Why do stars twinkle

Stars send us light for millions of years. This light arrives as almost perfectly flat "wavefronts", basically like parallel lines. But in the last 100 km, in the Earth’s atmosphere, everything goes to shit....(Yes, as you told yesterday, smart asses😌😌)

The atmosphere is not fucking uniform, there are different temperatures and densities. Where the air is warmer, it’s less dense and the refractive index of light changes going from cold to warm and back.

Formula for this (very simplified):
n≈ 1 + 77.6 * 10^(-6) *P/T
where P is pressure (Pascal), T is temperature (Kelvin).
(Physicists mostly use Kelvins, it’s more convenient when working with low temperatures.)

So, if temperature jumps, pressure jumps too (just like yours with the weather), and the star’s light jumps as well. As a result, the light from the star reaches us sometimes a bit brighter, sometimes weaker. We see this as twinkling (scintillation).

For telescopes this is fucked. Even if you have a 10-meter mirror, the atmosphere will still blur the picture down to about 1 arcsecond. This limitation is called seeing.

Small insert about arcminutes
Imagine you look at a full circle around you. That’s 360°.
If you cut it, then 1° = a piece of that circle, like the width of your pinky at arm’s length.
1 arcminute = that pinky divided by 60, fuck me, already complicated…
1 arcsecond = divided by 60 again.
Basically nothing left of the pinky.

The Moon in the sky is about 1800 arcseconds across.
1 arcsecond = same as taking a 2 cm coin and moving it 4–5 km away. From the ground it would look the same size as 1 arcsecond in the sky.

You ask how twinkling and angular resolution are connected?
Answer: twinkling blurs an already shitty image and instead of a sharp dot we see a blurry blob. It refracts, goes crooked, gets messed up and you can’t see shit.

Let’s continue
But sneaky physicists came up with a solution: adaptive optics. Scientists shine a laser into the sky, get an "artificial star" and track how its light is distorted, basically calibrating. Then the telescope mirror bends hundreds of times per second, compensating all the dents of the air. As a result they get a perfectly sharp image, almost like in space.