ISM

Fig. 18.5

Interstellar Matter

Interstellar matter (medium) = ISM

ISM is material located between the stars

distribution of ISM is patchy

Cold, T=100K

Composition:

Gas, mostly atomic and molecular H, He

Dust: silicates, graphite, iron, and dirty ice

Emission Nebula

UV radiation from O and B stars ionizes H

When electrons recombine with proton, emit visible radiation

Red color is from the prominent transition of the Ha line

Ha line occurs at 656.3 nm (red)

Extinction

Dust in the ISM absorbs light from distant stars, causes dimming effect

Dimming effect is known as extinction

Dust absorbs primarily blue light

This causes the observed spectrum to be much redder

Reddening

Dust scatters blue light more than red

This causes the observed spectrum to be much redder

Fig. 18.2

Scattering of Blue Light

Dust scatters primarily blue light, reddening the observed spectrum.

What Earthly event does this remind you of?

Are the two processes related?

Dark Clouds

Dust scatters and blocks light from background objects Fig. 18.15 (a) dark cloud in visible (b) CO molecular radio emission

Polarization of Light

Unpolarized light has randomly oriented electric (and magnetic) fields

Polarization is when all the electric are aligned in a common direction

Effect is similar to Polaroid filter

Polarization

Fig. 18.4

What We Learn from Polarization

Size and orientation of scattering molecules determines degree of polarization

Dust particles in ISM orient similarly to Polaroid filter molecules

Magnetic field of ISM causes dust particles to align

Absorption by IS Clouds

Fig. 18.14

Forbidden Transitions

In space, densities are low enough that electron has time to drop down naturally

Forbidden lines cannot occur on Earth because of the high densities leading to collisions that kick electron into another energy state

Fig. 18.11

Molecular Emission

Formaldehyde molecule , rotational transition

Fig. 18.18

Dust may protect molecules or aid in their formation

Molecular Clouds

Molecular clouds are found embedded in dense clouds

Dust may protect molecules from destruction by UV radiation or aid in their formation

Fig. 18.19 and 18.20: formaldehyde absorption and emission

Observing Atomic Hydrogen

21 cm radiation

Neutral Hydrogen

Alignment of electron spin relative to proton spin

Spin is quantized

Atomic collisions cause higher energy state to be populated

Observing Atomic Hydrogen

21 cm radiation

Line is red and blueshifted due to motion of gas

21cm radiation is not scattered by ISM

Important Concepts

ISM composition and distribution

Emission nebulae, dark clouds

Effects of dust and gas on spectra

Reddening, extinction, polarization

How do we observe ISM?

Observing atomic and molecular hydrogen (21 cm line and molecular tracers)

Indirect methods (absorption lines)

Forbidden lines