Why do galaxies collide more often than stars?


Typical stellar radius: 	R = 1 A.U. (e.g., red giant)
Typical stelalr distance:	D = 1 light year


D/R = 9.5 x 10^12 km / 1.5 x 10^8 km

    = 6x10^4

	a very big number

Typical galaxy radius: 		R = 20 kpc
Typical galaxy distance:	D = 1 Mpc


D/R = 1 x 1-^6 pc / 20 x 10^3 pc

    = 50


Relevant cross-section for collision goes as (D/R)^-2