A gravity simulation I made.

function setup() {
    createCanvas(800, 800, $("#main"))
    setInterval(draw2, 5);
    noStroke()
}

let particles = [[0, 0, 0, 0, 50]];
let newParticles = [...particles];
const gravitational_constant = 2;

//Randomly spawns particles on the canvas
function addDust(num) {
    for (let i = 0; i < num; i++) {
        let x = [Math.random() * 400 - 200, Math.random() * 400 - 200, Math.random() * 2 - 1, Math.random() * 2 - 1, 0.1]
        particles.push(x)
        newParticles.push(x)
    }
}

//Randomly spawns particles in a disk around the star, and gives them a random velocity that should make most of them orbit the same direction
function addAccretionDisk(num) {
    for (let i = 0; i < num; i++) {
        let a = Math.random() * 2 * Math.PI;
        let x = [(Math.random() * 200 + 100) * Math.cos(a), (Math.random() * 200 + 100) * Math.sin(a), (Math.cos(a) - Math.cos(a + 0.1)) * Math.random() * 10, (Math.sin(a) - Math.sin(a + 0.1)) * Math.random() * 10, 0.05]
        particles.push(x)
        newParticles.push(x)
    }
}

addAccretionDisk(600)


function draw2() {
    console.log(particles.length);
    background(0, 20)
    push()
    translate(400 - particles[0][0], 400 - particles[0][1])

    let count1 = 0
    for (p of particles) {
        count1++;


        if (p[4] > 30) {
            //draw stars
            colorMode(HSB, 100, 100, 100, 100)
            fill(p[4] - 30, 100, 100)
            ellipse(p[0], p[1], Math.sqrt(p[4] / Math.PI) * 10)
            fill(p[4] - 30, 100, 100, 5)
            ellipse(p[0], p[1], Math.sqrt(p[4] / Math.PI) * 11)
            fill(p[4] - 30, 100, 100, 5)
            ellipse(p[0], p[1], Math.sqrt(p[4] / Math.PI) * 12)
            fill(p[4] - 30, 100, 100, 5)
            ellipse(p[0], p[1], Math.sqrt(p[4] / Math.PI) * 13)
            fill(p[4] - 30, 100, 100, 5)
            ellipse(p[0], p[1], Math.sqrt(p[4] / Math.PI) * 14)
            fill(p[4] - 30, 100, 100, 5)
            ellipse(p[0], p[1], Math.sqrt(p[4] / Math.PI) * 15)
            fill(p[4] - 30, 100, 100, 5)
            ellipse(p[0], p[1], Math.sqrt(p[4] / Math.PI) * 16)
            fill(p[4] - 30, 100, 100, 5)
            ellipse(p[0], p[1], Math.sqrt(p[4] / Math.PI) * 17)

            colorMode(RGB)
        } else {
            //draw planets
            fill(255)
            ellipse(p[0], p[1], Math.sqrt(p[4] / Math.PI) * 10)
        }

        let count2 = 0;
        for (p2 of newParticles) {
            count2++
            if (count1 != count2) {
                let dist2 = Math.pow(p[0] - p2[0], 2) + Math.pow(p[1] - p2[1], 2)

                //gravity
                let xgravity = p2[4] * gravitational_constant / dist2
                if (p2[0] > p[0]) {
                    p[2] += xgravity
                } else if (p2[0] < p[0]) {
                    p[2] -= xgravity
                }
                let ygravity = p2[4] * gravitational_constant / dist2
                if (p2[1] > p[1]) {
                    p[3] += ygravity
                } else if (p2[1] < p[1]) {
                    p[3] -= ygravity
                }

                //collsions
                if (Math.sqrt(dist2) < Math.sqrt(p[4] / Math.PI) * 5.5) {
                    let x = particles.indexOf(p2)
                    p[2] = (p2[2] * p2[4] / (p[4] + p2[4])) + (p[2] * p[4] / (p[4] + p2[4]))
                    p[3] = (p2[3] * p2[4] / (p[4] + p2[4])) + (p[2] * p[4] / (p[4] + p2[4]))
                    p[4] += p2[4]
                    particles.splice(x, 1)
                    newParticles.splice(x, 1)
                }
            }
        }


    }


    particles = [...newParticles]
    for (p of particles) {

        //update position based on velocity
        p[0] += p[2]
        p[1] += p[3]

        //Removes particles if they get too far
        if (dist(p[0], p[1], particles[0][0], particles[0][1]) > 100000) {
            let x = particles.indexOf(p)
            particles.splice(x, 1)
            newParticles.splice(x, 1)
            console.log(particles.length)
        }

    }
    pop()

}

Collisions are kind of fixed, though some are still broken. I also have separate roche limit code, but that doesn't work well at all

I use p5.js for drawing stuff and jQuery to get the canvas on line 2

@SpiritOfWrath @Usseewa