page_title: Interstellar medium
hierarchical_section_title: Interstellar medium
caption_attribution_description: Voyager 1 / Voyager 2 English: NASA photograph of one of the two identical Voyager space probes Voyager 1 and Voyager 2 launched in 1977. The 3.7 metre diameter high-gain antenna (HGA) is attached to the hollow ten-sided polygonal body housing the electronics, here seen in profile. The Voyager Golden Record is attached to one of the bus sides. The angled square panel below is the optical calibration target and excess heat radiator. The three radioisotope thermoelectric generators (RTGs) are mounted end-to-end on the left-extending boom. One of the two planetary radio and plasma wave antenna extends diagonally left and down, the other extends to the rear, mostly hidden here. The compact structure between the RTGs and the HGA are the high-field and low-field magnetometers (MAG) in their stowed state; after launch an Astromast boom extended to 13 metres to distance the low-field magnetometers. The instrument boom extending to the right holds, from left to right: the cosmic ray subsystem (CRS) above and Low-Energy Charged Particle (LECP) detector below; the Plasma Spectrometer (PLS) above; and the scan platform that rotates about a vertical axis. The scan platform comprises: the Infrared Interferometer Spectrometer (IRIS) (largest camera at right); the Ultraviolet Spectrometer (UVS) to the right of the UVS; the two Imaging Science Subsystem (ISS) vidicon cameras to the left of the UVS; and the Photopolarimeter System (PPS) barely visible under the ISS. Suggested for English Wikipedia:alternative text for images: A space probe with squat cylindrical body topped by a large parabolic radio antenna dish pointing upwards, a three-element radioisotope thermoelectric generator on a boom extending left, and scientific instruments on a boom extending right. A golden disk is fixed to the body.
context_page_description: In astronomy, the interstellar medium is the matter and radiation that exists in the space between the star systems in a galaxy. 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 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, of 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.
context_section_description: In astronomy, the interstellar medium (ISM) is the matter and radiation that exists in the space between the star systems in a galaxy. 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 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, of 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 (1 million 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 at sea level, and 10¹⁰ molecules per cm³ (10 billion 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 8.9% 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, which ultimately contributes to molecular clouds and replenishes 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.
  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 mission's end in 2025. Its twin, Voyager 2 entered the ISM in November 2019.
caption: Voyager 1 is the first artificial object to reach the ISM.
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