Sunday, April 21, 2013

Stellar density and star catalogs

I've done a little analysis of XHIP that illustrates the issues with any current parallax catalog.

I took the stars with good distances in the new reduction, and determined which 5-parsec thick "shell" the fell into around sol.  While the number of stars in each shell is stable, of course the volume of each shell is greater, and the density drops off considerably.

This should not come as a surprise to anyone, of course.  Visual magnitude is a key factor in being able to make any observation about a star, and that (of course) drops off with square of distance.

With the current state of the art, that means that any catalog I produce which attempts to reproduce the "real" density of stars in local space will require a fair amount of guesswork.  Keep in mind, my objective is to produce something suitable for a game or for an SF story.  Story lines can be preserved by emphasizing the things we know, and playing down what what is made up.  For a game, the MST3K Mantra comes to mind.

How many stars do we need to backfill?  Well, taking the local density as "correct" - which is a poor base of itself, and will need refinement, we should have 361179 stars out to 105 parsecs, and only have distances for 24183.  This means that we will need to generate (or at least assign distances for) 336996 stars - overall, 93% or so of objects. 

Graphically by distance this looks like:

All of which will be invalidated by the ESA in about five years, but I'll survive.

A side note: if I want my space to be a 200 parsec  cube centered on Sirius, I should re-work this to 180 parsecs radius or so.

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