An aggregation of hydrogen sufficiently massive to engender thermonuclear reactions where

the radiation pressure balances out the gravitational force of said hydrogen mass, which determines the radius of the star. It will eventually go (super)nova (depending on mass) when the gravitational forces are greater than radiation pressure (i.e. the fuel runs out, which is by now not just hydrogen but goes through the periodic table for very massive stars - Fred Hoyle postulated the carbon-nitrogen as a fusion process in our sun).
There's an awful lot more space than stars, so easily avoidable if you can steer.

The above off the top of my head, so no guarantees.

Posted By: dennis moore, Aug 21, 22:12:20