Hertzsprung was born in Frederiksberg, Denmark, the son of Severin and Henriette.

He studied chemical engineering at Copenhagen Polytechnic Institute, graduating in 1898.

He began making astronomical observations in Frederiksberg in 1902, and within a few years had noticed that stars with similar spectral type could have widely different absolute magnitudes.

In this determination he made a mistake, possibly a slip of the pen, putting the stars 10 times too close.

[5] His wife Henrietta (1881–1956) was a daughter of the Dutch astronomer Jacobus Kapteyn.