<p>It is an optical illusion. Our brains associate brighter with bigger.  This is how software like stellarium are able to make convincing views of the night sky: they play this in reverse and use larger &quot;stars&quot; for brighter ones.  Your brain sees them the same.</p>
<p>Your brain is not tuned as a scientific device, there are lots of tricks that we can play on our visual system.</p>

CC BY-SA 4.0
2024-10-11T01:17:22.513Z
The same effect is seen with photographs.  So it is a real physical effect.
Greg Miller
CC BY-SA 4.0
2024-10-11T05:21:10.357Z
Or it is an effect of light on photographic films
James K
CC BY-SA 4.0
2024-10-11T11:34:30.583Z
Photographic “films” are not relevant; see @ProfRob’s answer.
Peter Erwin
CC BY-SA 4.0
2024-10-11T19:07:16.643Z
Okay, so "the effect of light passing through an appature and being detected on a light sensitive medium.  The point is that brighter stars are not actually bigger.  And the visual appearance of size is not actually anything to do with the airy disc on the retina of the eye, but the brain's processing of signals from the retina.
James K
CC BY-SA 4.0
2020-10-18T07:26:44.260Z
2020-10-18T07:26:44.260Z
James K
<p>It's a question of the angular resolution of your eye and the sensitivity of the retinal cells that detect light.</p>
<p>The image of a star on the back of the eye is not a dot, even with perfect vision. That is because the pupil has a finite diameter and therefore the best possible image is roughly an Airy disc with a characteristic angular size of about <span class="math-container">$1.2\lambda/D$</span>, where <span class="math-container">$D$</span> is the physical diameter of the pupil. This answer is in radians; for <span class="math-container">$D = 8$</span> mm, <span class="math-container">$\lambda = 500$</span> nm, this is about 15 arcseconds - not a point. And for many people the image is more blurred than this because of imperfect sight.</p>
<p>Now the term Airy <em>disc</em> is a bit misleading. The light distribution is not a uniform disc; it has a peaked distribution with fainter light in wings that are even broader than the angular size calculated above.</p>
<p><a href="https://i.sstatic.net/oJ3VqxeA.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/oJ3VqxeA.png" alt="Airy disc" /></a></p>
<p><em>Airy disc from <a href="https://commons.m.wikimedia.org/wiki/File:Airy_disk_D65.png#mw-jump-to-license" rel="nofollow noreferrer">here</a>. Image by &quot;SiriusB&quot;.</em></p>
<p>If the star is bright, then these fainter wings trigger retinal cells surrounding the central point and become visible, and the image of the star appears broader on the retina. Conversely, for a faint star, only the centrally peaked emission is seen.</p>
<p>In the picture above you can imagine that the light intensity has to reach a certain threshold to trigger a non-black response. If you scale the brightness of the Airy disc up or down then the non-black area would become larger or smaller.</p>

CC BY-SA 4.0
2024-12-24T16:57:18.953Z
Whew very late to this but thank you for the answer!
user177107
CC BY-SA 4.0
2024-10-11T06:53:00.707Z
2024-10-11T09:45:36.010Z
2024-10-11T09:45:36.010Z
ProfRob
<p>Look at Sirius and then some 2nd magnitude star. Sirius appears physically larger despite both stars subtending approximately the same angular diameter.</p>
<p>Callisto is far enough from Jupiter at its greatest elongation to be seen with the naked-eye, but a common reason it still isn't visible is because Jupiter's glare hides it.</p>
<p>To be clear, I'm not talking about Jupiter's additional reflected light making it harder to spot dim Callisto the same way the Moon makes nearby stars harder to see. Jupiter literally appears larger in the sky because of how bright it is. Why is that?</p>

CC BY-SA 4.0
2024-10-11T01:16:29.177Z
The short answer is diffraction.  Longer answer is to read about the Airy Disk: https://en.m.wikipedia.org/wiki/Airy_disk
Greg Miller
CC BY-SA 4.0
2024-10-11T12:14:17.180Z
And there's this little issue, that they *are* larger. Jupiter is huge next to Callisto.
Mithoron
<brightness>
Why do brighter stars/planets appear physically larger?
user177107
# Why do brighter stars/planets appear physically larger?

(Asked by: username_0 on 2020-10-18)

Look at Sirius and then some 2nd magnitude star. Sirius appears physically larger despite both stars subtending approximately the same angular diameter.

Callisto is far enough from Jupiter at its greatest elongation to be seen with the naked-eye, but a common reason it still isn't visible is because Jupiter's glare hides it.

To be clear, I'm not talking about Jupiter's additional reflected light making it harder to spot dim Callisto the same way the Moon makes nearby stars harder to see. Jupiter literally appears larger in the sky because of how bright it is. Why is that?

**Tags:** brightness

> **Comments:**
> **username_1**: The short answer is diffraction.  Longer answer is to read about the Airy Disk: https://en.m.wikipedia.org/wiki/Airy_disk  
> **username_2**: And there's this little issue, that they *are* larger. Jupiter is huge next to Callisto.  

---

## 2 answers

### Answer by username_5 on 2024-10-11. Score: 6 | Accepted answer

It's a question of the angular resolution of your eye and the sensitivity of the retinal cells that detect light.

The image of a star on the back of the eye is not a dot, even with perfect vision. That is because the pupil has a finite diameter and therefore the best possible image is roughly an Airy disc with a characteristic angular size of about $1.2\lambda/D$, where $D$ is the physical diameter of the pupil. This answer is in radians; for $D = 8$ mm, $\lambda = 500$ nm, this is about 15 arcseconds - not a point. And for many people the image is more blurred than this because of imperfect sight.

Now the term Airy *disc* is a bit misleading. The light distribution is not a uniform disc; it has a peaked distribution with fainter light in wings that are even broader than the angular size calculated above.

[![Airy disc](https://i.sstatic.net/oJ3VqxeA.png)](https://i.sstatic.net/oJ3VqxeA.png)

*Airy disc from [here](https://commons.m.wikimedia.org/wiki/File:Airy_disk_D65.png#mw-jump-to-license). Image by "SiriusB".*

If the star is bright, then these fainter wings trigger retinal cells surrounding the central point and become visible, and the image of the star appears broader on the retina. Conversely, for a faint star, only the centrally peaked emission is seen.

In the picture above you can imagine that the light intensity has to reach a certain threshold to trigger a non-black response. If you scale the brightness of the Airy disc up or down then the non-black area would become larger or smaller.

> **Comments:**
> **username_0**: Whew very late to this but thank you for the answer!  

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### Answer by username_3 on 2020-10-18. Score: 1

It is an optical illusion. Our brains associate brighter with bigger. This is how software like stellarium are able to make convincing views of the night sky: they play this in reverse and use larger "stars" for brighter ones. Your brain sees them the same.

Your brain is not tuned as a scientific device, there are lots of tricks that we can play on our visual system.

> **Comments:**
> **username_1**: The same effect is seen with photographs.  So it is a real physical effect.  
> **username_3**: Or it is an effect of light on photographic films  
> **username_4**: Photographic “films” are not relevant; see @username_5’s answer.  
> **username_3**: Okay, so "the effect of light passing through an appature and being detected on a light sensitive medium.  The point is that brighter stars are not actually bigger.  And the visual appearance of size is not actually anything to do with the airy disc on the retina of the eye, but the brain's processing of signals from the retina.  

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