Herbig–Haro object

Lore & Background

The first HH object was observed in the late 19th century by Sherburne Wesley Burnham, who noted a small patch of nebulosity near the star T Tauri. It was later called Burnham's Nebula but was not recognized as a distinct class. In the 1940s, George Herbig and Guillermo Haro independently studied several similar objects in the Orion Nebula. Herbig found that Burnham's Nebula displayed an unusual electromagnetic spectrum with prominent emission lines of hydrogen, sulfur, and oxygen, while Haro found that all such objects were invisible in infrared light. They met at an astronomy conference in Tucson, Arizona in December 1949, after which Herbig carried out more detailed studies. The Soviet astronomer Viktor Ambartsumian gave the objects their name, Herbig–Haro objects, and suggested they might represent an early stage in the formation of T Tauri stars.

Reader's Guide

Herbig–Haro objects are significant because they provide direct observational evidence of the star formation process, specifically the ejection of material from young stars. Their study has revealed that stars form by gravitational collapse of interstellar gas clouds, and that accreting material is ejected along the star's rotational axis in collimated bipolar jets. These jets carry away excess angular momentum, preventing the star from rotating too rapidly. HH objects are shock-induced phenomena, with shocks driven by collimated jets from protostars. Hubble Space Telescope observations have revealed their complex evolution over a few years, as parts of the nebula fade while others brighten upon colliding with clumpy interstellar material. They are visible-wavelength phenomena, but many are only detectable at infrared wavelengths, where they are called molecular hydrogen emission-line objects (MHOs). Their transient nature, lasting only tens of thousands of years, and their rapid changes make them key to understanding the early stages of stellar evolution and the interaction between young stars and their environment.

Did You Know?

• HH objects are transient phenomena that last around a few tens of thousands of years.
• They can change visibly over timescales of a few years as they move rapidly away from their parent star.
• Around 20–30% of the gas in HH objects is ionized near the source star, but this proportion decreases at increasing distances.

The Long Road to a Name

In the closing decades of the 1800s, astronomer Sherburne Wesley Burnham pointed a 36-inch refracting telescope at the young, variable star T Tauri from Lick Observatory and noticed a faint smudge of nebulosity nearby. He catalogued it as a simple emission nebula, and for half a century it went unremarked. It was not until the 1940s that George Herbig and Guillermo Haro, working independently on star-formation research, began examining a cluster of similar star-like nebulae in the Orion Nebula. Herbig detected an unusual spectrum rich in hydrogen, sulfur, and oxygen emission lines, while Haro noted that the objects vanished entirely in infrared light. Their paths crossed at a December 1949 conference in Tucson, Arizona, where Herbig, initially focused on the nearby stars, was inspired to dig deeper. Soviet astronomer Viktor Ambartsumian subsequently coined the name Herbig–Haro objects and proposed they marked an early phase in T Tauri star formation. Early speculation that they hid low-luminosity hot stars was ruled out by the infrared silence, and it was not until 1975, when R. D. Schwartz linked T Tauri stellar winds to shock-generated visible light, and later the proto-stellar jet in HH 46/47 was found, that the true shock-driven origin became clear.

From Collapse to Jet

Stars are born when vast clouds of interstellar gas succumb to their own gravity. As the cloud compresses, rising opacity traps radiative energy, heating the material until a hydrostatic balance halts the free-fall. Yet gas keeps spiralling inward along a rotating disk, and the dense central core becomes a protostar. A fraction of the infalling material is flung outward along the protostar's rotational axis as twin jets of partially ionised plasma. The precise physics that collimates these outflows remains only partly understood, but the prevailing view is that the accretion disk interacts with the star's magnetic field, slinging material away from the disk plane within a few astronomical units. Close to the source the flow fans out at angles of roughly ten to thirty degrees; farther out, at tens to hundreds of astronomical units, it tightens into a narrow beam. Beyond their role in shaping the nebula, these jets serve a critical mechanical function: they shed the excess angular momentum that accretion would otherwise pile onto the star, preventing it from spinning apart. When the high-speed plasma finally slams into the surrounding interstellar medium, the resulting shock fronts produce the luminous knots we recognise as Herbig–Haro objects.

Reading the Shocks

The light we see from a Herbig–Haro object is generated at what astronomers call terminal working surfaces—regions where the jet-driven shock wave slams into the ambient interstellar gas. The resulting spectrum is a blend of a continuous background and intense emission lines from both neutral and ionised atoms. Doppler-shift measurements reveal that the emitting material is racing away from the parent star at several hundred kilometres per second. Yet the emission lines are noticeably weaker than pure high-speed collision theory would predict, suggesting that some of the surrounding gas is already drifting along the jet axis at a lower velocity, softening the relative impact. Most HH knots sit within roughly one parsec, about 3.3 light-years, of their source star, though a few have been traced several parsecs out. They cluster in active star-forming regions, and multiple knots are frequently spotted around a single young star, neatly aligned with its rotational axis. The Hubble Space Telescope has been especially valuable here: by imaging the same nebulae over a span of just a few years, it has captured individual bright spots fading while new ones blaze to life as the jet ploughs through the clumpy, inhomogeneous material of the interstellar medium.

Fleeting and Hidden

Despite their brilliance in certain wavelengths, Herbig–Haro objects are fundamentally ephemeral. Each knot persists for only a few tens of thousands of years before the jet that powers it wanes, making them among the shortest-lived structures in the stellar nursery. On human timescales the change is still visible: over a period of just a few years, the Hubble Space Telescope has documented parts of a nebula dimming while other regions flare up as the advancing shock encounters fresh clumps of interstellar gas and dust. This rapid evolution means that the detailed morphology of an HH object is a snapshot of a process in motion, not a static portrait. Complicating ground-based and optical studies, many of these objects are effectively invisible at visible wavelengths because intervening dust and gas absorb their light. In such cases, astronomers turn to the near-infrared, where the emission from molecular hydrogen becomes detectable. Objects identified through this infrared signature are classified as molecular hydrogen emission-line objects, or MHOs, a designation that highlights the different observational window through which the same underlying jet-and-shock physics can be studied.