Setting up the stage#


It is commonly assumed by the public that appart from stars, our galaxy is empty, filled by void (as our eyes can see we we observe the night sky). And even though the vaccum we can achieve on Earth is higher than what we found in space, assuming that the medium between the stars (rightfully named the Interstellar Medium) is empty, is a big misconception.


  • Molecular Clouds

    • Observation

      • Extinction of star light

      • Distribution throughout the Galaxy

    • Properties

      • Molecular factories

      • Star nurseries



  • Status: flag alt >

  • Reviewed: ❌

  • Updated: 15/02/2023

The Interstellar Medium#

Galactic Ecocystem and Star lifecycle #

It is not unlikely in life that birth story start with death. So is the case for plants, whose death replenish the earth with organic matter that will make the terrain fertile for a new plant to emerge. Stars are no different in that respect and their life goes in cycle and the ISM is their ecocystem.


Fig. 8 Figure caption#

The Big Bang produced all the Hydrogen (as well as Helium and Lithium) present today in the universe. The rest of the elements of the periodic table (called metal by Astronomers), up to iron (Z = 26), has been produced in the center of stars in a process called nucleosynthesis. However, it is only when the star die in a massive explosion like supernovae, that those heavy elements are ejected throughout the Universe. We are now 14.6 Billion years post Big Bang and at the third generation of stars, galaxies have had time to form and they will be our host for the rest of this discussion. So what are galaxies made of ? Well, a supermassive Black hole at the center and then stars (may be solar systems) as far as our eyes can see, but this would be omitting the major constituent of the


Introduce the ISM

  • cycle

  • Stars produce Ashes during their lifetime that are dispersed into the Diffuse ISM

  • Big dust production when they die (supernovae)

Structure and environments #

ISM environments are interconnected and affect each other through a variety of physical and chemical processes, making the ISM a complex and dynamic medium.


Fig. 9 Figure caption#

Fig. 9 show those different environments represented in an Onion like structure.

flag alt > Onion layers

Coronal gas, also known as coronal plasma, is a type of hot, highly ionized gas that is found in the outermost atmosphere of stars, known as the corona, that is the source of the solar wind and of coronal mass ejections. It is characterized by its extremely high temperatures, typically around 1 to 10 million degrees Celsius. This is much hotter than the surface of the star, which is typically around 5,500 to 5,800 degrees Celsius for the sun. Coronal gas is composed mostly of hydrogen and helium, as well as small amounts of other elements such as carbon, nitrogen, and oxygen. The high temperatures of this gas cause the atoms to become highly ionized.

Coronal gas is a subject of ongoing research in astrophysics and heliophysics, as scientists try to understand how the corona is heated and how this relates to the solar wind and other solar activity



Temperature is around 8000K, density is around 0.1 to 1 particle per cubic centimeter, hydrogen is mostly neutral (H)


Temperature is around 8000 K, density is around 0.1 to 1 particle per cubic centimeter, hydrogen is mostly ionized (H+)













Evolution of material through the ISM

  • Diffuse ISM - radiation - no ice

  • Interconnectivity of the different environments - material exchange

  • From low to high density environments

  • Finally end up in the densest environments of the ISM, molecular clouds

Interstellar Dust Grains#

We have seen that dust grains are present in the ISM and play a key role in it's evolution but what are they and how do they come from

Formation and evolution #

Interstellar dust form through a combination of processes in the interstellar medium (ISM). One of them is the condensation from the ejecta of evolved stars. As stars come closer to the end of their lives, they often experience mass loss through winds and outbursts, which can lead to the formation of dust grains. We had a recent great example of such processes with the star Betelgeuse that is explain below:


Fig. 10 Betelgeuse Outburst (Artist impression) Source#

Scientific Observations

Sphere Instrument - ESO VLT


Fig. 11 Source#


Fig. 12 Source#

Review: Carbonaceous grains [Herrero et al., 2022]

Interstellar dust is the Ariane thread connecting star death with star formation after having evolved through the ISM.

Structure #


Fig. 13 Interstellar Dust#


  • Core

Silicate or carbonaceous material …

  • Ice layer

check formation processses

Condensation temperature

sub-µm dust grains

[Williams, 2000] - from ashes to dust

Here is a typical dust grain that can be found inside a Molecular clouds. Because low temperature the grain is surrounded by an icy layer.

This ice layer is responsible for most of the chemistry observed in the ISM

  • link to molecular zoo …



Introduction to dust

  • Ice - link to intro Thesis.A with more details

  • Dust processing within MC: preliminary growth up to µm sizes.

  • Role in chemistry

  • Ice formed by atomic deposition, diffusion, reaction of atomic species to form molecules.

Astrophysics of dust grains

  • physical optics of small particles

  • charging of dust grains

  • heating and cooling of dust grains

  • chemistry on dust grain surfaces

  • forces and torques on dust grains

  • sputtering and shattering of dust grains

  • alignment of dust with the magnetic field

introduction to these topics can be found elsewhere (Kr¨ugel 2002, Draine 2003a)

Molecular Clouds#

Observation #

‣ ‣ Extinction of star light #


Images source: ESO - Apod

B68 was first observed in 1785 by the great astronomers William Hershell, nonetheless he described it unapropriately as

" Hole in the sky "

Indeed, our dear William couldn’t have been more wrong (yes, it happens to everybody, including the greatest) [Steinicke et al., 2016]. Molecular Clouds are full gus and dust particles like we have described abova and this it the reason why they obstruct the light of stars in the background, a process called extinction. It is a very important concept because most of our knowledge of interstellar dust is derived from the interaction of dust particles with electromagnetic radiation:

‣ ‣ ‣ How does it work ? #

Absorption / Scattering

  • Attenuation of starlight Aλ

Wavelength dependance of interstellar extinction gives information about the size and composition of the grains. Best determined using the pair method.

  • RV

‣ ‣ Distribution #

Hover the figure to switch between images - Image 1 Gaia image of the Galactic plane - Image 2 Molecular cloud mapping

For an interactive view of the Galaxy at multiple wavelength over the whole electromagnetic spectrum, check Chromoscope .

The most famous Molecular Clouds (and some of their science)







Properties #


Filament structure

  • Article to extract and put in bib

‣ ‣ Molecular factory #

Quiescent over long timescales. Shield dust from radiation …

‣ ‣ Star nursery #


Fig. 14 Source#

link to teaching JWST to see more pictures

Star nurseries

Our fascinating story of stars and planet formation begins in the darkest and coldest places of the Interstellar Medium, Molecular Clouds. Those clouds, made of gas and dusts are the cocoons within which star forms. They evolve for […] years, …

  • Clumps

  • Critical mass - Jeans instability


Star nursery

  • Gas + Dust, High density, protected from radiation

  • Diverse molecular content, Chemistry - How ?

  • Surface of dust grains

Triggering star formation#

How does the process of star formation beggin ? not out of the blue

  • Contraction of molecular cloud OK but what else

A Brief History of our own Solar system #

4.6 Billion years ago …

The local Bubble#

[Zucker et al., 2022]


Go check the various star formation region in Figure X


Fig. 15 Source#

Pre Stellar Cores #

3D Modelling#

Arxiv - to download when published


Filament structure …


Hover the figure to switch between images - Image 1 Visible - Image 2 Infrared

We have seen that …

Next Steps