Composition of interstellar medium
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Basic Composition of the Interstellar Medium: Gas and Dust
The interstellar medium (ISM) is primarily made up of about 99% gas and 1% dust. The gas component is mostly hydrogen, existing in different forms: neutral atomic hydrogen (H I), ionized hydrogen (H II), and molecular hydrogen (H₂). The dust consists of tiny solid particles that contain heavier elements and play a key role in the chemistry and physics of the ISM Dick2019Gerin2012.
Phases of the Interstellar Medium: Atomic, Ionized, and Molecular
The ISM is structured into several phases:
- Cool atomic clouds: Mainly neutral hydrogen (H I).
- Hot ionized regions (H II regions): Composed of ionized hydrogen, often found around young, hot stars.
- Cold molecular clouds: Dominated by molecular hydrogen (H₂) and containing other molecules, these are the sites of star formation Dick2019Gerin2012Cox2005.
Elemental Abundances and Heavy Elements
The ISM contains a variety of elements beyond hydrogen and helium, including oxygen, carbon, nitrogen, magnesium, silicon, sulfur, iron, and others. However, the abundances of these heavy elements (metals) are generally lower than their values in the Solar System, as a significant fraction is locked up in dust grains rather than in the gas phase Gnaciński2006Fitzpatrick1996Witt2001.
- Gas-phase abundances: Elements like sulfur have gas-phase abundances similar to solar values, but others such as iron, manganese, chromium, and titanium are found at much lower levels, often less than one-third of solar abundance. This is because these elements are efficiently incorporated into dust grains .
- Dust composition: A large portion of elements like iron and oxygen is found in dust, with 65–90% of iron and 15–25% of oxygen bound in solid form. Dust grains are a major reservoir for these metals Pinto2013Pinto2010.
Chemical Homogeneity and Abundance Gradients
Studies using X-ray spectroscopy and other methods show that the ISM is relatively chemically homogeneous on large scales, with similar gas ionization ratios and dust mixtures along different lines of sight. There are, however, abundance gradients within the Galaxy, with the inner regions being more metal-rich Pinto2013Pinto2010.
Interstellar Molecules and Grains
The ISM also contains a rich variety of molecules, from simple diatomic species to more complex polyatomic molecules. These molecules are mostly found in dense molecular clouds and are important for the formation of new stars and planetary systems. Interstellar grains (dust) are composed of silicates, carbonaceous materials, and ices, and their exact composition is still being studied Aller1972Gerin2012Kwok2006.
Challenges in Determining ISM Composition
Determining the precise composition of the ISM is challenging because:
- A significant, but uncertain, fraction of heavy elements is locked in dust.
- The total mass, size distribution, and exact composition of interstellar grains are not fully known.
- Reference values from stellar atmospheres are becoming less reliable as more is learned about the diversity of stellar compositions .
Conclusion
The interstellar medium is a complex mixture of gas (mostly hydrogen in various forms), dust grains, and molecules. Heavy elements are present but often depleted in the gas phase due to incorporation into dust. The ISM is structured into different phases and appears chemically homogeneous on large scales, though abundance gradients exist within the Galaxy. Understanding the ISM’s composition is crucial for studying star formation, galactic evolution, and the lifecycle of matter in the universe Gnaciński2006Fitzpatrick1996Witt2001+6 MORE.
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Composition of the Interstellar Medium
The interstellar medium contains elements lower than their Solar System values, with Mg II, S II, Si II, and Zn II being more abundant in dense molecular clouds than in the Sun.
The chemical composition of the interstellar medium
In situ detection and chemical characterization of interstellar grains could improve our knowledge of heavy element abundances in the interstellar medium.
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