Holmberg radius

The Holmberg radius is a measure of the angular extent of a galaxy in the sky. It is named after the Swedish astronomer Erik Bertil Holmberg , who suggested this measure.

definition

The Holmberg radius is defined as the length of the major semi-axis of the galaxy up to the isophote (the curve of equal brightness) at which the surface brightness

${\displaystyle \mu _{H}=26{,}5{\frac {mag}{arcsec^{2}}}}$

is. Measurements are made in what is known as the B-band ( in astronomy, the “ band ” denotes a certain area of ​​the electromagnetic spectrum in which the brightness is measured. The B-band is in the area of blue light ). The Holmberg radius indicates the area of ​​a galaxy from which most of the light comes (26.5 magnitudes correspond to about 1 to 2 percent of the brightness of the night sky).