A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, H2), and the formation of H II regions. This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas.
Astronomer Henk van de Hulst first theorized hydrogen could be traceable in interstellar space using radio signals.
Plaque commemorating the discovery of 21-cm radiation from the Milky Way
Left to right: Jan Oort, Hendrik C. van de Hulst, Pieter Oosterhoff. Jan Oort had a pivotal role in the research that lead to the discovery of molecular clouds.
Penzias and Wilson with the Holmdel horn antenna used to detect microwave emissions from the Big Bang
Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", collapse and form stars. As a branch of astronomy, star formation includes the study of the interstellar medium (ISM) and giant molecular clouds (GMC) as precursors to the star formation process, and the study of protostars and young stellar objects as its immediate products. It is closely related to planet formation, another branch of astronomy. Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of binary stars and the initial mass function. Most stars do not form in isolation but as part of a group of stars referred as star clusters or stellar associations.
The W51 nebula in Aquila - one of the largest star factories in the Milky Way (August 25, 2020)
Hubble Space Telescope image known as Pillars of Creation, where stars are forming in the Eagle Nebula
Assembly of galaxy in early Universe.
ALMA observations of the Orion Nebula complex provide insights into explosions at star birth.