While I was looking at a wide-angle photograph of the Milky Way I wondered: Is it possible that some of the stars in the image have yet to be identified and/or cataloged? My answer to this is no, but I would like confirmation or counterexamples for my reasoning:

  • The usual cited number of stars in the entire night sky that you can see with the naked eye (up to magnitude 6) is from 2500 to 10000 [1]
  • With a telescope and long-exposure/stacking on a camera, stars of magnitude 10 to 14 can be seen, perhaps even fainter
  • The Gaia star catalog (although not fully released [2]) contains data for 1.7 billion stars brighter than magnitude 20.7

Assuming that a telescope/camera combination can detect 100 times as many stars than the naked eye can in the entire night sky, if we calculate the proportion of visible to cataloged stars: $$ \frac{n_{visible}}{n_{cataloged}} = \frac{100 \cdot 10e3}{1.7e9} \approx 0.05\% $$

I would consider this a low enough value to conclude that every star above a certain magnitude has been detected and cataloged. Even seeing a thousand times more stars than the naked eye would give you 0.5% - a negligible proportion of the star catalog.

Is this justifiable? Can these assumptions be challenged?

[1] - https://earthsky.org/astronomy-essentials/how-many-stars-could-you-see-on-a-clear-moonless-night

[2] - https://irsa.ipac.caltech.edu/Missions/gaia.html


1 Answer 1


Variable stars might challenge this. A distant Mira type variable or recurrent nova could have been at its minimum when the (for example) Gaia catalogue was being assembled, but appear in an amateur photographs.

These stars undergo very large variation in brightness. Mira can increase in brightness by 8 magnitudes, so it is not inconceivable that an uncatalogued Mira type variable could be found in an amateur photograph. Yet have been omitted from other catalogues by bad luck.

Recurrent novae are also "variable stars". They also undergo large changes in brightness. As they spend many years at minimum brightness and then suddenly flare, there is a greater chance that they could have been missed in the Gaia and other catalogues.

However, these are rather special cases. In general, any star that an amateur could image with reasonable equipment will have been photographed, identified and catalogued by one of the sky surveys. Automated image processing makes this possible. Many of the things that amateurs did in the 1980s and before are now done by robots and computers.

Supernovae have often been found in other galaxies by amateurs, with no catalogued progenitor (though supernovae might not be considered a "star")

  • $\begingroup$ New stars pop out of the nebula from which they formed too. Although with IR imaging, these may already be catalogued. Bok globules also move over time: en.wikipedia.org/wiki/Bok_globule to reveal background stars. $\endgroup$ Jan 6, 2021 at 17:12

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