Interstellar medium

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Molecular cloud of interstellar gas and dust that has separated from the Carina Nebula . The image taken with the Hubble Space Telescope shows a section of about two light years .

The interstellar medium ( ISM ) is the name given to matter and the radiation and magnetic fields caused by its dynamics in interstellar space , the space between the stars of a galaxy .

Important components of the interstellar medium are gas in ionized , atomic and molecular form as well as dust , which are collectively referred to as interstellar matter . In addition, there are cosmic and electromagnetic radiation as well as the galactic magnetic field. All of these components interact with one another and have comparable energy densities. The term “interstellar matter” is sometimes used when actually referring to the entire interstellar medium - for example when it comes to the interaction with the solar wind .

The ISM plays an essential role in astrophysics , as stars are created from interstellar matter, which with stellar winds and supernovae also release matter into interstellar space. It causes the interstellar absorption and discoloration of starlight.


The ISM does not reach the stars. It is separated from the spheres of influence by the astropause of the stars, the astrospheres , which are filled by their stellar wind and have other properties. The interplanetary medium (IPM) is located within the astrospheres .

Matter, radiation and magnetic fields between galaxies are also not counted as part of the ISM, but rather referred to as the intergalactic medium (IGM).

The local interstellar medium ( LISM ) describes the interstellar medium in the vicinity of the sun . In the history of the solar system , the sun has crossed different regions in the Milky Way with different compositions of the interstellar medium. The sun is currently passing through the local flake .

Composition and distribution

The origin of the interstellar matter lies in the big bang , stellar winds and supernova explosions, whereby their mass fraction in the Milky Way is only a few percent. In the Milky Way, it consists on average of around 90% hydrogen , 10% helium (proportions) and traces of heavier elements, which are referred to as metals in astronomy , with 99% of the matter being present as gas and the proportion of dust being around one percent.

Neither the density nor the temperature of the interstellar matter are constant, rather it is very unevenly distributed between dense interstellar clouds and thin bubbles and super- bubbles . The density varies between 10 -4 atoms / cm³ in coronal gas and 10 5  atoms / cm³ in molecular clouds , the temperature range extends from 20 to 50  Kelvin in molecular clouds or infrared cirrus to several million Kelvin in coronal gas. Usually, three phases of interstellar matter are distinguished on the basis of the measured temperatures (according to McKee , Ostriker 1977):

  • hot - coronal gas with temperatures above a million Kelvin
  • warm - areas with temperatures of a few thousand Kelvin
  • cold - HI areas and molecular clouds with temperatures below 100 Kelvin
Composition of interstellar matter
component proportion of Temperature
(atoms / cm³)
Molecular clouds 20-50 10 3 -10 5 neutral hydrogen molecules
HI clouds 50-100 1-10 3 neutral hydrogen atoms
warm ionized medium (WIM) 50% 10 3 -10 4 0.01 partially ionized plasma
H-II clouds 10 4 10 2 -10 4 almost completely ionized plasma
coronal gas 10 5 -10 6 10 -4 -10 -3 fully ionized plasma


  • CF McKee, JP Ostriker: A theory of the interstellar medium - Three components regulated by supernova explosions in an inhomogeneous substrate . In: Astrophysical Journal 218, 1977, pp. 148-169.
  • Joachim Herrmann: dtv atlas astronomy . 15th edition. Deutscher Taschenbuch Verlag, Munich 2005, ISBN 3-423-03267-7 .
  • Alexander G. Tielens: The physics and chemistry of the interstellar medium. Cambridge Univ. Press, Cambridge 2005, ISBN 0-521-82634-9 .
  • Dieter Rehder: Chemistry in space - from interstellar matter to the origin of life. Wiley-VCH Verlag, Weinheim 2010, ISBN 978-3-527-32689-1 .
  • "Farewell to the Solar System" Voyager 2 left the heliosphere

Individual evidence

  1. Günther Hasinger: Introduction to Astrophysics - Interstellar Medium. (pdf) November 14, 2006, accessed May 31, 2018 .
  2. NZZösungen-von-der-heliosphaere-ld.1519717 removed. Nov. 19

See also