Stars appear larger than they are, could that be the stellar corona here around Sirius, making it appear n-times larger than the stellar body? NASA recently mapped the solar corona to extending 12 solar radii above the surface 1, the corona as a whole occupying 13x the suns diameter if viewed from a distance.

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1) NASA's STEREO Maps Much Larger Solar Atmosphere Than Previously Observed - nasa.gov

  • $\begingroup$ It's rather difficult to see the corona of a star since they're so far away. However, according to this article (scientificamerican.com/article/astronomers-find-optical), we've been able to detect them... It's just really hard to see. Since Sirius is rather close to us, that's why it's barely visible. $\endgroup$
    – MystaryPi
    Oct 24, 2018 at 22:06
  • $\begingroup$ you can write an answer if you want. I guess this platform has at least more than a few people, or, are you the go-to authoritarian? a plasma atmosphere surrounding a star, seems like a better model than Halley's "optical illusion" to explain why they appear larger than their stellar body is $\endgroup$
    – user24634
    Oct 24, 2018 at 22:10
  • $\begingroup$ Oh I was just coincedentially online when the question popped up. I'll leave some room for other people to answer - I'm a little confused now on your question. Are you asking for viewing an actual corona of a star? If so, the corona is actually very small and probably won't be visible from here on Earth. Hope that helps? $\endgroup$
    – MystaryPi
    Oct 24, 2018 at 22:15
  • $\begingroup$ before the 1700s, people believed they were seeing the stellar body, and that the stars were larger than they are. in the 1700s, Edmond Halley and others suggested that they appeared larger because of an "optical illusion". a better model seems to be that the plasma atmosphere, that has in just the past few years been shown to for our sun be 13x the diameter of our sun, is scattering light, appearing as a glowing circle $\endgroup$
    – user24634
    Oct 24, 2018 at 22:22
  • $\begingroup$ as for if you are online or not, do not care. i'm interested in the scientific method. if I am wrong, then that is that. I am interested in the possibility that I am right, scientism is the belief that current models are never wrong. $\endgroup$
    – user24634
    Oct 24, 2018 at 22:24

1 Answer 1


Stars other than the Sun do not have photospheres or coronae that can be directly imaged (the exception would be the red giant Betelgeuse that can just be resolved from the HST). They "appear larger than they are" as you put it because of the limitations of physical optics or turbulence in the atmosphere. There is a minimum-sized "point spread function" image that will be convolved with the intrinsic image of a star, whatever its size. Thus even a point-like source will appear to have a finite size when viewed with a telescope with a finite aperture. Images of very bright objects may also be affected by "saturation" of the detector - the counts in saturated pixels can "bleed" into surrounding pixels. All stars (bar Betelgeuse) are too small and far away to be anything but intrinsically "point-like", even when observed with space telescopes.

The image of Sirius in your question is the saturated image of a very bright, point-like object blurred by the imperfect optics (and perhaps a turbulent atmosphere) on a telescope of finite aperture. Any "corona" you see on this picture is the extended wings of the telescope point spread function. The size of Sirius cannot be directly inferred from this picture. (It is also the case that Sirius, an A-type star, does not have a corona like the Sun's, because it lacks the magnetic dynamo and strong magnetic fields that are responsible for heating the solar corona, and it lacks the very strong winds that shock heat coronae in much hotter stars).

That said, there are clever, interferometric techniques to synthesise a large telescope using extended baselines between multiple telescopes. These are capable of directly measuring the angular diameters of stars. The assumption is that what is being measured is the angular diameter of the photosphere, since this is what dominates the light at optical and near-infrared wavelengths from most stars. In particular, the corona - a regions of tenuous, optically thin, ionised gas - emits hardly any visible/infrared light (it is of order a billion times less bright than the solar disk and due mainly to Thomson scattering of photospheric light from free electrons and dust scattering) and can safely be neglected. That is why the optical/infrared corona of the Sun can only be seen during total solar eclipses on Earth or from spacecraft with occulting disks that can totally block out the dominant light from the photosphere.

Distant stars are too far away for this occultation technique to work. The presence of any coronae around them can only be inferred from the X-ray emission due to the hot coronal gas. In some cases it has been indirectly argued that the coronal gas can extend several radii or more above the stellar surfaces (e.g. by analysing the fall and rise times of stellar X-ray flares or from the lack of X-ray eclipses in some eclipsing binary systems).

  • $\begingroup$ Jupiter for example, that atmosphere, in gas, reflects a lot of light. close to it it doesn't appear luminescent, but from Earth it does. the suns corona is up to 30 sun diameters, NASAs measurement was 13 sun diameters and science news reported 30 (have not gone to the source of that yet), that must be diffracting a significant amount of light, and must be visible from a distance if the surrounding region is dark enough $\endgroup$
    – user24634
    Oct 25, 2018 at 16:06

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