The expanding photosphere method (EPM) is a method used to measure distances 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 ...

e. It was developed by Robert Kirshner and John Kwan in 1974, based on the Baade–Wesselink method (1926). EPM is a geometrical method that compares a supernova's angular size to its physical size determined from spectroscopic measurements. The method works by comparing a supernova photosphere's angular radius ''θ'' to its linear radius ''R'' to calculate its distance ''d'' through direct geometric calculation. This calculation requires determining two key parameters: the temperature of the ejecta's

photosphere The photosphere is a star's outer shell from which light is radiated. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately , or equivalently, a depth from which 50% of light will esc ...

and its expansion velocity. The temperature is typically found by fitting a

blackbody A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The radiation emitted by a black body in thermal equilibrium with its environment is ...

curve to the continuum spectrum. The photosphere's expansion velocity is calculated from the Doppler blueshift of specific absorption lines that form at the photosphere. Isolated, easily identified spectral lines should be used for calculations, because blended or misidentified lines can introduce significant uncertainties into the velocity measurement. Because the method relies on intrinsic luminosity of supernova, it is independent from the

cosmic distance ladder The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A ''direct'' distance measurement of an astronomical object is possible ...

, and doesn't require external calibration. The method is "observationally demanding" and requires a good quality spectra. The method requires correcting for atmospheric dilution effects using theoretically calculated dilution factors. These account for electron scattering in the supernova's atmosphere causing it to deviate from a perfect blackbody. Different sets of dilution factors have been published, notably by Eastman et al. (1996) and Dessart & Hillier (2005). EPM relies on several assumptions regarding supernovae photospheres: # the expansion of the ejected material is spherically symmetric # the ejecta is expanding homologously # the ejecta is optically thick # the photosphere radiates as a blackbody Modified EPM was used to measure the

Hubble constant Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther a galaxy is from the Earth, the faste ...

References

{{Reflist Supernovae