World Generation in 2300AD

[Tanuki]

Originally posted by Aramis:
Seriously, the Titus-Bode relationship is easy enough to use, but the problem becomes "In what units do you generate A and B? (A and B are orbital distances for 1st and second body...)

If you have different scales for different sized stars, where do you place those breaks?

How granular should the orbital data be for ballancing reality and play ballance? 0.1 AU? 0.05 AU? 0.01 AU?

Well, one can chew on "Dynamical Derivation of Bode's Law" by Robert W. Bass at http://arxiv.org/pdf/astro-ph/0209276 and calculate probable orbits using mass ratios and orbital resonances and suchlike -- if you can follow the math... basically the orbital radii of the planets converge to r+1 = 1.8 x r. Actual values in the solar system range from 1.4 to 1.9 with most of the variation in the inner system (that range isn't random, it shows up in the derived table with a close match to the real Solar System).

http://astrosun.tn.cornell.edu/courses/astro201/bodes_law.htm shows with a couple of diagrams that the orbital periods of the planets and of the moons of Uranus form an equidistant set of points when plotted on a logarithmic scale. Again, this reflects the orbital resonances involved.

For our purposes, once an arbitrary inner orbit is chosen (outside of the star's Roche and vaporized-rock limits), the next orbit out should be around 1.8 times the radius of the innermost one. Varying that by +/- .4 would add variety, but the 2300 rules are complicated enough already. Given the first two orbits, the rest follow with the formula or we can do Yet Another Table.

Granularity? For sunlike stars or above, .1 AU should be fine. For dimmer stars (K and M class) we should go to .01 AU. The life zone for an M9 (luminosity of .001) runs from .023 AU to .046 AU. Given that we only have one significant digit on the luminosity, the answer should match so that would be .02 to .05 AU for the habitable zone of an M9 main sequence star.

Class VI stars are so rare (and the classification seems to have fallen out of use) that they shouldn't even appear in the tables.

More on my next rock.

 

[Tanuki]

I mentioned significant figures. 2300 suffers from a real bad case of bogus accuracy in running everything out to umpty decimal places. Given that planetary diameters step in units of a thousand kilometers this is silly. None of the tables should have more than 2 significant digits.

It's like giving travel time to the nearest hour, and milage to the nearest mile, and then calculating speed in inches per millisecond. That level of accuracy is impossible given the units we started with. The result may look nice and accurate with all those decimal places but the accuracy is an illusion.

Yet the tables in 2300 go out to four decimal places and Pi is given to the fifth decimal place. 3.14 is more than enough given the level of accuracy possible with thousand kilometer steps in planetary diameter.

Cutting all the tables back to two decimal places will make the whole thing look cleaner and simplify calculations.

2300 suffers from malignant spreadsheet-itus. They put everything into spreadsheets and gave us ALL the results, even the ones that make no sense whatsoever. I've got to do some digging in Hartmann's Moons and Planets and the like first, but I'll post recommendations for practical limits on the tables and values therein.

More on my next rock.

 

[EvilDrGanymede]

Originally posted by Tanuki:
Class VI stars are so rare (and the classification seems to have fallen out of use) that they shouldn't even appear in the tables.
[/QB]

I thought about 15% of all G,K, and M stars wre subdwarfs? They *should* be, assuming stars have been forming at a roughly constant rate since the earliest time they could start forming... (but subdwarfs stop forming in the galaxy after about 2 billion years, so all we're left with now are the subdwarfs that haven't died out plus all the main sequence stars that have formed since then)

 

[Tanuki]

Originally posted by Evil Dr Ganymede:
</font><blockquote>quote:</font><hr />Originally posted by Tanuki:
Class VI stars are so rare (and the classification seems to have fallen out of use) that they shouldn't even appear in the tables.

I thought about 15% of all G,K, and M stars wre subdwarfs? They *should* be, assuming stars have been forming at a roughly constant rate since the earliest time they could start forming... (but subdwarfs stop forming in the galaxy after about 2 billion years, so all we're left with now are the subdwarfs that haven't died out plus all the main sequence stars that have formed since then) [/QB]</font>[/QUOTE]Last time I went out looking for information there was virtually nothing on Type VI. I'll do another search and dig around in my references and see if I can scare anything else up.

15%? Hmmm... If we're talking first generation, metal-poor stars, they're pretty unlikely to have much in the way of planets unless they've captured something.

 

[EvilDrGanymede]

Last time I went out looking for information there was virtually nothing on Type VI. I'll do another search and dig around in my references and see if I can scare anything else up.

There are definitely subdwarfs around Sol, I've seen a few listed in the near star catalogues.

15%? Hmmm... If we're talking first generation, metal-poor stars, they're pretty unlikely to have much in the way of planets unless they've captured something. [/QB]

Well, that proportion wasn't the number of stars that had planets, just the total number of stars, period - all of those are late G-M subdwarfs, though there are probably a few giant stars that are evolved from F/early G subdwarfs. Though that was based on a very crude simulation I ran.

 

[Tanuki]

Looks like there's a little more out there now.

This from Paul Drye over on the TML: http://lists.travellerrpg.com/pipermail/tml/2001-May/119582.html

A bit of further investigation suggests to me that class VI stars are weird birds: they're rare (about 1 in a 1000 to 1 in 2000 in the local neighborhood), actually main sequence (which normally is equivalent to class V), but old and metal-poor so that their spectral lines look different. They also have low luminosities for their mass because of their unusual composition. Hence the different classification.

So the two Traveller-related issue here are that:
- They shouldn't be in binary systems with any other class than VII -- they form differently at a different time. CT/MT allowed this.
- It's unlikely they have planets anyway -- no metals, in fact nothing but hydrogen and helium to work with.

Contrary to my prior belief that they've been removed from more recent editions of Traveller because they don't exist, it would appear that they've been removed because they're inappropriate stars for the planet generation rules.

This from Jim Vassilakos at http://www.cs.ucr.edu/~jimv/list4.txt

Subject: sfrpg: more gliese data questions
Date: Mon, 18 Mar 1996 11:40:01 -0800 (PST)
One of the key problems with the 2300AD data was that there were too many type VI stars (subdwarfs). The reason we knew this was because Joerg said that nearly all type VI stars are 1st generation (this being the main sequence for the low metalic stars), and Ken Croswell (a guy who writes astronomy books) said that this population should only account for about 0.1% to 0.2% of the stars in our area. The 2300AD data gave a number closer to 5%, which we reasoned was way off base. Well, the Gliese-3 data has about 3800 stars, and of those, only 7 are reported as subdwarfs (which is right in that 0.1% to 0.2% ballpark). So, in short, the Gliese-3 data is looking pretty good on this count.

And:

David Knisely (dk84538@ltec.net) indicated that the "d" before the spectral type indicates a dwarf star and that the "e" at the end means that there are few emission lines in the star's spectra. I mentioned that Gliese uses both dM & MV and queried about whether the two were interchangable or whether the "d" indicated that the star might be a Subdwarf (class VI).
He responded:
> Usually a dM5 is M5 luminosity class V (main sequence), but
> occasionally, you will see the same "d" designation given for sub-dwarfs.
> That is why the I like the luminosity class better (also known as the MK
> system). But watch out for the capital letter "D" in front. That usually
> means a white dwarf (DA would be a white dwarf with spectral class A, ect).
> I hope this clears things up a bit.
John Gizis (jeg@astro.caltech.edu) got into even more detail:
> A lower case d prefix means it's a dwarf (g is giant)-- it's
> basically the same as a Roman Numeral V but is an earlier system.
> Dwarfs are on the main sequence -- the Sun is a dwarf.
> A DA star is white dwarf (Degenerate star) which shows Hydrogen
> absorption lines, a DB is a white dwarf with Helium lines,
> DC shows no lines. Often when viewed at higher resolution
> or high signal-to-noise weak features become evident, like
> metal lines (Ca,Mg, Fe) which is DZ or Carbon (DQ).

So it looks like they do exist, but on the order of .1% or 1 in a thousand. Rare.

Still looking.

 

[Tanuki]

This from astronomer Ken Crosswell at http://www.cs.ucr.edu/~jimv/list1.txt

From: galaxy@ccnet.com (Ken Croswell)
Subject: Re: Star age question
Date: 18 Sep 1995 15:29:08 -0700

james vassilakos (jimv@cs.ucr.edu) wrote:
: The chart below (which isn't entirely above suspicion, but which
: I think is probably fairly accurate) shows forty-five subdwarfs
: (of the approximately 750 stars which are within fifty lightyears
: of Sol). That indicates to me that some 6% of the stars in the
: local vicinity are of Population II (all subdwarfs being of that
: population). Does all of this sound reasonable?

OK, hang on; this is gonna get complicated.... "Population I" and "population II" originated with Baade, who in 1943 divided stars into two broad populations. Today, we know the Galaxy is considerably more complicated, and we recognize four different stellar populations. To make a long story short, they are:
THE THIN DISK--metal-rich, various ages
THE THICK DISK--old and somewhat metal-poor
THE STELLAR HALO--old and very metal-poor; home of the subdwarfs
THE BULGE--old and metal-rich

To make a long story longer, I'll engage in some self-plagiarism from Alchemy (pages 62-63): As astronomers presently understand the Milky Way, every star falls into one of four different stellar populations. The brightest is the thin-disk population, to which the Sun and 96 percent of its neighbors belong. Sirius, Vega, Rigel, Betelgeuse, and Alpha Centauri are all members. Stars in the thin disk come in a wide variety of ages, from newborn objects to stars that are 10 billion years old. As its name implies, the thin-disk population clings to the Galactic plane, with a typical member lying within a thousand light-years of it. Kinematically, the stars revolve around the Galaxy fast, having fairly circular orbits and small U, V, W velocities. Thin-disk stars are also metal-rich, like the Sun.

The second stellar population in the Galaxy is called the thick disk, which accounts for about 4 percent of all stars near the Sun. Arcturus is a likely member. The thick disk is old and forms a more distended system around the Galactic plane, with a typical star lying several thousand light-years above or below it. The stars have more elliptical orbits, higher U, V, W velocities, and metallicities around 25 percent of the Sun's.

The third stellar population is known as the halo. Halo stars are old and rare, accounting for only 0.1 to 0.2 percent of the stars near the Sun. Kapteyn's Star is the closest halo star to Earth. These stars make up a somewhat spherical system, so most members of the halo lie far above or far below the Galactic plane. Kinematically, halo stars as a group show little if any net rotation around the Galaxy, and a typical member therefore has a very negative V velocity. The stars often have extremely elliptical orbits: a halo star may lie 100,000 light-years from the Galactic center at apogalacticon but venture within a few thousand at perigalacticon. Metallicities are even lower than in the thick disk, usually between 1 and 10 percent of the Sun's.

The fourth and final stellar population is the bulge, which lies at the center of the Galaxy and can be seen in other edge-on spiral galaxies as the bump that extends above and below the galaxy's plane at the center. The Galactic bulge is old and metal-rich. Most of its stars lie within a few thousand light-years of the Galactic center, so few if any exist near the Sun. Consequently, the bulge is the least explored stellar population in the Milky Way.

Now, since halo stars make up 0.1 to 0.2 percent of nearby stars, this is roughly the percentage of nearby stars that should be subdwarfs, if the thick disk is not also contributing subdwarfs. So 6 percent is too high.

: star class dist star class dist
: ------------------------------------------------------
: AC+10 95-26 M4VI 46.6 AC+19 1165-38 M5VI 46.5
: AC+20 1463-148 M2VI 27.3 AC+20 1463-154 M2VI 27.3
: AC+22 308-605 M2VI 42.2 AC+23 468-46 M2VI 21.6
: AC+41 726-154 M4VI 39.7 AC+79 3888 M4VI 16.6
: DM-2 2902 K0VI 45.9 DM-4 2226 M3VI 33.9
: DM-14 5936 B M0VI 49.4 DM-17 6768 M5VI 42.3
: DM-29 8019 M4VI 44.1 DM-31 9113 M2VI 35.8
: DM-34 4036 B K3VI 44.6 DM-37 10765 A M4VI 24.9
: DM-38 1058 M5VI 42.8 DM-39 10940 B M0VI 49.5
: DM-40 9712 M4VI 19.3 DM-42 249B M0VI 43.4
: DM-45 1184 M4VI 38.8 DM-45 5378 M4VI 29.9
: DM-45 5627 M5VI 43.5 DM-46 11540 M4VI 15.2
: DM-46 12902 B M0VI 46.6 DM-48 12818 M4VI 39.3
: DM-51 13128 M0VI 47.2 DM-54 9222 B M0VI 39.8
: Ac-48 1595 89 M3VI 23.5 G 5-43 M3VI 38.2
: G107-69 A M6VI 43.4 dm+38 2285 G8VI 28.7
: L489 58 A G0VI 43.5 L1064 75 M5VI 49.4
: L1113 55 M5VI 49.4 Mu Cassiopei A G5VI 25.0
: Ross 52 A M5VI 33.9 Ross 555 M4VI 36.9
: Ross 974 K0VI 41.3 Theta Bootis B M3VI 47.3
: Wolf 414 M5VI 41.8 Wolf 629 A M4VI 20.2
: Wolf 906 M3VI 39.7 Wolf 918 M3VI 35.0
: Wolf 1039 M4VI 40.7

This list looks fishy. Most of these stars are spectral type M, and it is very difficult to determine their metallicities. On your list, I checked at random half a dozen of the M stars here and found that not a single one had halo kinematics.

Most of the subdwarfs that astronomers study are spectral types F or G, because these stars' metallicities can be determined from their ultraviolet excesses. The two most famous subdwarfs are Groombridge 1830 (listed here as DM+38 2285) and Mu Cassiopeiae, both spectral type G.

If you want LOTS of subdwarfs, see Laird, Carney, and Latham (1988), AJ 95, 1843. There are over 800 stars, with roughly a couple hundred having halo metallicities and being subdwarfs. Also, Sandage has long searched for subdwarfs. See Sandage and Fouts (1987)--AJ 93, 74--for a list of over a thousand stars, again with metallicities (in the form of the UV excess) listed. Over 300 have halo metallicities and are subdwarfs.

: My 2nd question: Do you think subdwarfs have any planets other
: than captured worlds? You said they had a low metallicity (not a
: zero-metallicity). I assume that if the metal-content is low
: enough, there would not be enough higher elements for planets
: to form. What do you think?

Earthlike planets need metals; it would be tough to form such planets around subdwarfs. Jupiter and Saturn might still form, since they are mostly hydrogen and helium. But the planets' cores are mostly heavier elements, so they might have trouble forming, too.

: 3rd question: Some of these subdwarfs are companions to type V
: main-sequence stars. Does that present any sort of inconsistency,
: a Population II star being a companion to a Population I star?

Yes.

: 4th question: Dirk mentioned that some subdwarfs are class O or
: B. I thought the only possible subdwarf classes were G, K, and M.

Subdwarf O and B stars exist, but as Dirk mentioned they don't have anything to do with the stars you have been asking about. They are not, in general, halo stars.

: Thanks again for the reply. I hope you don't mind the additional
: questions.

Questions from non-crackpots are always welcome.

 

[EvilDrGanymede]

Yep. All that stuff makes sense... evidently my crude assumption that the first stars to form in the disk are going to be subdwarfs was wrong

.

 

[Tanuki]

And in the "Why didn't I notice this before?" department:

The star data tables in 2300 are exactly the same bogus tables as the ones in Book 6. All the "stellar mass" data seems to have been generated by running the main-sequence mass-luminosity equations on a table of stellar luminosities gathered from who knows where.

We have GOT to come up with valid star data to replace them.

Looks like there's some good stuff at http://curriculum.calstatela.edu/courses/builders/lessons/less/les1/StarTables_Z.html

 

[Tanuki]

One thing I found peculiar about 2300AD's world creation was that there were no tables for generating stars. System generation began with "pick a star on the map." Whereupon we are presented with a huge pile of tables of stellar data, mostly for stars that are not on the map.

I expect that Dr. Ganymede will keep me honest here and let me know if I come up with anything too outragious.

By far, the majority of stars are Main Sequence (type V). Most of the others are so rare that devoting that much space to idealized/average versions of them is silly IMHO.

For today's astronomy class: Percentages from the real universe (taken from: http://www.physics.gmu.edu/classinfo/astr103/CourseNotes/str_hr.htm ):

Blue and Red Supergiants (type I & II): 0.000025% of stars. This is a staggeringly smal percentage. Given 400 billion stars in the galaxy, just 100,000 would fall in these two classes or about 1 star in 4 million. And most of these will be type IIs. Yet three columns of the star tables are devoted to stats for these beasts. These are massive stars near the end of their lifetime. Unlikely to have planets. Likely to have a LOT of gas, dust, and rubble. Whole shells of ejected star stuff will englobe the system. Intense UV and other radiation. Not a healthy place for living things.

Red Giants (type III): .5% of stars in our galaxy are red giants -- about 2 billion of the 400 billion stars in our galaxy or one star in 200. These stars average about 1.2 solar masses and are near the end of their lifetimes, fusing helium, oxygen, nitrogen, carbon, etc. in their cores instead of hydrogen. Remaining planets will have originally been in the mid to outer system of the star. Waterballs (ex-iceballs) are likely -- much water which may or may not have a solid core waaay down in the depths. Given the increased insolation, smaller gas giants may be losing significant amounts of hydrogen, etc.

White Dwarfs: 8.75% of stars in our galaxy are white dwarfs -- about 1 star in 11. White dwarfs average about 1 solar mass compressed by gravity into a sphere about the size of the Earth. Basically burned out stars. Inner planets, if any, will be very cooked. Planets further out will have been thawed and refrozen and may have lost volitiles during the red giant stage of the star. Might have some nice real estate if you could warm it up. Some of our favorite hellholes might circle white dwarfs and wilderness refueling is likely to be possible from remaining gas giants -- they're probably somewhat reduced, but still there. The system should be generated as for the original parent star (likely an F, G, or K), less the innermost planets, which were probably cooked or vaporized during the star's giant stages.

Main Sequence Stars (BTW all these classes formed a logical progression from A to whatever when they were first designated, based on spectral lines):

Class O: .0000025% of stars -- only about 10,000 of the 400 billion stars in the galaxy are class O or about one in 40 million. Ten times rarer than supergiants. Average around 25 solar masses. These are very short-lived stars. During their brief lifetime they will throw off a lot of mass, first from tremendous stellar winds and later in the giant phase they will throw off their outer layers. The remnant that goes supernova will be much less massive than it started out. As with supergiants (which they will become), likely to have a LOT of gas, dust, and rubble. Intense UV and other radiation. Not a healthy place for living things.

Class B: .075% of stars -- about one star in 1300. Average of 5 solar masses. Hot and short lived. Not all that different from class O stars, but will last a little longer. Ditto for junk, and UV. The finale will pretty much toast anything there.

Class A: .75% of stars -- about 1 star in 130. Average around 1.7 solar masses. Still too hot and short lived for life and real planets, but they're getting closer. Though useful planets are unlikely to form during their lifetime, hot, cratered protoplanets might form by the time things are winding up. Go out with a hot, bright scream instead of a bang.

To this point, excluding white dwarfs, we've covered less than 1% of the stars in our galaxy, but they occupy the majority of the star tables. I suggest that the tables for Class 1-IV be simplified greatly, giving a range of masses/luminosities/etc. for each class. They are so rare that it's not worth wasting enormous amounts of space on. Perhaps a few representitive stars for each class and their stats might be more useful. If anything, the stars from this point on deserve more coverage.

Class F: 3% of stars -- about one star in 33 is an F. Average about 1.2 solar masses. Likely to form planets, but early (biggest) Fs probably won't be around long enough for any life beyond soup. Later (smaller) F's are more likely to be around long enough for life-bearing planets to evolve. Higher UV levels than we're used to.

Class G: 6.5% of stars -- about 1 star in 15. Average about .9 solar masses. Our sun is a G. From our point of view, the perfect star for life-bearing planets. Other species in the universe may disagree.

Class K: 13% of stars -- about 1 star in 8. Average mass about .5 solar masses. Life zones for later (smaller) Ks fall into the realm of tide-locked planets, but recent research suggests strongly that tide-locked planets are much more friendly to life than we used to think. The majority of life-bearing planets in the universe may be tide-locked.

Class M: 67.5% of stars -- about 2 out of 3 stars. Average around .25 solar masses, shading down to a few dozen Jupiter masses. Life-zone planets will almost certainly be tidelocked in one way or another. 3:2 locks and the like may still produce rotating planets, though tides would be high. Again, tide-locked does not mean lifeless or boring, and a decent atmosphere will ameliorate the effects of flares considerably. The mass of planet, water, and atmosphere mean that flare-heating will be moderated and spread out over time.

Beyond class M we move into the realm of brown dwarfs and ease on down from there into gas giants.

More on my next rock.

 

[Colin]

I'll be honest: I've pretty much decided to cut stellar generation rules, and go with (slightly) modified T20 rules for that, instead concentrating on world generation. This is to ensure that special flavor of 2300 world generation is retained. Stellar generation is largely unnecessary, as the NSL outlines all the stars (yes, Gliese II.) in the 50 LY around Earth, the Kafer star list outlines all the stars in the Kafer sphere, and only the Bayern corridor reaches out of these areas.
I do want to try and minimize mechanics in the book, concentrating on setting instead. Of course, having said that, vehicle and starship design are more than double my original word count estimate...

 

[Tanuki]

Originally posted by Colin:
I'll be honest: I've pretty much decided to cut stellar generation rules, and go with (slightly) modified T20 rules for that, instead concentrating on world generation. This is to ensure that special flavor of 2300 world generation is retained. Stellar generation is largely unnecessary, as the NSL outlines all the stars (yes, Gliese II.) in the 50 LY around Earth, the Kafer star list outlines all the stars in the Kafer sphere, and only the Bayern corridor reaches out of these areas.
I do want to try and minimize mechanics in the book, concentrating on setting instead. Of course, having said that, vehicle and starship design are more than double my original word count estimate...

Makes sense. How about this?

Stars, table one: Common Stars (2D6)
11-13 -- White Dwarf (earth-sized, incandescent dead star)
14-53 -- class M (dim red star)
54-62 -- class K (orange star)
63-65 -- class G (yellow Sol-like star)
66 -- class F (green) or (6 on 1d6) Rare star

That table alone gives a reasonably accurate distribution. That much can occupy a small sidebar box. If you cut back the stellar tables to reflect what's actually available that will save you some more room.

And for another column inch you can include:

Stars, table two: Rare Stars (2D6)
11-42 -- class A (yellow-white star)
43-44 -- class B (blue-white star)
45-65 -- Red Giant (post main-sequence star)
66 -- Red or Blue Supergiant star, class O star, Neutron Star, or Black hole

Another paragraph on multiple stars should suffice. And you've got the whole random star generation thing in a half sidebar or so.

Let me see what I can do with the other world creation tables. I think you can save a lot more space by eliminating the ridiculous from the huge spreadsheet tables for worlds and stellar data.

As a planetary scientist, I'm sure the justly infamous Evil Dr. Ganymede might have some ideas there.

 

[EvilDrGanymede]

Originally posted by Tanuki:
As a planetary scientist, I'm sure the justly infamous Evil Dr. Ganymede might have some ideas there. [/QB]

Well, I'm sitting on several excel files based on the Geneva Stellar Structure/Evolution grids, which are supposed to be what the astrophysicists use to model stars with. Your distributions sound about right - though do note that F stars aren't 'green', they're white. Their light may peak in the green part of the visible spectrum but that just means there's enough blue and red light to keep it white rather than bias the light towards either end. There is therefore no such thing as a 'green star'.

But as Colin says, for 2300AD purposes they might be a bit redundant, since it's based on a near star list anyway and you're not actually likely to be making any stars. Is that list being updated using what we know of the nearby stars today? The Near Star List was one of the most generally (in astronomy terms) useful things in 2300AD, and not updating it would be a missed opportunity IMO.

 

[Erik Boielle]

--The Near Star List was one of the most generally (in astronomy terms) useful things in 2300AD, and not updating it would be a missed opportunity IMO.--

I understand that adding in everything new does really nasty things to the whole 'Arms' conept by liking everything togeather and loosing the choke points, which is a shame as I really like them.

I was kinda wondering if you could redo it using a slightly shorter maximum stutter distance.

 

[EvilDrGanymede]

Originally posted by Erik Boielle:
I understand that adding in everything new does really nasty things to the whole 'Arms' conept by liking everything togeather and loosing the choke points, which is a shame as I really like them.[/QB]

Nuts. There is that, isn't there.

One could probably keep the arms (since hopefully the existing stars would be in the same place in the new data), but they'd be denser and can potentially extend further since you'd have more stars within stutterwarp range.

 

[Colin]

One of the big problems with an updated star list isn't just the additional stars, but the adjusted position of already extant (NSL) stars. Wolf 359, in particular, ends up being out of range, effectively making the French Arm as tortuous as the Chinese Arm.

Given the rate at which new planets and stars are being discovered, and changes in the charts (Gliese II to III to Hipparcos) it's difficult to determine where a cutoff should be. Thae easiest one, and the one that does the least damage to the setting, is the current NSL. Some of the star types may get corrected, as that doesn't affect the game (note I said 'may'), but the stellar frequency and positions will not be changed. In part because it's simpler, and in part because I was told to do it that way.

Later,

Colin

 

[Tanuki]

Originally posted by Evil Dr Ganymede:
</font><blockquote>quote:</font><hr />Originally posted by Tanuki:
As a planetary scientist, I'm sure the justly infamous Evil Dr. Ganymede might have some ideas there.

Well, I'm sitting on several excel files based on the Geneva Stellar Structure/Evolution grids, which are supposed to be what the astrophysicists use to model stars with. Your distributions sound about right - though do note that F stars aren't 'green', they're white. Their light may peak in the green part of the visible spectrum but that just means there's enough blue and red light to keep it white rather than bias the light towards either end. There is therefore no such thing as a 'green star'.[/QB]</font>[/QUOTE]In truth they all look pretty much white when we look at them. I thought about including a note about the spectrum peaking in red, orange, green, etc. but decided not to. If I remember aright, type Fs are represented on the map as green dots, though. But you're right, calling them "green stars" is misleading.

 

[Tanuki]

Evil Dr Ganymede -- What can you tell us about the density table? I have a hard time believing that you can pack five or six earth masses of even pure hydrogen into a ball and come up with a density of .1 -- gravitic compression alone should bump the density up. Besides, something that "small" really needs a core to gather H2 around so a ball of pure hydrogen that small seems rather unlikely.

Where does the limiting curve fall on density?

 

[EvilDrGanymede]

Originally posted by Tanuki:
Evil Dr Ganymede -- What can you tell us about the density table? I have a hard time believing that you can pack five or six earth masses of even pure hydrogen into a ball and come up with a density of .1 -- gravitic compression alone should bump the density up. Besides, something that "small" really needs a core to gather H2 around so a ball of pure hydrogen that small seems rather unlikely.

Where does the limiting curve fall on density?

Unfortunately my 2300AD Directors Guide is at home (about 5,000 miles away) and I don't remember the details of the world generation system (*wishes FFE would get on with reprinting it*).

But going by what you've said here, 0.1 is definitely way too low (assuming that's 100 kg/m3). The lowest possible density for an iceball like a Saturnian moon would be around 1000-1500 kg/m3.

Larger objects would be like Ganymede or Triton or Pluto - made from ice/rock mixes so they'd have densities between 1500 and 2500 kg/m3. If differentiated enough (e.g. Ganymede) they may have small iron cores.

Small silicate worlds (Io, Europa, Luna, Mars) would have densities between about 2500 and 4000 kg/m3. They may have iron cores too, and they may have a thin veneer of ice on top (eg Europa).

'Metal Core' worlds are generally large silicate worlds (eg Venus, Earth) but can be smaller too (eg Mercury). They have densities between 4000 and 7000 kg/m3). The higher the density, the larger and more iron-rich the metal core. Self-compression in the core plays a big role in elevating the bulk density of the planet.

Beyond that, we have more putative worlds that may or may not exist in reality. Denser worlds are unusual and are probably the result of early planetary collisions that blew off the crust/mantle of the planet, or odd planetary chemistry. The 'Heavy Core' worlds (densities between 7000 and 10000 kg/m3) have HUGE metal cores that take up about 90% of their mass. They'd have thin silicate crusts/mantles, but most of the planet would consist of the metal core.

"Cannonball" worlds with densities over 10000 kg/m3 would be exceedingly rare - they'd pretty much be entirely metal core with no silicates, and would almost certainly be the result of a major planetary calamity. At that density, they'd have to have a core rich in dense refractory rare earth elements.

I made this classification scheme while I was figuring out the Sword Worlds in Traveller for the upcoming GT book... some of the planets were quite ridiculously dense as a result of silly random UWPs (small worlds with breathable atmospheres at habitable temperatures are especially wrong) and needed some pretty unlikely explanations to make them work. Of course, the large, dense worlds are the ones more likely to hold onto hydrogen. Even a size A world of any density and blackbody temperature would probably be able to hold onto helium at least.

 

[snrdg082102]

Hello Evil Dr Ganymede,

I happen to have, thanks to William, an almost complete, dice missing no big deal I've got plenty , boxed set of 2300 AD. Here is a copy of the 2 or 3 Desnity Tables from p. 88 of the Director's Guide (copyright 1988).

</font><blockquote>code:</font><hr /><pre style="font-size:x-small; font-family: monospace;"> WORLD DENSITY
Density of Density of
1D10 Rocky Core 1D6 Icy Core
1 .4 1 .1
2 .5 2 .2
3 .6 3 .3
4 .7 4 .4
5 .8 5 .5
6 .9 6 .6
7 1.0
8 1.1
9 1.2
10 1.3
Density is in Earths (Earth=1) </pre>[/QUOTE]</font><blockquote>code:</font><hr /><pre style="font-size:x-small; font-family: monospace;"> Equivalent Densities
Material g/cc Earths
Water 1.0 .18
Ice .9 .16
Carbon 2.3 .41
Rock 3.5 .65
Iron 7.9 1.43
Gold 19.3 3.50</pre>[/QUOTE]</font><blockquote>code:</font><hr /><pre style="font-size:x-small; font-family: monospace;">Body g/cc Earths
Sun 1.0 .18
Mercury (Rock Ball) 5.4 .98
Venus (Hot House) 5.2 .94
Earth (Garden) 5.5 1.00
Luna (Rock Ball) 3.3 .60
Mars (Desert) 3.9 .71
Jupiter (Gas Giant) 1.3 .23
Saturn (Gas Giant) .7 .13
Uranus (Gas Giant) 1.6 .29
Neptune (Gas Giant) 1.7 .30
Pluto (Ice Ball) 1.0 .18</pre>[/QUOTE]Does the above help or is there something else you need?

Originally posted by Evil Dr Ganymede:
</font><blockquote>quote:</font><hr />Originally posted by Tanuki:
Evil Dr Ganymede -- What can you tell us about the density table? I have a hard time believing that you can pack five or six earth masses of even pure hydrogen into a ball and come up with a density of .1 -- gravitic compression alone should bump the density up. Besides, something that "small" really needs a core to gather H2 around so a ball of pure hydrogen that small seems rather unlikely.

Where does the limiting curve fall on density?

Unfortunately my 2300AD Directors Guide is at home (about 5,000 miles away) and I don't remember the details of the world generation system (*wishes FFE would get on with reprinting it*).

But going by what you've said here, 0.1 is definitely way too low (assuming that's 100 kg/m3). The lowest possible density for an iceball like a Saturnian moon would be around 1000-1500 kg/m3.

Larger objects would be like Ganymede or Triton or Pluto - made from ice/rock mixes so they'd have densities between 1500 and 2500 kg/m3. If differentiated enough (e.g. Ganymede) they may have small iron cores.

Small silicate worlds (Io, Europa, Luna, Mars) would have densities between about 2500 and 4000 kg/m3. They may have iron cores too, and they may have a thin veneer of ice on top (eg Europa).

'Metal Core' worlds are generally large silicate worlds (eg Venus, Earth) but can be smaller too (eg Mercury). They have densities between 4000 and 7000 kg/m3). The higher the density, the larger and more iron-rich the metal core. Self-compression in the core plays a big role in elevating the bulk density of the planet.

Beyond that, we have more putative worlds that may or may not exist in reality. Denser worlds are unusual and are probably the result of early planetary collisions that blew off the crust/mantle of the planet, or odd planetary chemistry. The 'Heavy Core' worlds (densities between 7000 and 10000 kg/m3) have HUGE metal cores that take up about 90% of their mass. They'd have thin silicate crusts/mantles, but most of the planet would consist of the metal core.

"Cannonball" worlds with densities over 10000 kg/m3 would be exceedingly rare - they'd pretty much be entirely metal core with no silicates, and would almost certainly be the result of a major planetary calamity. At that density, they'd have to have a core rich in dense refractory rare earth elements.

I made this classification scheme while I was figuring out the Sword Worlds in Traveller for the upcoming GT book... some of the planets were quite ridiculously dense as a result of silly random UWPs (small worlds with breathable atmospheres at habitable temperatures are especially wrong) and needed some pretty unlikely explanations to make them work. Of course, the large, dense worlds are the ones more likely to hold onto hydrogen. Even a size A world of any density and blackbody temperature would probably be able to hold onto helium at least. </font>[/QUOTE]