The very small radius of Sirius B was recognized in the 1920s, and several efforts were made to measure the gravitational redshift of light reaching us from Sirius B, particularly by Walter S. It is not an object visible with the naked eye, but it can be observed with telescopes – the following image was taken with the Hubble Space Telescope Sirius B is the small object visible on the left-hand side the cross-like structure and the small ring around Sirius B are artefacts caused by the telescope’s optical systems: Perhaps the best known white dwarf is Sirius B, the white dwarf companion to the star Sirius you might have seen in the night sky. The sun is so large that we can only show some part of its disc here the earth is the pale blue dot and the White Dwarf the pale grey dot in front of that disc:Ĭonsequently, the shift should be much larger for them than for our sun – parts in 10,000 in wavelength, rather than parts in 1,000,000. The following illustration shows the relative sizes of our sun, the earth, and a White Dwarf star. This means that the stars astronomers call White Dwarfs, which are formed when low-mass stars like our sun have exhausted their nuclear fuel, are interesting candidates for observation: White dwarfs have masses close to that of the sun, but radii smaller by factors near 100. The expected amount of redshift for light from the surface of a massive object reaching a distant observer is proportional to the object’s mass divided by its radius. Gravitational redshift in white dwarf stars The gravitational redshift of light coming to us from the sun has also been observed, but the accuracy is not very good because of gas motions on the solar surface: Whenever light is emitted by a moving source, there is a motion-dependent frequency shift called a Doppler shift, and in the case of the sun, the Doppler shifts due to the moving gas are somewhat larger than the gravitational redshift due to the light having to climb out of the field of the sun. The gravitational redshift was first measured on earth in 1960-65 by Pound, Rebka, and Snider at Harvard University, who examined gamma rays emitted and absorbed by atomic nuclei. It is, therefore, perhaps best regarded as a test of that principle rather than as a test of general relativity. A combination of Newtonian gravity, a particle theory of light, and the weak equivalence principle (gravitating mass equals inertial mass) suffices. However, in contrast to the other two tests – the gravitational deflection of light and the relativistic perihelion shift –, you do not need general relativity to derive the correct prediction for the gravitational redshift. One of the three classical tests for general relativity is the gravitational redshift of light or other forms of electromagnetic radiation. Getting Sirius with the Hubble Space Telescope.
Gravitational redshift in white dwarf stars.