distant galaxies, and the appearance of galaxies can change depending on the wavelength of light in which they are observed. Nonetheless, the Hubble sequence is still commonly used in the field of extragalactic astronomy and Hubble types are known to correlate with many physically relevant properties of galaxies, such as luminosities, colours, masses (of stars and gas) and star formation rates. In June 2019, citizen scientists through Galaxy Zoo argued that the usual Hubble classification, particularly concerning spiral galaxies, may not be supported by evidence. Consequently, the scheme may thus need to be updated. See also Galaxy color–magnitude diagram Galaxy morphological classification References External links Galaxies and the Universe – an introduction to galaxy classification Near-Infrared Galaxy Morphology Atlas, T.H. Jarrett The Spitzer Infrared Nearby Galaxies Survey (SINGS) Hubble Tuning-Fork, SINGS Spitzer Space Telescope Legacy Science Project Galaxy Zoo – Galaxy classification participation project. Sequence Astronomical classification systems Extragalactic astronomy