The interstellar medium is composed of multiple phases, distinguished by whether matter is ionic, atomic, or molecular, and the temperature and density of the matter. The interstellar medium is composed primarily by hydrogen followed by helium with trace amounts of carbon , oxygen , and nitrogen comparatively to hydrogen.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 is.
In all phases, the interstellar medium is extremely tenuous by terrestrial standards. In cool, dense regions of the ISM, matter is primarily in molecular form, and reaches number densities of 106 molecules per cm3. In hot, diffuse regions of the ISM, matter is primarily ionized, and the density may be as low as 10−4 ions per cm3. Compare this with a number density of roughly 1019 molecules per cm3 for air, and 1010 molecules per cm3 for a laboratory high-vacuum chamber. By mass, 99% of the ISM is gas in any form, and 1% is dust. Of the gas in the ISM, by number 91% of atoms are hydrogen and 9% are helium, with 0.1% being atoms of elements heavier than hydrogen or helium, known as "metals" in astronomical parlance. 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 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, molecular clouds , and replenish the ISM with matter and energy through planetary nebulae , stellar winds , and supernovae . 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.
ISM - useEdit
Union Fleet ships have until recently been equipped with emergency scooper equipment to gather hydrogen for fuel. However in the entire history of the Union fleet , only one ships (USS PT'Gara ) has used the scooping equipment in an emergency situation and was rescued before it could gather enough Hydrogen to even attempt to start the engines.
In 99.99 % of space there is simply not enough hydrogen to fuel a modern Union ship.
However there are regions with very dense matter nebulae where commercial operations use special scooper ships to gather dust and micro matter.
A special branch of the science corps is conducting ISM research as ISM research gives insights into star evolution. All Union ships, but especially Explorer and survey units are equipped to take ISM samples and analyze them (Sending the results to the ISM Institute on Petra )