Why stars blink and planets don't

[likephysics]

Why do starts blink and planets do not. There are some stars that are just as bright as a planet(unless my observation is wrong).
I know the light from the star enters Earth's atmosphere and due to the variation in density(which means change in dielectric constant of the medium in which light travels), the intensity varies which appears as blinking. Kinda of low frequency modulation.
Why doesn't this affect the light from the planet.

 

[mgb_phys]

Stars twinkle because the atmosphere bends the light so one moment it's hitting your eye the next moment it isn't

Planets are large enough that they are bigger than the size of the atmosphere twinkle patch (with apologies to Fried!) - so when one bit of the planet moves out of your eye the chance is that another bit moves in. On average the planet appears constant - although if you looked at a feature on the planet that feature would twinkle.

 

[likephysics]

Ok. Thanks. Why does the start light bend? Refraction?

I couldn't stop laughing at your irony definition.

 

[mgb_phys]

Yes
Picture the atmosphere as the top of a pan of boiling water.
There are bubbles (called cells) of stable air which act rather like lenses.
When you look through one of those you get a good image, when the edge of one passes over you see a twinkle, typically in the visible this happens many times/second.

You can use this effect to take twinkle free images from the ground - http://www.ast.cam.ac.uk/~optics/Lucky_Web_Site/index.htm

ps. the example we used to use (before the university introduced rules about this sort of thing) is imagine the obscured glass in a bathroom window. If you look through it at a distance object - the image would move around randomly as your head moved and you were seeing it through a different bit of glass. But if you pressed a body upto the glass - although each bit would be distorted you would accurately see the overall shape of the body.

 

[pmacias]

The appearance of blinking in stars is due to a phenomenon called scintillation. This is caused by the Earth's atmosphere acting as a lens, causing the light from the star to be refracted and its intensity to vary. This effect is more noticeable in stars because they are much farther away from Earth than planets, and therefore their light has to travel through more of the atmosphere.

On the other hand, planets appear relatively stable in brightness because they are closer to Earth and their light does not have to travel through as much of the atmosphere. Additionally, planets emit their own light, while stars only reflect light from a distant source, making the light from planets more steady and less affected by atmospheric variations.

Furthermore, the scintillation effect is more prominent in brighter objects, which is why some stars may appear to blink even though they are just as bright as planets. The intensity of light from stars is much higher than that of planets, making the scintillation effect more noticeable.

In conclusion, the reason why stars blink and planets don't is due to their distance from Earth, their source of light, and the intensity of their light. While both stars and planets can appear bright in the night sky, their differences in distance and light emission result in varying levels of scintillation.