Interstellar Medium - GasASTR 2120

Sarazin

Interstellar MediumCovered in part last semester – shortened review

in lectures here. (No star formation – covered with planet formation last semester)

Interstellar Medium - GasASTR 2120

Sarazin

Interstellar GasClassify based on physical state of hydrogen

Molecular H2

Atomic H I = H0

Ionized H II = H+

Neutral, Atomic Hydrogen (H I, Ho)

~50% of mass in local ISM

Interstellar Clouds

Warm Neutral Intercloud Medium

Most are fairly cold (~100 K), atoms in ground state, no normal atomic emission lines

How to see?

Interstellar Absorption Lines

Narrower than stellar lines

→ Cold neutral gas

Most atomic lines in UV, can�t be seen except from space

Neutral Atomic Hydrogen Gas21 cm Hyperfine Line of Hydrogen1944 – van de Hulst predicts 21 cm line of atomic H1951 – Ewen & Purcell detect

21 cm Hyperfine Line of HydrogenDE = 6 x 10-6 eVn = DE/h = 1.42 x 109 Hz = 1420 MHz

= 1.42 GHzl = c/n= 21.1 cm3/4 of atoms in upper state, 1/4 in lower state1 decay per 1.1 x 107 years

n=1 gs

n=2

n=3

3

1

p e

hydrogen

Neutral Atomic Hydrogen Gas21 cm Hyperfine Line of Hydrogen

Just Counts H atoms!!Image of Milky Way

Molecular GasThree kinds of energy levels and lines• Electronic transitions

– Just like individual atoms– Mainly in UV and optical

• Rotational transitions– Mainly in mm and submm radio– Except H, in IR

• Vibrational Transitions– Mainly in mid-IR– Except H, in near IR

C O+ -

C O+ -

C O+ -

e

Molecular Gas1969 – H2CO Formaldehyde detected (Snyder,

others)1970 – CO (3 mm) detected in radio

Molecular make emission lines due to rotation, in radio

Molecular Gas H2

~50% of mass

Often in dense clouds

n(H2) ~ 105 molecules/cm3

T ~ 10 K

AV > 10

> 100 molecules now known

Orion: visible and CO

Photo-ionized HydrogenAlways at T ~ 104 K

H II regions around OB stars

Planetary nebulae (?)

Diffuse ionized gas

Ionized Hydrogen (H II, H+)

Emission Nebulae

1930�s – Strömgren, Menzel, Baker, Goldberg, . . .emission nebulae = photoionized hydrogen

gas

Emission nebulae:Emission lines from atomic hydrogen H I, helium He I

Why atomic H in ionized H region?Made by recombination, H+ + e- → Ho

HgHa

Hb

Emission nebulae:Emission lines from atomic hydrogen H I, helium He I

Why atomic H in ionized H region?Made by recombination, H+ + e- → Ho

n=2

n=3

n=1 gs

hydrogencontinuum

e-

Ha

Lya

Emission nebulae:Emission lines from atomic hydrogen H I, helium He I

Why atomic H in ionized H region?Made by recombination, H+ + e- → Ho

Forbidden lines of common elements (O, N, etc.) but only occur at very low densities, never in lab

O III = O+2

O II = O+

Collisionally Ionized GasGenerally at T ~ 106 to 108 K

Due to shocks from SNe and stellar winds

Supernova remnants

Diffuse hot gas

Cas A – Chandra X-ray

Star Formation

Density r (stars) ~ 1 gm/cm

ISM <r> ~ 10-24 gm/cm

Stars form from the densest clouds

Star Formation

Internal pressure balances gravity, external pressure Pext

Unstable (collapse) if:

Gravity too strong

External pressure too strong

Star FormationPressure Pext

Gravity

Pressure Pin

€

M ≥MJeans ≡kTµ

$

% &

'

( )

4π

G 3Pext

+

, - -

.

/ 0 0

1/2

€

Pext ≥kTµ

#

$ %

&

' (

4π

G 3M 2

*

+ , ,

-

. / /

Triggers:

1. Clouds coalesce, grow until M > MJeans, collapse, form star or stars

2. External pressure on cloud increases suddenly

1. Nearby supernova

2. Nearly HII region, heating by OB stars

3. Spiral arm shocks in galaxies

4. Collapse and formation of galaxy or merger of galaxies

Star Formation

As cloud collapses, density and pressure increase

Unless temperature increases too rapidly …

Jean’s mass decreases

èParts of cloud become unstable, cloud fragments

Make cluster of many stars, rather than just one star

Fragmentation & Star Clusters

Pressure Pin

€

M ≥MJeans ≡kTµ

$

% &

'

( )

4π

G 3Pext

+

, - -

.

/ 0 0

1/2

Fragmentation & Star Clusters

Orion star cluster Pleides star cluster

Fragmentation & Star Clusters

Open star cluster Globular star cluster

The Milky WayOur Galaxy - Love It or Leave It!

ASTR 2120Sarazin

The Milky Way – Our Galaxy

�In� the Milky Way

• Stars• Open star clusters• Gas

– Emission nebulae– Atomic hydrogen– Molecular hydrogen

• Dust– Dark nebulae– Reflection nebulae

�Out of� the Milky Way

• Globular star clusters• Galaxies

– Spiral nebulae– Elliptical nebulae

Is the Milky Way the Whole Universe?

Everybody knows their place in it?

What is the Size and Shape of the Milky Way?

Where are we located in it?Early 1900�s -> this is the whole Universe?

Kapteyn Model & Star Counts• Counted number of stars vs.

brightness in all directions• Didn’t know about extinction!• Milky Way is small, flattened

disk, Sun close to center?Pre-Copernicus?

0.5 kpc

2 kpc8

Globular Cluster Distribution

Centered in Sagittarius

This is center of Milky Way

But, how far away?

Shapley

Cepheid Variable Star Distances

Pulsating variable stars

Ppul ≈ (Gr)-1/2

Giants → bigger = lower density, longer period, and brighter

Henrietta Leavitt

Cepheid Variable Star Distances

Distance to Galactic CenterR0 = 8 kpc

Shapley

Structure of Milky Way

Structure of Milky Way

Milky Way – IR COBE Picture

Structure of Milky Way - Bar

Galactic Coordinatesl = longitude, measured from Galactic Center

b = latitude, measured from Galactic plane

Is the Milky Way the Whole Universe?

Everybody knows their place in it?

What are Spiral and Elliptical Nebulae?

• Part of Milky Way, small objects, Milky Way is the whole Universe?

• Similar to Milky Way (�island universes�), big objects, very big Universe?

Edge-On Spirals ≈ Milky Way?