# How bright would an object need to be in order to block out stars at night?

I am wondering just how bright a star or other object would have to be before it starts blocking out the night sky. I'm looking for an answer in apparent magnitude.

I'll note the Full Moon gets to magnitude -12.74 but doesn't come close to causing a blue sky like you see right before dawn.

The Sun has magnitude -26.74.

I also tried to gather info from other planets. Wikipedia has an article on extraterrestrial skies. There is an article on the martian sky too and I noticed the martian day has an illuminated sky with no stars, but unfortunately this is not relevant to me because I'm only considering the Earth's atmosphere. Mars' atmosphere is too different.

I also noticed this question and answer. It's really great info, but still missing a few key numbers needed to use the equations. For example, what is the sky's luminance (in mag/arcmin^2) when the sun is directly overhead, when the sun is at x angle, and ideally for any magnitude object at any angle.

• 'block out stars' is not a hard yes or no question. The moon does inhibit us to see stars, at least some as it rises the brightness-level of the sky so that we will not be able to see the faint stars anymore. So is your question about not seeing any star anymore? Commented Oct 23, 2023 at 20:21
• So is your question about not seeing any star anymore? Well i realize every star has a different magnitude. I guess the target mag is 0. How bright does a star/sun have to be before it blocks out all Mag 0 stars? Commented Oct 23, 2023 at 20:59
• This question and answers may be of use. worldbuilding.stackexchange.com/questions/250524/… Commented Oct 25, 2023 at 19:58

About magnitude -17, or about 100 times brighter than the moon now.

I've found reference to the sky brightness at full moon as being about magnitude 18 per square arcsecond (and equivalent to light pollution in the suburbs)

I've also found reference to the sky brightness twenty minutes after sunset as about 13 magnitude per arcsec², and this is roughly the point at which magnitude 0 stars appear.

Magnitude is a logarithmic scale and a difference of five magnitudes (from 18 to 13) corresponds to a hundred-fold difference in brightness. Hence if the moon were five magnitudes brighter it could hide most stars. The moon is magnitude -12 when full, so a magnitude -17 body would scatter enough light to hide most stars. Very bright stars (Sirius, Canopus etc) would still be visible, as would the bright planets.

The actual ability to see stars is against a bright sky is quite individual. And so to be reasonably sure to hide all stars from all observers you would need a moon that was much brighter than this.

Sources:

• I think i get it. So you want sky magnitude to go from 18 to 13, therefore Moon magnitude would have to go from -12 to -17. Very nice find on the data, thank you. Commented Oct 25, 2023 at 20:54
• That is my logic... I think it works out, but I'm ready to get owned by the real physicists here. Commented Oct 25, 2023 at 21:01

Here is a link to a question about the visibility of stars in the moon light of a fictional planet. It might be of some use to you.

https://worldbuilding.stackexchange.com/questions/250524/how-to-find-visibility-of-stars-given-a-certain-moon-brightness

• Your link is broken, it just leads to your userpage on worldbuilding. Anyway, link only answers are to be discouraged. This could be expanded to an actual answer or converted to a comment (to suggest further lines of research to the OP) Commented Oct 25, 2023 at 15:35