Galactic Astronomy
Interstellar medium
The interstellar medium (ISM) is the gas, dust, magnetic fields, radiation, and cosmic rays that occupy the space between stars within a galaxy. In the Milky Way it exhibits multiple thermally and dynamically distinct phases spanning densities from less than 10⁻³ to more than 10⁶ particles per cm³ and temperatures from about 10 to over 10⁶ K. Its composition, structure, and energy balance regulate star formation and connect stellar feedback to galactic evolution.
Sagittarius A*
Sagittarius A* is the compact radio source identified as the Milky Way’s central supermassive black hole, located about 26,700 light-years from Earth. Its mass is roughly four million times that of the Sun, and in May 2022 it became the second black hole ever imaged directly by the Event Horizon Telescope. In 2024, horizon-scale polarimetric imaging revealed strong, organized magnetic fields near its event horizon.