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Interstellar Medium and Molecular Clouds

Interstellar space — the region between stars inside a galaxy — is home to clouds of gas and dust. This interstellar medium contains primordial leftovers from the formation of the galaxy, detritus from stars, and the raw ingredients for future stars and planets. Studying the interstellar medium is essential for understanding the structure of the galaxy and the life cycle of stars.

The Contents of Interstellar Space

Most of the space between stars is much emptier than the best vacuum we can make in a lab on Earth. However, it’s not truly empty: the interstellar medium (ISM) is complex, with a variety of physical processes governing its behavior. This is the place where stars are born, and where stars recycle their atoms when they die.

Hydrogen and helium make up about 98% of the mass of the ISM, with the remaining 2% being heavier elements like carbon, oxygen, and other elements astronomers call “metals”. Most of these atoms are in the form of gas, but about half of the heavier elements form dust: relatively large grains composed of carbon-containing molecules. These include polycyclic aromatic hydrocarbons, which on Earth are often produced by burning organic matter: their smell is the smell of charred hamburger on a grill. Even though dust only makes up about 1% of the ISM, it’s very good at absorbing visible light, particularly toward the blue end of the spectrum. As a result, starlight shining through a cloud of dust appears redder, and sometimes can be blocked entirely. For that reason, astronomers study these dust clouds in radio and infrared light, which passes through dust clouds more easily.

The ISM is made up of many parts:

  • Roughly half of interstellar gas is spread out through 98% of the space between stars. Though this “intercloud gas” is incredibly low density, it’s heated by the light from stars. The hottest part of the ISM can reach millions of degrees, and even the cooler parts are hotter than the surface of the Sun. The low density means intercloud gas only emits low levels of visible light. The hot ISM produces a diffuse X-ray glow, while hydrogen in the warm intercloud gas emits radio light with a wavelength of 21 centimeters (8.3 inches). This 21 cm emission allows researchers to map much of the hydrogen in galaxies, and even measure how fast galaxies rotate.

  • The other half of interstellar gas is compressed into 2% of the volume, and forms interstellar clouds. Most of these clouds are cold but still relatively low in density, which means hydrogen atoms don’t meet up to form molecules.

  • The densest interstellar clouds are the molecular clouds. Molecular clouds are very dense by ISM standards, and very cold: only about 10º above absolute zero. The most common component of these clouds is molecular hydrogen (H2). Other molecules include carbon monoxide (CO) — which astronomers often use to trace molecular cloud structure — and organic compounds such as methanol. While dust is present throughout the ISM, their concentrations are high enough in molecular clouds to make the clouds appear like a black opaque splotch. Molecular clouds range greatly in size and density, from small clouds less than a light-year across up to the giant molecular clouds (GMCs), which are over 100 light-years across and contain enough material to make several hundred thousand stars.

  • The densest, most opaque molecular clouds are where stars are born; when something disturbs the cloud, it can cause the gas and dust to collapse under its own gravity.

The rapidly-moving giant star Zeta Ophiuchi produces a shock wave in the interstellar dust, as seen in this infrared image from NASA's Spitzer Space Telescope.

Credit: NASA/JPL-Caltech