Herbig–Haro object

Herbig–Haro objects, often called HH objects, are bright patches of glowing gas found in space.

These objects are closely linked to the birth of new stars. They form when narrow jets of partially ionized gas are ejected from newborn stars. This gas travels at speeds of several hundred kilometers per second. When these high-speed jets collide with nearby clouds of gas and dust, they create shock waves. These collisions cause the material to glow brightly, creating the visible patches we call HH objects. They serve as important markers for astronomers studying how stars are born and evolve.

The process begins with the gravitational collapse of massive interstellar gas clouds. As the cloud collapses, its density increases and its temperature rises. This eventually creates a dense core known as a protostar. As material falls toward this core in a rotating disk, some of it is redirected. This material is ejected from the star along its rotational axis in two narrow, collimated bipolar jets.

These jets are made of plasma, which is partially ionized gas. It is believed that the interaction between the accretion disk and the stellar magnetic field accelerates this material. These jets are vital because they carry away excess angular momentum. Without this process, the star might rotate too rapidly and disintegrate.

HH objects are transient phenomena, meaning they do not last forever. They typically exist for only a few tens of thousands of years. Because the jets move so rapidly through the interstellar medium, HH objects can change visibly over just a few years. Observations from the Hubble Space Telescope have shown that parts of these nebulae can fade while others brighten. This happens as the jets strike clumpy material in space. Sometimes, faster streams of gas catch up to slower ones. These collisions create what scientists call "internal working surfaces." This constant movement and pulsing makes the objects appear to change shape and brightness over time.

Humans have been studying these objects for over a century. In the late 19th century, Sherburne Wesley Burnham first observed a patch of nebulosity near the star T Tauri. At the time, it was not recognized as a unique class of object. In the 1940s, astronomers George Herbig and Guillermo Haro independently studied similar objects in the Orion Nebula. They realized these nebulae were a byproduct of the star formation process. The Soviet astronomer Viktor Ambartsumian eventually gave them the name Herbig–Haro objects. He suggested they represented an early stage in the life of T Tauri stars.

These objects have very specific physical properties. Their temperatures typically range from 9,000 K to 12,000 K. This is similar to the temperatures found in other ionized nebulae. However, HH objects are much denser, with particle counts between a few thousand and tens of thousands per cubic centimeter. The composition of the gas is mostly hydrogen, which makes up about 75% of the mass. Helium accounts for about 24% of the mass. The remaining 1% consists of heavier elements like oxygen, sulfur, nitrogen, iron, calcium, and magnesium.

These elements help astronomers identify the objects through their unique emission lines.

There are many different examples of these cosmic displays. For instance, HH 46/47 is located about 450 parsecs from our Sun. It is powered by a binary protostar and features two emission caps spaced 2.6 parsecs apart.

Another example is HH 34, located in the Orion A molecular cloud. This object features bright bow shocks and a complex that stretches about 3 parsecs in length. While we have identified more than a thousand HH objects, scientists estimate there may be as many as 150,000 in the Milky Way galaxy. Most are simply too far away for us to see clearly with current technology.

Understanding HH objects helps us understand the broader lifecycle of stars. They are always found in star-forming H II regions and are often located near Bok globules. These are dark nebulae that contain very young stars. By studying the jets and the resulting shocks, astronomers can learn about the magnetic fields and disks surrounding protostars. They also learn how stars interact with the interstellar medium. This connection shows how the birth of a single star can influence the chemistry and structure of the entire surrounding space.