2.5 Interstellar Medium

Question 1

What is the observational arument for the ISM?

Answer 1

Some MW regions are not resolved into individual stars - they retain a “diffuse appearance”
- Must be material filling in the volume between stars

Question 2

What is the origin of the ISM?

Answer 2

Comes from

• Unprocessed material (never been in stars)
• Material processed by previous stellar generations released via supernovae
• Material expelled by very massive/young stars via winds

Question 3

What is the ISM made of?

Answer 3

Gas component: Atoms+molecules at very low density. Complex spectrum that absorbs/emits radiation at specific wavelengths

Dust component: Everything else - mainly micrometre Si-C aggregates
Absorbs radiation (extinction) and radiates it as a BB

Question 4

How is the gas in the ISM produced?

Answer 4

It is made by atoms (neutral or ionised) and molecules

Question 5

State the composition of the gas in the ISM

Answer 5

H: X = 0.73
He: Y = 0.25
Metals: Z = 0.02

Question 6

What are quantised frequencies?

Answer 6

Specific wavelengths which atoms can emit/absorb light

Question 7

What is fine structure?

Answer 7

The interaction between the spin of the electron and the electron orbital angular momentum

Question 8

What is the hyperfine structure?

Answer 8

The interaction between nuclear spin and electron spin

Question 9

What counts as a metal in astronomy?

Answer 9

Any element heavier than helium

Question 10

What percentage are metals in the ISM, and what process contributes to it?

Answer 10

At most 2% of mass of ISM

• Mostly due to output of the CNO cycle in stars
• C N O will dominate the composition

Question 11

How can we get heavier elements than the CNO cycle in the ISM?

Answer 11

They are processed by more massive stars and released via a supernova

Question 12

What is the notation for an ionised element, and how many times can something be ionised?

Answer 12

Notation is e.g. O-III (double ionised)
Can get FE XXV (24th ionisation level)
Elements can be ionised multiple times

Question 13

What is the notation for a forbidden transition?

Answer 13

It is enclosed in square brackets e.g. [ O-III ]

Question 14

Why do we get forbidden transitions?

Answer 14

Obseved in the density of lab experiments

- Collisions de excite atoms so the transitions cannot be observed effectively

Question 15

What are semi forbidden transitions, and what is their notation?

Answer 15

Lines that are rare, but not completely absent e.g. O-I ]

Question 16

Are forbidden transitions allowed in the ISM, and why?

Answer 16

Yes

- Density is high enough to excite the atom, but low enough for collisions to be negligible

Question 17

Where do the energy levels of the molecules in the ISM come from?

Answer 17

Quantised angular momentum (J transitions)

Question 18

What are molecular bands?

Answer 18

The very rich energy spectrum of molecules due to all the possible rotational and vibrational modes

Question 19

What part of the spectrum are molecular spectra observed in, and why?

Answer 19

Observed in the IR

- Energy transition related to rotation and vibration occur at much lower energy than orbital transitions