F2 - general properties of ISM gas

• hydrogen makes up $73\mathrm{\%}$ by mass, $X=0.73$ ($0.7-0.75$)
• helium makes up $25\mathrm{\%}$ by mass, $Y=0.25$ (%0.24-0.28%)
• 'metals', which are atoms heavier than He, make up the remaining $2\mathrm{\%}$, $Z=0.02$ ($0.015-0.02$)

metals

• metals are mostly due to the output of the CNO cycle in stars, ie. the fusion of $H\to He$ for stars with $M>M\odot$ using carbon, nitrogen, and oxygen as catalysts

• thus, carbon, nitrogen, and oxygen dominate the composition

• heavier elements are processed by massive stars and expelled via supernova explosions, with iron dominating

• metals can be ionised multiple times, indicated by adding roman numbers to the atomic symbol, eg: O I for neutral, O III for double-ionised

• forbidden transitions are indicated with [], eg: [O III], which are hard to get at lab conditions

• semi-forbidden are indicated with ], eg: O I], which are rare, but not completely absent at lab conditions

• extreme ionisation states exist, eg: Fe XXV in the solar corona

atomic spectra

• atoms can absorb/emit photons at very specific frequencies (hence, energies), quantised according to QM

• electrons can shift between orbital levels with structures defined by the quantum numbers $n,l,m_l$

• the principle quantum number $n$ gives the energy shells

• $m_l$ produces degenerate energy levels unless an external magnetic field is present

• fine structure of atomic levels arises from the interaction between the electron spin and the orbital angular momentum

• hyper fine structure arises from the interaction between spins of the nucleus and the electron

molecular spectra

• energy levels of molecules come from quantised angular momentum ($J$ transitions)

• molecules vibrate and/or rotate along many different degrees of freedom

• thus, they have a rich spectrum originating from these modes

• thus, rather than referring to absorption or emission lines, refer to bands

• energy transitions due to rotation and vibration occur at much lower energies than orbital transitions in atoms

• thus, the molecular spectra are mostly formed in the infrared

• similar to how GHGs in earth's atmosphere let sunlight (visible) in but block thermal emission ($10\mu$m) due to energy levels occurring at energies compatible with the infrared