Most stars are much like the Sun—giant balls of gas burning billions of miles away. These spherical stars pump out a steady stream of light that crosses vast stretches of space before it illuminates the night sky. Down here on the ground, though, stars appear not as unwavering and blazing spheres of plasma, but as gently twinkling stars.
Why is our perception of stars so distorted? Stars twinkle for a fairly intuitive reason: The movement of the air in Earth's atmosphere can momentarily dim a star's light. This is why, says NASA, stars on the horizon seem most twinkly—“because there is a lot more atmosphere between you and a star near the horizon than between you and a star higher in the sky.”
But what about stars' characteristic pointy star shape? The science behind that is surprising and has less to do with the stars or the Earth or with space than it does with us. Stars are shaped like stars, says Henry Reich in the Minute Physics video above, because of imperfections in the back of our eyeballs. Most intriguingly, says Reich, this biological explanation means that every one of us sees stars slightly differently.