The "Classic VPython" import statement: from visual import * is obsolete and no longer works in the latest versions of VPython. The latest versions of Vpython use the import statement from vpython import * and there are also some syntax changes between the new vpython and the older "Classic VPython".

Ask your teacher if you are expected to install an older version of Python for this assignment. In order to not interfere with your existing Python installation you should install the required packages for your homework assignment in a sandboxed virtualenv (python3 -m pip3 install virtualenv) Python virtual environment. It's also easier to install Python packages with pip when you're starting off in a fresh, clean Python virtual environment.

There exists a conversion program to convert from Classic VPython to the new VPython 7. See details here.

http://www.glowscript.org/docs/VPythonDocs/index.html

where is says ...

"Many programs written in Classic VPython 6 will run in GlowScript VPython or VPython 7 without change after being run through a conversion program written in Python. This program converts (x,y,z) => vector(x,y,z) and obj.x => obj.pos.x. These changes are necessary because GlowScript does not recognize (x,y,z) as a vector nor obj.x as a shorthand for obj.pos.x. The program also converts display => canvas and gdisplay => graph. The program also converts scene.mouse.getclick() => scene.waitfor('click'), which works in both environments."

the python conversion program is.

http://www.glowscript.org/docs/GlowScriptDocs/VPtoGS.py

There is no python-visual package in the 18.04 LTS repository:

So consider to install using pip3:

sudo apt-get install python3-pip
pip3 install vpython
python3 -c "from vpython import *; sphere()"

and you will get sphere in web-browser:

visual module has been renamed to vpython lately.

So to run this now, you first install vpython like:

sudo pip3 install vpython 

then replace the line:

from visual import *

with

from vpython import *

That worked for me.

First things first. Your code uses list concatenation to add stuff to the list. It is better to use the .append() method of lists. Also, the last loop could iterate directly on the objects instead of using an index. It is more elegant and easy to understand this way.

The pseudo-code below is equivalent to yours, but with the above corrections applied:

from visual import *
stars = []
galaxies = []    
for i in  range(10):
   stars.append(sphere(...))
for j in range(20):
   galaxies.append(sphere(...))
for star, galaxy, starpos, galaxypos in zip(stars, galaxies, 
                                            position, G_position):
   star.pos = starpos
   galaxy.pos = galaxypos

With that out of the way, I can explain how visual works.

Visual module updates the screen as soon as the object is changed. The animation is done by that alteration, in realtime, there's no need for a show() or start_animation() - it happens as it goes. An example you can run on python command line:

>>> from visual import sphere
>>> s = sphere()

That line creates a sphere, and a window, and shows the sphere in the window already!!!

>>> s.x = -100

That line changes the sphere position on x axis to -100. The change happens immediatelly on the screen. Just after this line runs, you see the sphere appear to the left of the window.

So the animation happens by changing the values of the objects.