pc. If I did my math right, there are 6(N(N+1)/2)+1 hexes [3N(N+1)+1 hexes] within distance N, so 30,301 hexes within 100 parsecs. That's an average of 10 M-subdwarf stars per hex, folks.

Just for scale, an area of 5x5 sectors is 160x200 hexes, or 32,000 hexes, which is pretty close to that 30,301 number. 300,000 stars in 25 sectors.

 

[Adam Dray]

Furthermore, adding in known M-dwarf stars within 10 pc of Earth yields more than 400 objects. There are 331 hexes within 10 pc of Earth, so just over one star per hex. If you account for the idea that about half the systems out there are binary/multiple, then you can cut those densities by half.

Basically, the neighborhood around Sol is pretty sparse compared to the rest of nearby space.

 

[aramis]

a single hex is .577 square parsecs.

Just taking a slice with a radius 100... 30301 hexes in the 100 hex "radius" hexagon. 3D would be FAR more.

 

[Adam Dray]

Sure. We're squashing three dimensions down to two, but trying to preserve somewhat accurate star density, right?

 

[Carlobrand]

Just as an update, I found this gem in my astronomy research:



This researcher is reporting over 300,000 M dwarf stars within 100 pc. If I did my math right, there are 6(N(N+1)/2)+1 hexes [3N(N+1)+1 hexes] within distance N, so 30,301 hexes within 100 parsecs. That's an average of 10 M-subdwarf stars per hex, folks.

Just for scale, an area of 5x5 sectors is 160x200 hexes, or 32,000 hexes, which is pretty close to that 30,301 number. 300,000 stars in 25 sectors.

He is searching a spherical volume with a hundred-parsec radius, yes? Ergo the volume is 4/3 Pi R³, or in excess of 4 million cubic parsecs: 1 star in 14 cubic parsecs.

We're trying to interpret that in two dimensions. One star in a spherical volume of 14 cubic parsecs is one star at the center of a sphere of a bit under 1.5 parsec radius. That is not 10 stars per hex; the stars are averaging about 3 parsecs apart - jump-3. Of course, he's looking only at M-dwarfs.

 

[JimMarn]

Probably out of date, but http://www.atlasoftheuniverse.com/12lys.html shows 33 stars within 12.5 light years. And 260 000 within 250 light years. http://www.atlasoftheuniverse.com/250lys.html only shows the 1500 most luminous stars within 250 light years.

 

[Adam Dray]

Very out of date. The latest astrography is finding all kinds of smaller, M-dwarf stars, many within 100 pc of Earth, that were not detectable before.

 

[Adam Dray]

He is searching a spherical volume with a hundred-parsec radius, yes? Ergo the volume is 4/3 Pi R³, or in excess of 4 million cubic parsecs: 1 star in 14 cubic parsecs.

We're trying to interpret that in two dimensions. One star in a spherical volume of 14 cubic parsecs is one star at the center of a sphere of a bit under 1.5 parsec radius. That is not 10 stars per hex; the stars are averaging about 3 parsecs apart - jump-3. Of course, he's looking only at M-dwarfs.

So I realized that we're doing different things, and we're both right.

My model fixes the numbers of stars within 100 pc by compressing the mean distance between stars.

Your model fixes the mean distance between stars by decreasing the number of stars within 100 pc.

I'd argue that my model is more interesting, as it creates more reachable stars for gaming.

My model is also less interesting, because no one wants to deal with 10 stars per hex. It does let us change up our hex system to 1-ly hexes and stick a star in almost every one of them, though!

 

[whulorigan]

The other option is to attempt to do a 3D-starmap of the region by assigning each star a "+/- # of parsecs" above/below the plane of the page (noted next to the star in the hex). You still might have multiple stars in a given hex, but they wouldn't necessarily be adjacent depending on their z-coordinate.

 

[Carlobrand]

So I realized that we're doing different things, and we're both right.

My model fixes the numbers of stars within 100 pc by compressing the mean distance between stars.

Your model fixes the mean distance between stars by decreasing the number of stars within 100 pc. ...

Decreasing the number of stars? I divided the number of stars you reported by the volume calculated from the radius you reported to get a star-per-volume figure. How did I decrease the number of stars?

What you did was to take all the stars reported in that 3-dimensional volume and squish them into a flat 2-dimensional disk. It's an interesting approach but, as you observe, it results in a massive and unrealistic increase in stellar density on the game map.

 

[Adam Dray]

Decreasing the number of stars? I divided the number of stars you reported by the volume calculated from the radius you reported to get a star-per-volume figure. How did I decrease the number of stars?

What you did was to take all the stars reported in that 3-dimensional volume and squish them into a flat 2-dimensional disk. It's an interesting approach but, as you observe, it results in a massive and unrealistic increase in stellar density on the game map.

If you divide the number of stars I reported into a smaller volume than I reported, that reduces the number of stars. You're maintaining density (and therefore distance between stars) but reducing the overall number of stars reachable within 100 pc. That's one valid way to squash 3D into 2D, by prioritizing star distances over reachable systems.

I flipped things around. I wanted to prioritize the fact of 300,000 systems within 100 pc. To do that, you have to increase star density.

Any way you squash 3D into 2D will be, as you say, unrealistic. Do you want your stellar density to be unrealistic or do you want the number of reachable stars to be unrealistic?

Now, I totally get why star density / mean distance between stars was chosen over reachable stars, because it speaks to the technology used to reach them, which is what Traveller has focused on.

 

[Carlobrand]

...Now, I totally get why star density / mean distance between stars was chosen over reachable stars, because it speaks to the technology used to reach them, which is what Traveller has focused on.

Yep. 300 thousand stars in a flat 100 parsec radius disk means the stars are around 1.2 light years apart. Accelerate at 1G for about a month, coast for about 12 years, decelerate for another month. Not exactly easy, but a lot easier than trying to hit the speeds or travel times needed for the Alpha Centauri sublight run.