Å* unit

A very small unit of length used to measure the wavelengths of X rays and the distances between atoms in crystals (such distances are measured by shining X rays through the crystal). Symbol, Å* (no period). The unit is read as “angstrom star”.

J. A. Bearden¹ introduced this unit in 1965 to replace the X unit, defining its length by taking the wavelength of the tungsten Kα₁ line as exactly 0.209 010 0 Å*, a value chosen to make 1 Å* equal to 1 angstrom within 5 parts per million. To quote Bearden:

“This numerical value of the wavelength is now proposed for use with the W Kα₁ line to define the x-ray wavelength standard by the relation

λ_W
kα1 = 0.20290 10 0 Å*.

“This is a new unit of length which may differ from the angstrom by ±5 ppm (probable error), but as a wavelength standard it has no error. In order to clearly indicate that this unit is not exactly an angstrom, it is suggested that it be designated Å*.”

Later work has shown that the Å* unit is about 15 parts per million bigger than an angstrom. The value of the unit according to the 1986 CODATA recommendations is
1.000 014 81 × 10^-10 meters, with a one-standard-deviation uncertainty of ± 0.000 000 92 × 10^-10 meters.²

1.
J. A. Bearden.
Selection of the W Kα₁ as the X-Ray Wavelength Standard.
Physical Review 2nd series, volume 137, no. 2B, pages 455B – B461 (1965).

2.
E. Richard Cohen and Barry N. Taylor.
The 1986 CODATA recommended values of the fundamental physical constants.
Journal of Research of the National Bureau of Standards, volume 92, no. 2, page 1. (March-April 1987)

Table 3.