Stellar Masses for use with GURPS SPACE?
 

Hello Folks,
I'm just a bit curious - how accurate would these tables be for stellar masses with regards to GURPS SPACE?


http://isthe.com/chongo/tech/astro/H...le-bymass.html

One problem I have with the data is that it seems to be that if the stellar mass is less than .1 Solar Masses, that the table doesn't say what it should be. Consequently, an M8 main sequence star is listed as .1 solar masses, as is an M7 main sequence star. An M6 Main sequence star is listed as .2 solar masses. Seems that the data rounds up to nearest tenth of a solar mass.

Thoughts? Comments? Would building a GURPS SPACE like interface using those solar masses and temperatures be worth the effort?

Thanks.

Hal

Re: Stellar Masses for use with GURPS SPACE?
 

I am a bit perplexed by your question. Both that table and GURPS Space use solar masses, and end at 0.1 solar masses.

Re: Stellar Masses for use with GURPS SPACE?
 

GURPS SPACE tables on page 103 end at M7, not M9. The temperatures in K also do not match.

This is why I was wondering if the web page is worth using for its data as compared/contrasted with GURPS SPACE.

Mind you, I've also tried using the material from the author of GURPS SPACE that he published AFTER SJGames published GURPS SPACE, but I run into some minor issues overall.

Re: Stellar Masses for use with GURPS SPACE?
 

Those figures are missing the effect of stellar age.

Re: Stellar Masses for use with GURPS SPACE?
 

Not worth the effort. The relationship between solar mass and temperature is unlikely to have a major effect on most campaigns.

Re: Stellar Masses for use with GURPS SPACE?
 

The GURPS Space (4th) system generation sequence will work fine if you plug some other masses and luminosities into the later steps rather than using the procedures in steps 16, 17, and 18. I use actual astronomical data for real stars (from the XHIP catalogue), and the only difficulty I have is that you need a figure for the system age at a few places (e.g. the Ocean➞Garden transformation of the world type in Step 25; volcanic and tectonic activity in Step 31). And then for realism, the age and mass of the star determine its luminosity class and luminosity, so you can't just generate a figure at random.

One thing that people often fail to mention is that the ice line and life zone of a star and the black-body temperatures of its planets are not determined by its visual luminosity (the figure cited as unqualified "luminosity" in astronomical usage) but by its bolometric luminosity. You ought to use bolometric luminosity, not visual luminosity, throughout the GURPS Space (or any other) star system generation sequence. Use visual luminosity only for calculating the illumination of planet surfaces.

Much depends on whether you have players who know a bit about astronomy and who check your figures, or who use your figures to work out further values. I was scarred for life because my gaming group in the late Eighties included an astrophysics student, and a few other science students who were fans of "real" SF. Otherwise it is as Anders says. The science only matters to the extent that anyone is going to check your figures.

Re: Stellar Masses for use with GURPS SPACE?
 

So that with a stellar mass of 1.0 there are entries for six different spectral classes.

Re: Stellar Masses for use with GURPS SPACE?
 

I guess what I'm going to have to do is go back to Jon's more recent material as far as coding a random generator for star systems. I'm not even sure if you can download the copyrighted material that was his ARCHITECH OF WORLDS piece any more. But it at least has stellar masses and temperatures for the worlds.

I find it interesting that the temperature ranges given on the web page don't really match those given in the books. Ah well.

Re: Stellar Masses for use with GURPS SPACE?
 

You touched on it a later, but the age of a star has big impact on where it lies on the H-R diagram. Stars on the main sequence tend to move up and to the left as they age (getting brighter and hotter) and helium "ash" builds up.

Our sun will then shoot up and to the right as it grows to it's first red-giant phase, then move along the "horizontal branch" before hitting the asymptotic-giant-branch red giant phase. The first burns hydrogen around a inert core of helium, the second is now burning helium into carbon.

This chart shows all the parts of the H-R diagram a 1 solar mass star will touch in its life-time.

Long story short, the chart you give just gives a snap shot of real stars at their current age. It is excellent for giving real data on what real stars are like, but you can't use the chart to say what a 1.5 solar mass star will be, without also knowing how old the star is.

Re: Stellar Masses for use with GURPS SPACE?
 

When you refer to the chart that I give, do you mean the XHIP catalogue that I linked or the chart that Hal linked in the initial post?

Eventually, yes. That is to say, in about five billion years, and the changes you outline will take another 1.11 billion years or so. For the last four billion or so the Sun has been moving very slowly up and to the left, and it will go on doing that for another five billion years. Over the whole ten billion years of its main-sequence lifetime it will more than double in luminosity with only a negligible loss of mass. The upshot is that depending on its age, a star of 1.0 M☉ can have a luminosity anywhere from about 0.75 L☉ to 2.0 L☉ even while it is on the main sequence, and can be vastly larger and more luminous or smaller and dimmer as a red giant or white dwarf in its protracted death throes. So a chart that gives spectral class and luminosity as a function of mass without reference to age is a bit perplexing and I don't know what the use of it would be.

Well, during its main-sequence and post-main-sequence lifetime. That chart does not include the development of such a star along the Hayashi track (straight down in the HR diagram) and then the Henyey track (horizontally to the left) during its 100-million-year pre-main-sequence lifetime. You can find that bit in this chart.

Re: Stellar Masses for use with GURPS SPACE?
 

A 1.0 solar-mass star should not reach 2.0 L during its main sequence (it is moving into its subgiant phase at that point). Of course, there are stars that break the examples of Space and one of them is quite close. Mu Hercules is a 1.1 solar-mass star that is a G5IV with a L of 2.7 that is 7.8 Gyr. It is too bright for its age and its mass, especially given its metallicity, as a star at its mass and age should be 20% dimmer (a lower metallicity should have accelerated its fusion processes slightly, there is less junk in the way of the fuel, which would have increased its luminosity but shortened its life). Now Space gives workable approximations and averages, but it does not account for all of the stars, even one of the ones that humanity has used as a benchmark.