Herbig–Haro object
Bright patches of nebulosity from newborn stars' jets.
Herbig–Haro (HH) objects are bright, patchy nebulae linked to newly formed stars. They arise when narrow, high-speed jets of partially ionized gas, blasted out by a young star at hundreds of kilometers per second, slam into surrounding clouds of gas and dust. These objects are short-lived, lasting only a few tens of thousands of years, and they can visibly change in just a few years as they race away from their parent star into the interstellar medium.
HH objects are typically found in star-forming regions, and a single star often has several of them lined up along its rotation axis. Most lie within about one parsec (3.26 light-years) of their source, though some have been spotted several parsecs away. Observations by the Hubble Space Telescope have shown how complex their evolution is over just a few years: parts of the nebula fade while others brighten as they collide with clumpy interstellar material.
The first HH object was spotted in the late 1800s by Sherburne Wesley Burnham, who noticed a small patch of nebulosity near the star T Tauri using the 36-inch refractor at Lick Observatory. This patch, later called Burnham's Nebula, was initially thought to be an ordinary emission nebula and wasn't recognized as a distinct type. T Tauri itself turned out to be a very young, variable star—the prototype of T Tauri stars, which haven't yet reached hydrostatic equilibrium between gravitational collapse and nuclear fusion. About fifty years after Burnham's discovery, several similar nebular patches were found. In the 1940s, George Herbig and Guillermo Haro independently studied these objects in the Orion Nebula. Herbig also examined Burnham's Nebula and found its spectrum unusual, with strong emission lines of hydrogen, sulfur, and oxygen. Haro noted that all such objects were invisible in infrared light.
Herbig and Haro met at an astronomy conference in Tucson, Arizona in December 1949. Herbig had initially focused on the nearby stars rather than the nebulae, but after hearing Haro's results, he conducted more detailed studies. Soviet astronomer Viktor Ambartsumian gave the objects their name—Herbig–Haro (HH) objects—and, because they appear near very young stars (a few hundred thousand years old), suggested they might represent an early stage in T Tauri star formation. Early studies showed HH objects are highly ionized, and some theorists thought they were r
- discovered_by
- Sherburne Wesley Burnham (late 19th century)
- named_after
- George Herbig and Guillermo Haro
- type
- Emission nebula associated with star formation
- typical_velocity
- Several hundred kilometers per second
- typical_temperature
- 9,000–12,000 K
- typical_density
- A few thousand to a few tens of thousands of particles per cm³
- lifespan
- Around a few tens of thousands of years
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 using the 36-inch refracting telescope at Lick Observatory. This object, later known as Burnham's Nebula, was not initially 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 noted that all such objects were invisible in infrared light. They met at an astronomy conference in Tucson, Arizona in December 1949, after which Herbig conducted 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 and the ejection of material from young stars. Their study has revealed that jets from protostars collide with the interstellar medium, creating shock-induced emission. Hubble Space Telescope observations have shown their complex evolution over just a few years, with parts of the nebula fading and others brightening as they interact with clumpy interstellar material. HH objects also help astronomers understand how angular momentum is carried away from accreting stars, preventing them from spinning apart. Although visible-wavelength phenomena, many HH objects are obscured by dust and gas and can only be detected at infrared wavelengths, where they are called molecular hydrogen emission-line objects (MHOs). Their transient nature and rapid changes make them valuable for studying dynamic processes in star-forming regions.
Did You Know?
- HH objects can change visibly over timescales of a few years as they move rapidly away from their parent star.
- Most HH objects lie within about one parsec (3.26 light-years) of their source star, though some have been observed several parsecs away.
- The total mass ejected by stars to form typical HH objects is estimated to be 10⁻⁸ to 10⁻⁶ solar masses per year.
- Around 20–30% of the gas in HH objects is ionized near the source star, but this proportion decreases at increasing distances.
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