Download Interstellar Medium in Astronomy II: Composition, Structure, and Star Formation - Prof. Do and more Study notes Astronomy in PDF only on Docsity! ASTR-1020: Astronomy II Course Lecture Notes Section IV Dr. Donald G. Luttermoser East Tennessee State University Edition 4.0 Abstract These class notes are designed for use of the instructor and students of the course ASTR-1020: Astronomy II at East Tennessee State University. Donald G. Luttermoser, ETSU IV–3 b) Matter this diffuse is far less dense than the best vacuums obtained in laboratories on Earth (about 1010 atoms/m3). c) Two distinct regions exist in the ISM: i) Giant Molecular Clouds (GMC) where densi- ties range between 107 to 109 particles/m3 (10 to 1000 particles/cm3) and temperatures range be- tween 10 to 50 K. ii) Hot Diffuse Gas where densities typically lie in the 104 to 105 particles/m3 (0.01 to 0.1 particles/cm3). Hot Gas [dark shaded area] (0.1 atoms/cm3) (T = 106 K) Giant Molecular Clouds (10 - 1000 atoms/cm3) (T = 10 - 50 K) B. Emission Nebula 1. The GMCs are frequently associated with stellar nurseries — regions in space where stars are born. a) Historically, astronomers have used the term nebula to refer to any “fuzzy” patch (bright or dark) on the sky. IV–4 ASTR-1020: Astronomy II b) We have already mentioned the “dark” nebula above. There are however two different types of bright (also called emission) nebula. These nebula are bright since they ale located near hot, luminous, massive stars. i) Reflection nebula result when starlight from these massive stars scatter from dust particles in the nebula. Some of the starlight gets scattered towards the direction of the Earth (the light is actually scattered in all directions). Such nebula typically have a bluish appearance from the Mie scattering by dust. ii) H II Regions result when this portion of the ISM is close enough to the massive stars such that the starlight actually ionizes the hydrogen gas (as well as other species) in the cloud (note that Ro- man numeral 2, “II” is the ionization state of this atom: ‘I’ for neutral, ‘II’ for singly ionized, ‘III’ for triply ionized, etc.). These nebula glow as free electrons recombine with the ions and emit pho- ton as the electron cascade back down the various bound levels in the atom. Such nebula typically have a reddish appearance (when photographed) due to the Hα transition in hydrogen which is in the red part of the spectrum. 2. Besides seeing nebula, the existence of the ISM can be seen by: a) Narrow absorption lines seen in the spectra of stars — lines formed in stellar atmospheres are broader due to pressure broadening (see text for further details). Donald G. Luttermoser, ETSU IV–5 b) Forbidden lines — these lines are normally not seen in stellar atmospheres since they only be formed in low den- sity gas (see text for further details). 3. Stars are formed out of the ISM! We know this from noting that the following objects are all found where there is a lot of ISM: a) Young open star clusters are often associated with neb- ula. All stars form in clusters when they are first formed. b) Protostars are seen in GMCs as strong infrared sources. c) T-Tauri type stars (see the next section) are also seen near GMCs. 4. Within the last 60 years, we have learned how stars are born. We see star formation going on at the present time in vari- ous regions of our Galaxy (called star-forming regions or as previously mentioned, stellar nurseries) — brand new stars are currently coming on-line! The Sun (and whole Solar System) formed in its own stellar nursery some 4.6 to 5 billion years ago. We will describe this star-formation process in detail in the next section.
