Freelance Traveller

Expanded Stellar Radius Table

by Ewan Quibell (with Jeff Zeitlin)

This article originally appeared in the November/December 2023 issue. Mr Quibell originally developed the tables in 2009.

Classic Traveller Book 6: Scouts, as part of the process of detailing a system, provides a table of stellar radii, as a multiplier of Sol’s radius. As presented, the table only gave the values for the x0 and x5 classes (plus M9) for each luminosity class (I through VI); this table expands that to all of the integer numeric subclasses (B0 through M9), excepting only “white dwarfs” (whose classification works differently) and “carbon stars”.

In describing the original table and explaining its use, Book 6 also notes that the “safe” jump limit would be at 200 times the radius of the star (that is, 100 diameters, as was standard for that version of Traveller), thus implicitly requiring that one be clear of both star and planet, something which wasn’t stated in the original rules from Books 1-3.

We can do some reasonably simple math, and discover something else about this table. Sol’s radius, if we look it up, is given as 696,000km. Multiplying this by 200 (to get the 100-diameter distance) gives us 139,200,000km. One AU—the distance from Sol to Terra—is approximately 149,600,000km. Thus, we can also read this table as giving the distance in AU from the star that represents a safe distance to jump—the 100-diameter distance.

Knowing this, we can now use one of the other tables in Book 6—specifically, the planetary orbital distance table—to know whether a world in a particular orbit around the star is beyond the 100-diameter distance: if the distance given in AU is greater than the distance given in this expanded table, it is safe to jump when you reach 100 planetary diameters. (If the planet’s distance is equal to the star’s 100-diameter distance, you must move away from the planet in a direction that does not move you closer to the star).

For worlds that are satellites of another world (e.g., Regina, orbiting Assiniboia), the orbit number is the number of radii of the central world (that is, Regina being in orbit 55 means that its distance from Assiniboia is 55 times the radius of Assiniboia). Thus, a ship leaving Regina to jump must go out to beyond Assiniboia’s “orbit 200”, not just Regina’s, and must also be beyond the star’s jump limit read from the table here.

(In the Sol system as described in Book 6, only Saturn’s satellite Phoebe is outside both the Sun’s and Saturn’s 100-diameter distances. All other satellites in the system are too close to their primaries to ignore them when travelling to a jump point.)