Herbig–Haro (HH) objects are bright patches of nebulosity associated with newborn stars. They are formed when narrow jets of partially ionised gas ejected by stars collide with nearby clouds of gas and dust at several hundred kilometres per second. Herbig–Haro objects are commonly found in star-forming regions, and several are often seen around a single star, aligned with its rotational axis. Most of them lie within about one parsec of the source, although some have been observed several parsecs away. 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 into the gas clouds of interstellar space. 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.
Infrared spectrum of HH 46/47 obtained by the Spitzer Space Telescope, showing the medium in immediate vicinity of the star being silicate-rich
HH 2 (lower right), HH 34 (lower left), and HH 47 (top) were numbered in order of their discovery; it is estimated that there are up to 150,000 such objects in the Milky Way.
HH 49/50 seen in infrared by the Spitzer Space Telescope
Image: Hubble Sees the Force Awakening in a Newborn Star (23807356641)
A nebula is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral, or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regions, such as in the "Pillars of Creation" in the Eagle Nebula. In these regions, the formations of gas, dust, and other materials "clump" together to form denser regions, which attract further matter and eventually become dense enough to form stars. The remaining material is then thought to form planets and other planetary system objects.
True color image of the Trifid Nebula, showing complex gas and plasma structure
Portion of the Carina Nebula
NGC 604, a nebula in the Triangulum Galaxy
Herbig–Haro HH 161 and HH 164.