stars

local mon = peripheral.wrap("back")
mon.setCursorBlink(false)

local stars = {}
local w,h = mon.getSize()

-- local i = 1 -- I replaced many of your loops with for loops; seemed simpler.

for i=1, 21 do
    stars[i] = {math.floor((math.random()-0.5)*200), math.floor((math.random()-0.5)*200), math.floor(math.random()*1000)}
end

local function displayStars()
    for i=1, 21 do
        mon.setCursorPos(stars[i][1]/stars[i][3] + w/2, stars[i][2]/stars[i][3] + h/2)
        term.write(mon.getCursorPos()[1] .. mon.getCursorPos()[2])
        mon.write("*")
    end
end

local function updateStars()
    for i=1, 21 do
        if stars[i][3]<2 then
            stars[i][3] = stars[i][3] - 1
        else
            stars[i][3] = 1000
        end
    end
end

local run = true

while run do
    if true then
        updateStars()
        mon.clear()
        displayStars()
    end
    sleep(1)
end