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What causes stars to appear as strange blobs sometimes as in this image?

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Source: Nikon P900 Stars- Real Stars and Planets vs NASA images. Stars are not what they tell us!

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    \$\begingroup\$ For the same reason they twinkle. \$\endgroup\$
    – Weather Vane
    Commented Nov 9, 2022 at 19:29

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That's a combination of defocus and distortion, most commonly caused by "seeing" -- the astronomical term for the refractive conditions of the air column between the lens and the star.

A star is effectively a point source, but air movement and density variations (similar to the ripples over a metal roof on a hot day) can momentarily distort the image. This is why classical star photographs (made with long exposures and a tracking telescope) show brighter stars as larger -- because they can adequately expose a larger area of film as the "seeing" makes the star image dance around.

Take a short exposure, however, and you can wind up with this, a momentary capture of the distorted image, without the time averaging that would come from exposing film for an hour or more.

Use a laser as an "artificial star" and take a bunch of these images in rapid succession, record them in a way to preserves the phase of the light, and then let a well programmed computer work on the data for a while, and you can do what's called "speckle interferometry" -- which was used to make the first images of stars (other than our own Sun) as a disk. Record a conventional video, however, and you may be able to pick up the clearest frames and add or average them to get much clearer images than the overall seeing would permit -- this is called "stacking" and is used to, among other things, record images at higher resolution than the actual sensor pixel size.

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    \$\begingroup\$ Camera and tripod vibration can also be a factor. I need to place a hard foam brace between my tripod legs to reduce tripod vibration. Camera vibration reduction systems can also cause issues due to electronic noise of the VR system. \$\endgroup\$
    – qrk
    Commented Nov 9, 2022 at 21:32
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    \$\begingroup\$ This answer is completely wrong; it's clearly a giant alien space angler fish and Earth is about to get eaten \$\endgroup\$
    – Whelkaholism
    Commented Nov 10, 2022 at 10:28
  • \$\begingroup\$ larger area of sensor, not film. 99% of astrophotography is done with a CMOS or CCD camera, not film. \$\endgroup\$
    – chili555
    Commented Nov 16, 2022 at 22:43
  • \$\begingroup\$ @chili555 You're correct, nearly all astrophotography has been on digital sensors for the past twenty-five years or so -- before that, only large observatories and government agencies could afford digital sensors and the ancillary equipment, and they had few advantages over film aside from being able to literally add up short exposures. Also "classical" star photographs would have been made on film or glass plates, going back to the 1890s. \$\endgroup\$
    – Zeiss Ikon
    Commented Nov 17, 2022 at 0:34

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