"The interstellar medium (ISM) is the matter and radiation that exist in the space between the star systems in a galaxy."
The matter and radiation that exists in the space between stars in a galaxy. Topics include gas-phase and solid-state chemistry, dust grains, and the formation and destruction of molecules in space.
Molecular clouds: Giant clouds of gas and dust in space where stars and planets form.
Star formation: The process by which new stars are born in molecular clouds through gravitational collapse.
Extinction and reddening: The absorption and scattering of light by dust particles in the interstellar medium.
Spectroscopy: The study of the absorption and emission of light by molecules in the interstellar medium, used to identify their chemical composition and physical properties.
Astrochemistry: The study of the chemistry of the interstellar medium, including the formation and destruction of molecules and the reactions that occur in these environments.
Isotopes: Variations of chemical elements with different numbers of neutrons in their nucleus, used to study the history and evolution of the interstellar medium.
Ionization: The process by which atoms lose or gain electrons, driven by radiation from stars and cosmic rays.
Shock waves: Powerful disturbances in the interstellar medium caused by supernova explosions and other high-energy events.
Interstellar dust: Tiny particles of material in the interstellar medium, including silicates, carbonaceous materials, and ice.
Molecular ions: Charged molecules that play a key role in the chemistry of the interstellar medium, including ion-molecule reactions and the formation of complex organic molecules.
Atomic gas: This type of interstellar medium is composed mainly of neutral hydrogen and/or helium atoms.
Molecular gas: This type of interstellar medium is composed mainly of molecules such as H2, CO, and OH.
Dust: Interstellar dust is composed of small particles of various materials, including carbon and silicon.
Ionized gas: This type of interstellar medium consists of gas that has been ionized by radiation from nearby hot stars.
Magnetic fields: Interstellar magnetic fields play a critical role in shaping and controlling the interstellar medium.
Cosmic rays: These high-energy particles are constantly bombarding the interstellar medium, affecting its properties and dynamics.
Turbulence: Turbulence is an essential feature of the interstellar medium, driving the formation and evolution of stars and galaxies.
Shock waves: These powerful waves are generated by supernova explosions and other energetic events in the interstellar medium.
Stellar winds: Hot, high-speed winds from massive stars can have a significant impact on the surrounding interstellar medium.
Jets: Jets are narrow, high-speed streams of gas and particles generated by accretion disks around stars and black holes.
"This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays."
"It fills interstellar space and blends smoothly into the surrounding intergalactic space."
"The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field."
"...the ISM behaves as a gas (more precisely, as a plasma: it is everywhere at least slightly ionized), responding to pressure forces, and not as a collection of non-interacting particles."
"The interstellar medium is composed primarily of hydrogen, followed by helium with trace amounts of carbon, oxygen, and nitrogen."
"The thermal pressures of these phases are in rough equilibrium with one another."
"Magnetic fields and turbulent motions also provide pressure in the ISM, and are typically more important, dynamically, than the thermal pressure."
"In the interstellar medium, matter is primarily in molecular form and reaches number densities of 1012 molecules per m3 (1 trillion molecules per m3). In hot, diffuse regions, gas is highly ionized, and the density may be as low as 100 ions per m3."
"Compare this with a number density of roughly 1025 molecules per m3 for air at sea level, and 1016 molecules per m3 (10 quadrillion molecules per m3) for a laboratory high-vacuum chamber."
"By mass, 99% of the ISM is gas in any form, and 1% is dust... By mass this amounts to 70% hydrogen, 28% helium, and 1.5% heavier elements."
"The hydrogen and helium are primarily a result of primordial nucleosynthesis."
"While the heavier elements in the ISM are mostly a result of enrichment (due to stellar nucleosynthesis) in the process of stellar evolution."
"The ISM plays a crucial role in astrophysics precisely because of its intermediate role between stellar and galactic scales."
"Stars form within the densest regions of the ISM."
"This interplay between stars and the ISM helps determine the rate at which a galaxy depletes its gaseous content, and therefore its lifespan of active star formation."
"Voyager 1 reached the ISM on August 25, 2012."
"making it the first artificial object from Earth to do so."
"Interstellar plasma and dust will be studied until the estimated mission end date of 2025."
"Its twin Voyager 2 entered the ISM on November 5, 2018."