Gas too thin remains gas.

If it gets too dense collapses into stars

Jeans Mass -- gravitational attraction > thermal

Equilibrium / cycle

1 Msun/yr new stars

0.5-0.8 Msun /yr returned SN Heavy

Red Giants Element

O stars Enriched

gas in spiral arms has a high density, crashed in and becomes more dense. Star formation in young stars -- O stars in spiral arms

Basic energertics

HEATED by supernova ejecta shockwaves

COOLS by Bremstrahlung with lines


HOT -- x-rays (soft x-ray background)

WARM -- UV (can be seen in absorption, but not in emission)




Supernova Remnanats

SN explodes --> 2-10 Msun ejected at 3-10000 km/s

Energy released

Three Phases

1. Free expansion

2. Adiabatic

sweeping up matter

gas heats up 106-107K

3. Snowplough (momentum conserving)

Radiates away its thermal energy through x-ray emission

Shock front gets lost in cloud motion


Equations of Growth

(n1 inside, n0 outside)

Pressure Equilibrium

x-ray sources:

non thermal radio (trapped high energy particles)


Cyngus Loop

The classic SNR

770 pc away

~20,000 years old

Veil Nebula

dense clouds optically emit, shocked by passage of waves

Very bright x-ray source



Interstellar Bubbles

10-5 Msun / yr V~2000 km/s

E after 10^6 years

~ 0.1 SN -- large!!

Can affect ISM near star -- blow it away

When SN goes off -- 1051 ergs released

SNR control state of ISM: makes tunnels of very hot gas in warm and cold gas

BUT SN come for OB stars

OB stars come in clumps, therefore SN come in clumps




HII Regions

O star

photon flux near star very high ionization rate > recombination rate

Radiates energy away through recombination lines Ha, Hb

forbidden lines [OIII]

Lya trapped, therefore IR source from dust

Gas between stars

galaxy started as gas

some left over

some reprocessed

Known for hundreds of years


HII regions

photon l < 912 angstrom, can remove electron for atom

OB stars generate lots of l < 912 angstrom light since OB young stars, are usually still in clouds they formed from

start with

turn on star, ionizes gas around star

Gas recombines, hits balance at some radius.

Ha is very strong, results in a red appearance of HII region

Next aspect of ISM was dust

Dust -- next aspect of ISM was dust

light does not need to be absorbed to make a star appear fainter. Like a sunset on a dusty day things appear redder. Blue is scattered more. Infrared is not scattered. IR is the only way to see center of Milky Way. Clearly activity is there -- IR will reveal it.


Molecular clouds

Very high density regions -- stars formed.

High density therefore radiates fast, therefore cold

T falls to 50-100K

Molecular form -- CO Seen in IR and radio

Clouds in less dense regions exist too. 21 cm emission

Can be seen by absorbtion spectroscopy

Molecular hydrogen

Hot gas --- OVI 300,000K gas

Lots of elements

Motion of gas -- 100 km/s




Supernova Remnants

Explosion sends out ~10 Msun at 10,000 km/s. Blast wave -- 1051 ergs. Same equation as effects of atom bomb in atmosphere.

Phase I: Expands freely, picks up all gas in its path

~100 years

~few parsecs

Phase II: Energy conserving, momentum also conserved. Slows down, but heats up.

1-10 million K --> emits x-rays

~20,000 years

~50-100 parsecs

Phase III: X-rays carry energy away

~1 million yearrs

Waves roll through ISM, interact with other SNRs

Phase IV: Breaks up and loses intensity




Structure of Milky Way

This basic picture is hard to put together. Can’t just count stars -- too may. We also can’t see all of them because of interstellar dust. WE could look at other galaxies, but we didn’t know what they were.

Globular clusters are bright enough to be studied over dust. This raises the question of how far away they are. We can fit them on the HR diagram.

Centered around point in Sag - 10 kpc away. This wa the first break in understandign the Milky Way.

We can see local arms, these indicate that we are in a spiral galaxy.



Radio Astronomy

No obscuration by dust

Karl Jansky -- first radio telescope. The first radio source is Sag A. -- galactic center.

21 cm line -- tracer of neutral Hydrogen. We can deconvolve this and map. In the 1960’s, for the first time, observers were abole to recognize spiral structure of galaxy.

Tracing the rotation outward, we see that there is a lot of mass. IN fact, most of the mass is in giant, invisible halo. This raises aquestion of what the halo is made of... neutrinos? baseballs? blackholes?



Why is it spiral?

but would wind up too much.

Sun has time to go around 100 times, yet only several spirals -- it isn’t that simple.

Spiral arms -- density waves

Matter slows down, gas piles up --> prefered star formation.

It is OBA stars that light up spiral arms. Otherwise would hardly be able to see them.

Stellar generations:

galaxy formed first in middle.

more generations in center --> higher heavy element abundance



UV Astronomy

912-3500 (3500 --> optical)

Particularly 1200-2000 Angstrom

Why study UV?

Basic ground state transitions of the atoms

all ionizing potential 3eV < IP < 25 eV

most 4eV < IP < 14eV

If you can observe at 4-14 eV, will get fundamental data most notably Lyman alpha -- main line o f main elements 1216 A

Must be above atmosphere, but conventional optics work (just be careful)

1. Rockets Stellar Winds 1967

2. Copernicus High Resolution 1972-1980

3. IUE Spectrograph 1978-now

4. Space Telescope 1985

5. FUSE 900-1200 1990

Types of objects

1. O stars Photospheres

a) Stellar Winds

b) ISM in absorption

2. Late Stars -- Chronospheres

3. QSO’s

a) Emission Spectrum

b) Intergalactic Gas




Self gravitating

Instabilities, perturbations excite spiral node.

Computer simulations --> much heavier for spiral, low mass stars in halo?



Local group

Magellanic Clouds

Dwarf galaxies

M31, M33

Local Group

M31 Sb

Milky Way SBc

M33 Sc

Magellanic Clouds

1% mass of Milky Way

66,000 pc distant

Early type

HI stream -- orbits behind

30 Doradus

Dwarf Galaxies NGC 205, M32 orbit M31

Virgo cluster

falling toward it

x-ray source 10^8 K Bremsstrahlung

M87 supermassive galaxy

Missing Mass -- virial theorem vs mass/light

rotation curves

Super clusters

hierarchy of clustering

largest structure scal of universe