INTERSTELLAR MEDIUM AND STAR FORMATION

In our Milky Way galaxy there are about a hundred billion stars, and it's mostly stars what we see when we look at the sky at night. The space between the stars may seem empty - but it isn't. It is filled with tenuous gas, mostly consisting of hydrogen. The hydrogen gas is mixed with very small quantities of atoms and molecules of other heavier elements as well as with small amounts of dust particles (silicates, graphite). Gas and dust together are called the interstellar medium (or ISM). The ISM is an important part of a galaxy’s cycle of life, because stars form in and from it, and when they die, their material is recycled for a next generation of stars.

Most of the gas and dust in the Galaxy is at very low temperatures (10-100 degrees above absolute zero), and therefore observable mainly at radio, millimeter and infrared wavelengths. At the Istituto di Radioastronomia we study the process of star birth, for example in the dense shells of gas and dust that are being swept up at the edges of nebulae of expanding gas around young hot stars. We are also interested in studying the physical conditions and chemical properties of interstellar clouds just before and after a star is born: many interesting phenomena are associated with these very early moments in a star’s life, and they determine what the mass of the star is going to be. Spectra of molecular clouds are made to identify new molecules, with the aim to get a better understanding of the network of chemical reactions that take place in interstellar space. High-resolution spectra are also used to search for variations in fundamental physical parameters with cosmological time.

Near the end of its life, a star like our Sun will blow away its outer layers, leaving a stellar nucleus surrounded by a big expanding envelope of gas and dust. These circumstellar envelopes contain many molecules, such as water and carbon monoxide, that emit at characteristic wavelengths in the cm and mm region of the spectrum. Observing this emission allows us to follow the evolution of the envelope and to learn about the origin of the mass loss. The conditions in the expelled gas are such that it allows the formation of water masers: very intense, nearly monochromatic radiation; like a laser, but at mm- wavelengths. These masers, and those in regions of star formation, can be seen over large distances, and many of these objects are observed regularly with the 32-m antenna here at Medicina.