You can’t count what you can’t see. Just like door-to-door census takers have a hard time accounting for the homeless population of a nation, astronomers have a hard time understanding a total population that includes bright and faint objects. This goes for stars (we can see brilliant O giants much further away than dim M dwarfs) and for galaxies.
Diffuse galaxies have stars spread over a large area relative to their overall brightness making them hard to find. “With surface brightnesses up to 250 times fainter than the night sky, these galaxies can be incredibly difficult to detect.”
Fainter than the dark sky! That doesn’t mean that they’re undetectable from the ground (you can subtract the sky brightness which is very hard but doable). But it sure is easier from space where there’s no sky to interfere. At that point, solar system dust and galactic gas+dust start becoming the dominant contaminants.
Originally shared by SETI Institute
Hiding in the night sky
This striking NASA/ESA Hubble Space Telescope image captures the galaxy UGC 477, located just over 110 million light-years away in the constellation of Pisces (The Fish).
UGC 477 is a low surface brightness (LSB) galaxy. First proposed in 1976 by Mike Disney, the existence of LSB galaxies was confirmed only in 1986 with the discovery of Malin 1. LSB galaxies like UGC 477 are more diffusely distributed than galaxies such as Andromeda and the Milky Way. With surface brightnesses up to 250 times fainter than the night sky, these galaxies can be incredibly difficult to detect.
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