A star is born (Part 1: The Inter-Stellar Medium and dust clouds/nebulae)

In this series of post, I will try to understand how a star is created until it reaches the so-called “main sequence”. Another series of post will look at what is happening after the star leaves the main sequence.

In our galaxy, the Milky Way, the space between the stars is called the ISM or Inter-Stellar Medium. Note that, astronomers also talk about inter-galactic space outside our galaxy and between galaxies and inter-planetary space in the solar system between planets around the sun.

In this post, I mainly write about our local neighborhood of stars and the ISM that fills the volume of our own galaxy and I do not really look at what creates the ISM, I just assume that it is there.

Our Sun formed from the ISM, so it is not surprising that the chemical composition of the ISM in our region of the Milky Way is similar to the chemical composition of the Sun that is mainly hydrogen and helium.

In the ISM, about 90% of the atomic nuclei are hydrogen, and the remaining 10% are almost all helium. More massive elements together account for only 0.1% of the atomic nuclei, or about 2% of the total mass in the ISM. 99% of the matter in the ISM is in gaseous form, consisting of individual atoms or molecules moving freely about, as do the molecules in the air on earth.

Only 1% of the ISM matter is in solid form (solid grains/conglomerates of many molecules and atoms) also called interstellar “dust” but this type of dust has nothing to do with the dust you’ll find at home because it is much smaller. Sometimes this dust creates giant dust clouds or nebulae. In these clouds/nebulae, the dust can be C-based or “graphite”, Si-based or “silicate”, iron, water ice, etc. The analogy with the air and dust on earth ends here because the density of these cloud is on a quite different scale than what you would expect to find on earth.

In a volume of one cubic centimeter of air on earth you have about 10^19 molecules, a good vacuum pump can bring this density down to 10^10 molecules per cubic cm. By comparison, the ISM has in average a density of 1 ATOM per CUBIC cm (remember that one atom is small about 10^-8 cm) !!!!

Giant dust clouds (150-250 ly) within the ISM can have densities as high as 10 10^6 molecules per cubic centimeter. In these clouds the dust is extremely effective at blocking visible light. Because the average size of dust grain is about 10^-6m similar to the wavelength of visible light and therefore, can interfere with visible light. So, dust clouds/nebulae are opaque to visible light. To illustrate this, the picture below shows the Milky Way at different wavelengths. As we can see, gas and dust clouds, which are opaque and dark in visible light (c), are also glowing brilliantly in infrared radiation (b), the explanation for why this intense glow in the infrared is given below.


Dust clouds at millimeter wavelengths: the temperatures of interstellar dust particles depend on their optical properties and sizes (i.e., on the way they absorb and reemit radiation) as well as on the interstellar radiation field (the radiation that they receive from other stars). Most of the visible and UV radiation in galaxies from stars passes through clouds of particles and heats them. This heating leads to reradiation at much longer wavelengths extending to the millimeter (the millimeter wave spectrum lies between the microwaves and infrared portions). On the average, in spiral galaxies, ~ 25% to 33% of the total stellar radiation is converted into dust radiation/emission. The converted radiation / reemitted radiation, is a probe of the particles and the physical environments in which they find themselves. Recent studies have actualy shown that these dust clouds can contain simple organic molecules that can be found here on earth in a completely different environment.

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