micro "physics" simulation? Just testing pygame.

star.py

import pygame
import sys
import math
import random

#vector stuff http://stackoverflow.com/a/4114962
def magnitude(v):
    return math.sqrt(sum(v[i]*v[i] for i in range(len(v))))

def add(u, v):
    return [ u[i]+v[i] for i in range(len(u)) ]

def normalize(v):
    vmag = magnitude(v)
    return [ v[i]/vmag  for i in range(len(v)) ]


class star():
    def __init__(self, x,y,size):
        self.x=x
        self.y=y
        self.x_center=x+size/2
        self.y_center=y+size/2
        self.size = size
        self.mass = size*size*math.pi
        self.surface = pygame.Surface((size, size))

        self.v_x = 0
        self.v_y = 0
        pygame.draw.circle(self.surface, (75,2,10), (size/2, size/2), size/2) 

    def update(self):
        self.x=self.new_x
        self.y=self.new_y
        self.x_center=self.x+self.size/2
        self.y_center=self.y+self.size/2

    def draw(self, screen):
        screen.blit(self.surface, (self.x, self.y), None, (pygame.BLEND_ADD))

    #physics stuff, should probably be a seperate engine

    def physics_step(self, star_list):
        force_list = []
        for star in star_list:
            if star is not self:
                dist = self.distance((self.x_center, self.y_center),(star.x_center, star.y_center))
                power = dist*dist
                force = star.mass-(power*0.01)

                vect = [star.x_center - self.x_center, star.y_center - self.y_center]
                
                vect = normalize(vect)
                vect[0]*=force
                vect[1]*=force
                force_list.append(vect)


        final_force = [0,0]
        for force in force_list:
            final_force[0]+=force[0]
            final_force[1]+=force[1]

        #print final_force
        scaling = 0.000005
        self.v_x+=final_force[0]*scaling
        self.v_y+=final_force[1]*scaling
        #damping
        self.v_x*=0.99
        self.v_y*=0.99
        self.new_x=self.x+self.v_x
        self.new_y=self.y+self.v_y

    def distance(self, p0, p1):
        return math.sqrt((p0[0] - p1[0])**2 + (p0[1] - p1[1])**2)

########################################################################################################################################################
#main
DISPLAY = pygame.display.set_mode((1024, 768), (pygame.DOUBLEBUF), 32)
FPS = 60
fpsClock = pygame.time.Clock()

star_list = []

nr_stars = 100

for i in range(nr_stars):
    new_star = star(random.randint(0,1024),random.randint(0,728), random.randint(10,75))
    star_list.append(new_star)

while True: # the main game loop
    #events
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()
            sys.exit()

    #logic
    for star in star_list:
        star.physics_step(star_list)

    for star in star_list:
        star.update()    
        
    #drawing
    for star in star_list:
        star.draw(DISPLAY)
        
    fpsClock.tick(FPS)
    pygame.display.update()
    DISPLAY.fill((0,0,0))