Posts Tagged ‘twinkle-star effect’

Question: Why do stars twinkle?

November 9, 2007

On Nov. 3 a fellow stargazer posted a question for astronomy graduate student Matt Davis on the Interview with Matt Davis page of this blog.  The question:  Why do stars twinkle?  I e-mailed Davis the next day about the inquiry, as promised.   (Thanks, Jay!)

The answer:  “Stars twinkle because we are looking at them through a thick, constantly moving layer of air:  Earth’s atmosphere,” Davis said.

Davis gave a great analogy of the twinkle-star effect:  “Imagine you are out in the desert on a hot day, looking down a straight road,” he said.  “As you look down the road, the line on the road seems to shift around and the air looks like it’s shimmering and moving.  This is because the air next to the road is very hot and it’s rising, as hot air does.  This means that the air is changing density and moving along your line of sight, and this causes things you see through that air [to] appear to move and shimmer.”

“This is the same effect we see as we look at stars or the distant lights of a city,” Davis said.  “All of the air between you and a star or [the] light is moving around.   Different parts of [the air] have different temperatures and different densities so it bends the light around, making it look like it’s twinkling.”

The twinkle-star effect is the main reason astronomers put telescopes (like the Hubble Space Telescope) in orbit around Earth, he said.  Above the atmosphere “the stars … no longer twinkle and we can take much clearer pictures,” Davis said.

“Interestingly, planets do not appear to twinkle the way stars do because they have an actual finite size because they are so much closer to us than the stars,” he added.  “If you look at a planet through binoculars you will see that it has a disk, [whereas] stars still just look like points.  This makes it so that the light from a planet gets kind of averaged out so that we don’t see it twinkle.”

For another good answer to why stars twinkle, read what volunteers in the Astronomy Department of Cornell University had to say on the Ask an Astronomer Web site.

Again, please post any questions about stargazing to this blog.  I’m happy to work as an intermediary for other interested stargazers.  Davis, a self-proclaimed “extragalactic distances guy,” encourages inquiries about celestial phenomena outside of the Milky Way galaxy.

Oh, and check out StarDate Online’s seven-day star forecast for the nights Nov. 8 to Nov. 14.