Herbig–Haro object
Hubble Space Telescope observations have revealed the complex evolution of HH objects over the period of a few years, as parts of the nebula fade while others brighten as they collide with the clumpy material of the interstellar medium.
First observed in the late 19th century by Sherburne Wesley Burnham, Herbig–Haro objects were recognised as a distinct type of emission nebula in the 1940s.
Herbig also looked at Burnham's Nebula and found it displayed an unusual electromagnetic spectrum, with prominent emission lines of hydrogen, sulfur and oxygen.
Herbig had initially paid little attention to the objects he had discovered, being primarily concerned with the nearby stars, but on hearing Haro's findings he carried out more detailed studies of them.
[2] Studies of the HH objects showed they were highly ionised, and early theorists speculated that they were reflection nebulae containing low-luminosity hot stars deep inside.
In 1975 American astronomer R. D. Schwartz theorized that winds from T Tauri stars produce shocks in the ambient medium on encounter, resulting in generation of visible light.
[8] When these jets collide with the interstellar medium, they give rise to the small patches of bright emission which comprise HH objects.
[9] Electromagnetic emission from HH objects is caused when their associated shock waves collide with the interstellar medium, creating what is called the "terminal working surfaces".
[6] Spectroscopic observations of HH objects' doppler shifts indicate velocities of several hundred kilometers per second, but the emission lines in those spectra are weaker than what would be expected from such high-speed collisions.
[11][12] Spectroscopic observations of HH objects show they are moving away from the source stars at speeds of several hundred kilometres per second.
[14][15] As they move away from the parent star, HH objects evolve significantly, varying in brightness on timescales of a few years.
Around 1% of the mass of HH objects is made up of heavier chemical elements, including oxygen, sulfur, nitrogen, iron, calcium and magnesium.
[7] The number of known HH objects has increased rapidly over the last few years, but that is a very small proportion of the estimated up to 150,000 in the Milky Way,[25] the vast majority of which are too far away to be resolved.
The bipolar jet is slamming into the surrounding medium at a velocity of 300 kilometers per second, producing two emission caps about 2.6 parsecs (8.5 light-years) apart.
[7][26] Located around 460 parsecs (1,500 light-years) away in the Orion A molecular cloud, HH 34 is produced by a highly collimated bipolar jet powered by a class I protostar.
[17] Nuclear fusion has begun in the cores of Class I objects, but gas and dust are still falling onto their surfaces from the surrounding nebula, and most of their luminosity is accounted for by gravitational energy.
They are generally still shrouded in dense clouds of dust and gas, which obscure all their visible light and as a result can only be observed at infrared and radio wavelengths.
[17] Class III objects (Weak-line T Tauri stars) have only trace remnants of their original accretion disk.
[34][35] HH objects associated with very young stars or very massive protostars are often hidden from view at optical wavelengths by the cloud of gas and dust from which they form.
Such deeply embedded objects can only be observed at infrared or radio wavelengths,[36] usually in the frequencies of hot molecular hydrogen or warm carbon monoxide emission.
