Their spectra contain moderately strong ultraviolet absorption at wavelengths below about 0.4 Î 1\/4 m to 0.5 Î 1\/4 m , while at longer wavelengths they are very featureless but slightly reddish . Their spectra contain moderately strong ultraviolet absorption at wavelengths below about 0.4 Î 1\/4 m to 0.5 Î 1\/4 m , while at longer wavelengths they are largely featureless but slightly reddish .