The Baade-Wesselink method is a method for determining the distance of a

Cepheid variable A Cepheid variable () is a type of variable star that pulsates radially, varying in both diameter and temperature. It changes in brightness, with a well-defined stable period (typically 1–100 days) and amplitude. Cepheids are important cosmi ...

star suggested by

Walter Baade Wilhelm Heinrich Walter Baade (March 24, 1893 – June 25, 1960) was a German astronomer who worked in the United States from 1931 to 1959. Early life and education Baade was born the son of a teacher in North Rhine-Westphalia, Germany. He fin ...

in 1926 and further developed by Adriaan Wesselink in 1946. In the original method the color of the star at various points during its period of variation is used to determine its

surface brightness In astronomy, surface brightness (SB) quantifies the apparent brightness or flux density per unit angular area of a spatially extended object such as a galaxy or nebula, or of the night sky background. An object's surface brightness depends on ...

. Then, knowing the

apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...

at these points in time the

angular diameter The angular diameter, angular size, apparent diameter, or apparent size is an angular separation (in units of angle) describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the ''visual an ...

can be calculated. Measurements are also taken of the

radial velocity The radial velocity or line-of-sight velocity of a target with respect to an observer is the rate of change of the vector displacement between the two points. It is formulated as the vector projection of the target-observer relative velocity ...

using

Doppler spectroscopy Doppler spectroscopy (also known as the radial-velocity method, or colloquially, the wobble method) is an indirect method for finding extrasolar planets and brown dwarfs from radial-velocity measurements via observation of Doppler shifts in ...

. This allows one to determine the speed at which the front surface of the star moves toward or away from us at various points in the cycle. Since the difference between this and the average speed is the derivative of the radius, one obtains the variation in radius. Combining this with the change in angular diameter gives the distance. It is now possible to measure the angular diameter of the pulsating star directly using optical interferometers, allowing a more accurate measurement of the star's distance. This newer technique is known as the geometric Baade–Wesselink method. A closely related technique is the

expanding photosphere method The expanding photosphere method (EPM) is a method used to measure distances to Type II supernovae. It was developed by Robert Kirshner and John Kwan in 1974, based on the Baade–Wesselink method (1926). EPM is a geometrical method that compare ...

, which can be used to determine the distance to

Type II supernova A Type II supernova or SNII (plural: ''supernovae'') results from the rapid collapse and violent explosion of a massive star. A star must have at least eight times, but no more than 40 to 50 times, the mass of the Sun () to undergo this type ...

References

{{Reflist Astrometry Cepheid variables