What are Herbig-Haro objects? Why are they often found in pairs?

The cosmic phenomenon of star formation is a captivating and intricate process that begins in the vast, dense regions within the interstellar medium known as molecular clouds. These clouds, rich in gases such as hydrogen, act as stellar nurseries where new stars are born.

Under the influence of gravity, regions within these clouds begin to condense and collapse, leading to the formation of a protostar — the early stage of a star's life cycle. As the protostar continues to accrete matter, its core gradually heats up until the temperature and pressure conditions are sufficient to initiate nuclear fusion, giving birth to a new star.

Throughout this transformative process, excess angular momentum can cause the formation of a circumstellar disk around the protostar. It is from this rotating disk that planets, asteroids, and other celestial bodies can eventually form, contributing to the development of a solar system. As the young star evolves, it is common for material to be ejected in the form of astronomical jets, a phenomenon closely linked to the Herbig-Haro objects observed by astronomers.

Imagining the space between stars as an empty void is a common misconception. In reality, this interstellar medium (ISM) is filled with a mix of gas, dust, and cosmic rays that play a crucial role in the lifecycle of a galaxy. It comprises multiple phases, from hot ionized gas to cold molecular clouds, each with its unique properties and importance.

The ISM is not a stagnant entity; it is dynamic and constantly influenced by the life cycles of stars. Supernova explosions, for example, can inject energy into the medium, spreading elements heavier than hydrogen and helium throughout the galaxy. This process, known as nucleosynthesis, is essential for the formation of new stars and planets as it enriches the interstellar medium with the building blocks required for their creation.

Within the cold, dense regions of the ISM, molecules such as hydrogen can form, leading to the creation of molecular clouds where new stars can be born. This cyclical interaction between stars and the interstellar medium is a fundamental process that drives the evolution and structure of the universe.

Astronomical jets are among the most spectacular displays in the universe, consisting of focused beams of matter that are propelled at high speeds from the regions around various types of celestial bodies, including newborn stars, dying stars, and even supermassive black holes at the centers of galaxies.

In the context of star formation, these jets occur when magnetic fields channel and accelerate matter away from the protostar. This phenomenon is most dramatic in the earliest stages of a star's life, when the star is still gathering mass. The twin jets are typically ejected in opposite directions, perpendicular to the circumstellar disk. This configuration further supports the angular momentum conservation of the accreting material.

The high-velocity streams within the jets can collide with the interstellar medium, leading to the formation of Herbig-Haro objects. The luminosity and dynamic structures of these objects allow astronomers a unique opportunity to study both the properties of the jets themselves and the nature of the young stars that produce them, giving valuable insights into the process of star formation.