IX. What is between the Stars?

1. Boundary of the solar system

Solar wind is stopped by interstellar gas and plasma

	-> decelerated to sub-sonic speed
	-> heliospheric termination shock
Ions accelerated at the shock:		like playing Ping Pong between the two sides

2. Interstellar gas in the solar system

For a general introduction to the Interstellar Medium see also this site.

a) Neutral gas enters the solar system

from outside

b) Neutral gas is ionized		close to the sun
	by UV radiation
	impact of solar wind ions
	impact of solar wind electrons

c) New ions will feel the magnetic field

	which is imbedded in the solar wind
	-> ions gyrate around magnetic (Giant Swing around field line)
	-> ions will be swept out of the system

These ions have been measured!

-> local information about the gas outside the solar system (interstellar gas)

d) Sun moves with respect to the gas

	-> interstellar wind (like wind felt in cabrio)

To review the interaction of the interstellar gas with the solar wind you may want to view the the Movie "The Interstellar Gas Reveals Itself by the Giant Swing"

Sun acts as gravitational lens		-> gas focused on the downwind side
	Measure	Determine
	absolute ion flux	-> density
	density enhancement in focus	-> velocity and temperature
			of interstellar gas

3. Gas Between the Stars

a) absorbes light from stars

> observe dark lines in star spectra

• interstellar gas distinguished by Doppler effect (different motion)

b) emits light

• when in the vicinity of bright stars -> observe emission lines

c) cold gas emits radio lines

21 cm line of un-ionized Hydrogen

	Importance:		lots of hydrogen in the universe;
		spectral line	--> get velocity with Doppler effect
		radio line	--> can see through dust (see below)

Molecules:

	Cool clouds	--> molecules don't break apart when they collide
	Dense clouds	--> better chance for atoms to form molecules
Cool and Dense		Þ Molecular clouds are "stellar nurseries"

4. Dust Between the Stars

a) Light scattering:

b) Obstruction (extinction).

• With light, we can't see very far into the Milky Way

c) Reddening:

• red light goes through, blue is scattered

-> Radio & IR astronomy: see through dust clouds

d) Blue reflection nebulae

• blue scattered to the side

Same physics in Earth's atmosphere

--> sunsets are red "Alpenglow"; sky is blue

e) Why is there dust?

	Cool clouds	--> dust doesn't evaporate
	Dense clouds	--> more chance for molecules to come together

Go to Chapter X.