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A gravitational redshift is a redshift of electromagnetic radiation (EMR) from a source within a gravitational field that is pulling it away from the observer (i.e., within a gravitational potential well). This is in contrast to redshift due to the relative radial velocity between observer and source. Instead of gravity slowing down such EMR's speed, as it would a physical object, it redshifts the EMR. For example, light from Earth has a very-slightly-longer wavelength when viewed from space (further out in Earth's gravitational field) than when viewed from the Earth's surface. Higher mass and density increase the effect and black holes, neutron stars, white dwarfs, and galaxy clusters can produce a significant such redshift. Stars can produce a detectable redshift in their own spectral lines. Measurement of such a redshift can be useful observation data e.g., to determine its surface gravity, which can help determine the mass of the object. Gravitational redshift also underlies the Sachs-Wolfe effect.