More of a maths problem than a Processing question...  Do some research on trigonometry ;)

Rather than two fixed points you can also think of a line in terms of starting point, an angle and a length:

1. int lineLength = 100;
2. float centreX;
3. float centreY;
4. void setup() {
5.   size(400,400);
6.   centreX = width/2;
7.   centreY = height/2;
8. }
9. void draw() {
10.   background(0);
11.   // determine the angle
12.   float dx = mouseX - centreX;
13.   float dy = mouseY - centreY;
14.   float angle = atan2(dy, dx);
15.  
16.   // calculate the end point
17.   float newX = centreX + cos(angle) * lineLength;
18.   float newY = centreY + sin(angle) * lineLength;
19.  
20.   noFill();
21.   strokeWeight(2);
22.   stroke(255);
23.   line(centreX,centreY,newX,newY);
24. }

[edit] Though you do mention spheres rather than circles.  The problem is a little more complex if you are working in 3D....

Great thanks for your super fast answer! :)

one more question: if I click the length should be twice as long. That works just fine.

Yes, but you have to make a variable (or more) to evolve with time, ie. on each draw() call.

Thanks! I used millis to do that. Although it works not yet as I want, i'll experiment more with it.