Herbig-Haro object

A Herbig-Haro object is the glowing scar where a young star's jet smashes into the surrounding cloud. When a protostar fires its narrow jets outward, those supersonic streams eventually crash into the slower interstellar gas around them, and the violent collision heats the gas until it lights up in bright, knotty patches and arcs. Those luminous shock fronts, often hanging in space far from the star that launched them, are Herbig-Haro objects.

Named after George Herbig and Guillermo Haro, who studied them in the 1950s, HH objects are not stars themselves but the visible aftermath of stellar jets. A jet of gas from a young star travels at hundreds of kilometres per second; where it rams into a denser pocket of cloud, or where faster gas catches up with slower gas it ejected earlier, a shock wave forms that heats and excites the gas, which then glows, especially in the red light of hydrogen and the green light of ionized oxygen and sulphur. The result is a chain of bright knots and bow-shaped arcs, often strung out in a line that points straight back to the hidden young star, with a matching set on the opposite side from the other jet.

Herbig-Haro objects matter as signposts and as probes. Because they trace the direction and reach of a jet, they reveal newborn stars that are themselves still buried and invisible in ordinary light, and the pattern of knots records the star's recent history of accretion in fits and starts, like growth rings. They are short-lived on cosmic timescales, evolving visibly over decades, so astronomers can literally watch them move and change from one image to the next, giving a rare real-time view of star formation in action.