The reason is, the Universe becomes transparent to optical light at times between 500-to-700 million years of age, with the most distant known galaxy existing in a rare “pocket” where the Universe is transparent at only 400 million years old. But various estimates for the time of formation of the very first stars, at redshifts of 20, 30 and 50, correspond to ages of the Universe of 177 million, 98 million and 46 million years, respectively. Even if the Universe were transparent to begin with, the wavelengths of light we’d look for — that strong Lyman-α emission line at 121.567 nanometers (UV light) — will be redshifted to wavelengths of 2,553 nm, 3,769 nm or 6,200 nm, depending on how early these stars formed.
Ethan Siegel — Why Hubble Will Never see the First Stars
Image by Eurritimia