A galaxy with a very high redshift discovered from its red colour. Hydrogen is very effective at absorbing radiation with wavelengths shorter than 91.2 nm (the Lyman limit), and all galaxies contain large amounts of hydrogen; hence galaxies are virtually dark at wavelengths shorter than 91.2 nm. This dividing point in a galaxy’s spectrum is termed the Lyman break. For a galaxy at a redshift of about 3, the Lyman break falls between the U and B photometric bands. The galaxy should therefore be seen in B but be virtually invisible in U, an effect called the U-band dropout. By looking for galaxies with just these properties, astronomers have found many at very high redshifts. About 1000 Lyman-break galaxies are now known. They have been important in investigations of galaxy evolution.