Herbig–Haro Objects

Herbig–Haro objects (HH) are small patches of nebulosity associated with newly born stars, and are formed when gas ejected by young stars collides with clouds of gas and dust nearby at speeds of several hundred kilometres per second. Herbig–Haro objects are ubiquitous in star-forming regions, and several are often seen around a single star, aligned along its rotational axis. HH objects are transient phenomena, lasting not more than a few thousand years.

The objects were first observed in the late 19th century by Sherburne Wesley Burnham, but were not recognised as being a distinct type of emission nebula until the 1940s. The first astronomers to study them in detail were George Herbig and Guillermo Haro, after whom they have been named. Herbig and Haro were working independently on studies of star formation when they first analysed Herbig–Haro objects, and recognised they were a by-product of the star formation process.

Studies showed HH objects were highly ionised, and early theorists speculated they might contain low-luminosity hot stars. However, the absence of infrared radiation from the nebulae meant there could not be stars within them, as these would have emitted abundant infrared light. Later studies suggested the nebulae might contain protostars, but eventually HH objects came to be understood as material ejected by nearby young stars, and colliding at supersonic speeds with the interstellar medium, with the resulting shock waves generating visible light.

Literary Note
At the time Harry Turtledove wrote this story, astronomers still thought HH objects contained protostars and were unaware they were material ejected by nearby young stars. As such, the story describes the objects based on the older, incorrect theory.