Why do stars twinkle and what causes it?

[nhmllr]

Why do stars "twinkle?"

I can't think of a better word for it

Why are there 4 "spikes" coming out of the star? What optical phenomenon causes this? Why do some stars twinkle and others not?

Thanks

 

[cepheid]

The optical phenomenon is diffraction. Light, even from a point source, is never focused down to a perfect point, but is smeared out over some finite area in the focal plane. The specific shape of the image depends upon your optics. I'm not sure, but I think that the diffraction "spikes" might have something to do with how the secondary mirror is mounted in front of the primary. These features are too faint to show up except for the brightest stars.


Twinkling is caused by atmospheric turbulence. So all stars twinkle, but I think that it is more noticeable in brighter ones.

 

[HallsofIvy]

Right. Looks to me like the picture was taken through a reflecting telescope with the secondary mirror supported by 4 arms a spider.

But that is different from the "twinkling" of stars. Looking at stars through the atmosphere, we see a "point" of light that can be blocked by changes in the air between us and the light. Interestingly, one way of differentiating planets from stars is that planets do NOT twinkle. Even though we don't normally notice it, planets are close enough that we actually see a small disk and not just a point of light. Atmospheric disturbances may block the light from part of the disk, it is only a slight effect and does not noticably change the appearance of the planet.

 

[russ_watters]

I sometimes string thin wires across my scope objective to create diffraction spikes.

 

[russ_watters]

Interestingly, one way of differentiating planets from stars is that planets do NOT twinkle. Even though we don't normally notice it, planets are close enough that we actually see a small disk and not just a point of light.

I've always been uncomfortable with this because:
1. Planets come in different sizes.
2. Our eyes are not good enough to resolve a disk of any planet except perhaps Venus, in limited circumstances.
3. The amount of atmospheric distortion varies widely - if the air is very still, stars won't twinkle either.

I've never been able to identify a planet by them not twinkling - they look to me like they usually do.

 

[cepheid]

I've never been able to identify a planet by them not twinkling - they look to me like they usually do.

Interesting. For me it's a very stark difference. Planets don't seem to twinkle at all.

But yeah, I'm not so sure I'm convinced by the explanation.

 

[ideasrule]

I've always been uncomfortable with this because:
1. Planets come in different sizes.
2. Our eyes are not good enough to resolve a disk of any planet except perhaps Venus, in limited circumstances.
3. The amount of atmospheric distortion varies widely - if the air is very still, stars won't twinkle either.

Our eyes aren't good enough to resolve the disk, but that doesn't change the fact that light from different parts of the planet are taking slightly different routes through the atmosphere to reach our eyes. The distortion caused by turbulence averages out, and since we can't resolve the planetary disk, the entire planet seems to be one point of light with constant brightness. With a telescope, you can resolve the disk, and the effects of turbulence become very obvious. In fact, on a bad night turbulence completely dominates any feature you might be trying to observe.

 

[russ_watters]

Here's a description from the wiki on scintillation:

Both objects in the sky are bright, circular discs. The light from a star enters the pupil of our eye and comes to a focus as a point at the back of the eye. If that light is followed backwards to the star, it forms a cylindrical bundle the size of the pupil but gradually widening as it nears the star. But so gradual is this widening that even at the edge of the atmosphere, 100 km up, the bundle is still about the same as the pupil, say half a cm across. This is so narrow that wind currents of different temperature and refractive index can disturb the light and cause the star to twinkle. A planet is so much nearer that at the edge of the atmosphere the bundle is a disc metres across. The light from the planet that forms our image of it comes from hundreds of different pathways in this bundle and it is unlikely that many will be disrupted at the same time. The amount of disturbance is therefore much less and the planet appears as a steady point.

It would surprise me if the disk for a planet was meters across near the "top" of the atmosphere (I'll calculate it when I'm more awake) but either way, that implies there must be a changing cutoff based on angular size, distance and astronomical seeing conditions.

 

[Janus]

Here's a description from the wiki on scintillation: It would surprise me if the disk for a planet was meters across near the "top" of the atmosphere (I'll calculate it when I'm more awake) but either way, that implies there must be a changing cutoff based on angular size, distance and astronomical seeing conditions.

It works out. For instance, Saturn's smallest angular diameter as seen from Earth is 15"

If you use the formula

$$\delta = 2 arctan \left (\frac{d}{2D} \right )$$

with d being the visual diameter and D the distance.

Plug in 15" (0.0042°) for $\delta$ and 100,000 meters for D, you get an answer of ~7 meters for the visual diameter at an altitude of 100 km.

 

[ideasrule]

Here's a description from the wiki on scintillation: It would surprise me if the disk for a planet was meters across near the "top" of the atmosphere (I'll calculate it when I'm more awake) but either way, that implies there must be a changing cutoff based on angular size, distance and astronomical seeing conditions.

As Janus said, the "disk" is indeed meters across. I agree that the cutoff is based on angular size, distance, and astronomical seeing conditions. The "planets don't twinkle" guideline is not meant as a strict rule; it just happens to be true given average seeing, average sensitivity of the human eye, and the sizes of the naked-eye planets in our solar system.

 

[Rogerio]

Why are there 4 "spikes" coming out of the star? What optical phenomenon causes this? Why do some stars twinkle and others not?

Thanks

Because the lens used to take the photo has a 4 blades diaphragm.

 

[Rogerio]

If the diaphragm had 6 blade (which is very usual) it would produce a 6 spikes star.
And an odd number of blades would double the amount of spikes: so, 5 blades -> 10 spikes.
More information http://en.wikipedia.org/wiki/Diaphragm_%28optics%29"

 

[hypatia]

I say, its because they can.

 

[russ_watters]

Because the lens used to take the photo has a 4 blades diaphragm.

No, it really is because of support structures. Irises tend to cause non-round lens flares, not diffraction spikes. More to the point, some of my pictures have diffraction spikes, but my rig has no iris and I create the diffraction spikes on purpose, by stringing wires across the objective of my telescope.

...besides, 4 bladed diaphragms are pretty rare afaik.

 

[mingpiasan]

google says,twinkling stars twinkle because of turbulence in the Earth’s atmosphere. Light from distant stars passes through various layers in the Earth’s atmosphere, and it gets refracted depending on the temperature and density of the air at that point. So light will pass through one layer, be refracted at one angle, and then pass through a different layer and be refracted at a different angle. When you see a twinkling star, you’re seeing the accumulated refractions from all those layers, which change the position and size of the star – many times a second.