Herbig-Haro objects (HHOs) are astronomical phenomena consisting of jets of gas and dust emitted by young stars. They are named after George Herbig and Guillermo Haro, who independently discovered them in the 1950s.
HHOs form when a young star is still accreting material from its surrounding protoplanetary disk. As this infalling material collides with the star’s surface, it releases energy in the form of jets. These jets can be observed as bright nebulae when they interact with the interstellar medium (ISM).
The most famous example of an HHO is HH 30, which is located in the Taurus molecular cloud. This object was first studied in detail by John Bally and colleagues in 1987. HH 30 consists of two main lobes of emission, separated by a dark lane of dust. The southern lobe is brighter and closer to us than the northern lobe.
Bally and his team found that HH 30 was powered by a young star called IRS 4, which was surrounded by a disk of material. They also discovered that the outflow from IRS 4 had interacted with another nearby star, creating a shock front that heated up the gas and made it glow brightly.
Since then, many other HHOs have been discovered both in our own Galaxy and beyond. In 2010, for example, astronomers using the Atacama Large Millimeter Array (ALMA) found an HHO around a young star called HL Tauri. This object is much younger than HH 30; at only about 10 thousand years old, it represents one of the earliest stages in stellar evolution that we can observe directly.
HL Tauri’s HHO is very different from that of HH 30; it consists of just a single jet instead of two, and this jet appears to be precessing or wobbling as it moves away from the star. These observations suggest that HL Tauri’s HHO may be caused by something different than what powers HH 30; perhaps multiple jets are ejected periodically from different parts of the disk around HL Tauri, or maybe there is more than one young star involved in powering this outflow.