•Earlier in the course, I told you stellar spectra are black bodies•Why are there all these features?

High resolution solar spectrum

•In dense inner part, light gets randomized – black body spectrum•Light passes through cooler stellar atmosphere•But some wavelengths get absorbed!

+-

Spectral Lines

Stellar spectra•Approximate black body spectrum – color tells you temperature

•Red is cool, blue is hot•Spectral lines tell you a lot about the star

•Different lines for each element (and ionization state)•Strength tells you about composition and temperature

More about stars•Stars are formed from clouds of cool gas•They come in different sizes:

•High mass stars tend to burn hotter (bluer) and don’t last long•Low mass stars tend to burn cooler (redder) and last a long time

•The color of a collection of stars (clusters) tells you about them:•Young clusters look bluish•Old clusters look reddish

Doppler Effect Revisited

0

1 cos

ff

v c

0

0

1

1

v cf f

v c

0

1

1

v cf f

v c

•In astronomy, most of the motion is towards youor away from you

•Let vr = - v cos be the velocity away from you •In astronomy, we work with wavelength instead of frequency•Except they actually use z, the fractional increase in wavelength

•For non-relativistic:

0 1 rf f v c

0 0f c f

0

1 z

0f

f 1 rv c

rz v c

2

2

1 1

1 1r

zv

c z

Orbits•When a low mass object orbits a high mass object, there is a simple relationship between the distance and the velocity:

2

GMmF

R

2mv

R

GMv

R

0

5

10

15

20

25

30

35

40

45

50

0 5 10 15 20 25 30 35 40

Mean distance (AU)

Mea

n V

elo

city

(km

/s)

Planets

Fit to formula

Distance1 AU = Earth-Sun distance

= 1.496 1011 m1 ly = cy = 9.46 1015 m

1 pc = 3.086 1016 m

Mass1 MSun = 1.989 1030 kg

The Milky Way: Visible Light•Obscured by gas and dust

Center of Galaxy

Galactic Plane

The Milky Way

The central portionBaade’s Window

The Milky Way – From Outside

The Milky Way – From Outside

The Milky Way – Edge On

The Milky Way – Artist’s Conception

Youare here

•Our galaxy is hard to study because we are inside it

•We are in the galactic plane, filled with obscuring dust

•Other galaxies are hardto study because theyare far away•Generalize from far away to nearby and vice versa

The Sun

•Galaxies and larger structures are incrediblylarge compared to anything we havestudied up to now

•We need newunits to describe itDistances

ly = 0.931016 mkly = 103 lyMly = 106 lyGly = 109 ly

The Milky Way – Basic Structure•The disk•The bulge•The nucleus•The halo

•Globular clusters

The Disk – Dimensions and Structure•The disk•The bulge•The nucleus•The halo

•Globular clusters

The Sun

100 kly

3 kly

•A large, flat disk, shaped like a pancake•About 100 kly in diameter•About 3 kly thick

•We are about half way out•Has prominent spiral structure

27 kly

The Disk - Composition•The disk•The bulge•The nucleus•The halo

•Globular clusters

•Stars, mixture of young and old•Circular orbits in plane of galaxy•From 0 to about 10 Gyr•Open clusters

•The interstellar medium•Hot bubbles •Atomic hydrogen clouds•Molecular clouds•Ionization nebulae

•Dust•Obscures and reddens things•Causes “reflection nebulae”

M35NGC 2158

NGC 290

M36

M6Pleiades

Open Clusters

Hot Bubbles•The disk•The bulge•The nucleus•The halo

•Globular clusters

•Gas heated by supernovae and other violent events•Gas is very thin•Gas is very hot and ionized

•Temperatures up to 106 K•Can betraced outby X-rays

Hot Bubbles - Images

Atomic Hydrogen CloudsSlightly cooler regions of gas•Hydrogen atoms produce 21 cm line

•Electron and proton are spinning and have magnetic interactions•When electron spin flips over, 21 cm radio emission is seen

•21 cm line used to map outour disk•Can also get accurate Dopplershift

Radio waves

•The disk•The bulge•The nucleus•The halo

•Globular clusters

The 21 cm line

Approximate Map of Galaxy

Molecular CloudsColdest and densest regions•Atoms join together to make molecules

•Principally hydrogen (H2), but this is difficult to detect•Other molecules vibrate to produce characteristic radio waves•These regions are where new stars can form

Carbon monoxide (CO) emissions from cool clouds in our galaxy

Molecular Clouds – Eagle Nebula

Molecular Clouds

Molecular Clouds – Keyhole and Orion

Molecular Clouds

Molecular Clouds – Horsehead Nebula

•Light from hot stars ionizes hydrogen•Hot thin gas•When it recombines, light is made

Light

Ionization (Emission) Nebulae•The disk•The bulge•The nucleus•The halo

•Globular clusters

Ionization (Emission) Nebulae

Ionization (Emission) Nebulae

Labeled Eagle Nebula

Dust•Lots of dust in spiral galaxies•Dust absorbs and scatters short wavelength light

•The color of objects is reddened if we look through thin dust clouds•Dust clouds look blue

•You can peek through dust in nearinfrared light

Dust cloud

•The disk•The bulge•The nucleus•The halo

•Globular clusters

Dust•Dust is warmed by absorbing light from stars•It glows in the infrared

•The disk•The bulge•The nucleus•The halo

•Globular clusters

A reflection nebula – the Pleiades•The disk•The bulge•The nucleus•The halo

•Globular clusters

A reflection nebula – the Merope Nebula•The disk•The bulge•The nucleus•The halo

•Globular clusters

Witch Head NebulaIC 349

Reflection Nebulae

Spiral Arms•The disk•The bulge•The nucleus•The halo

•Globular clusters

Spiral Arms – Signs of Rotation•The disk•The bulge•The nucleus•The halo

•Globular clusters

•Bulge is flattened - rotation•Disk - rotation vs. gravity•Spiral arms - what causes them?

•Simple winding? No!•Density waves? Yes! Complicated!

Simple Winding – The Wrong Theory•Spiral arms wind up in one cycle•20 cycles since beginning of galaxy•Something else is going on

An Analogy from Driving•Knot in traffic causes slowdown•Slowdown causes other cars to slow down•Knot in traffic moves•Different cars at different times

•Gas is in a disk•Inevitably, some parts are more dense than others•Gravity/rotation causes dense parts to become denser

•Different gas goes in/out•Dense parts form new/bright stars

Density Waves – The explanation

The Bulge•The disk•The bulge•The nucleus•The halo

•Globular clusters

•Our view of it is (mostly) blocked by dust•We can see through the dust in near infrared

The Bulge•The disk•The bulge•The nucleus•The halo

•Globular clusters

•Flattened sphere approximately 20 kly across•Composition

•Mostly older stars•Little gas and dust

•Recent evidence indicates itis bar shaped, with left sidecloser to us than right side.

The Sun

The Bulge – Color and Appearance

•Older stars give it a redderappearance

The Nucleus•The disk•The bulge•The nucleus•The halo

•Globular clusters

The Sun

•At the center of our galaxy lies a complex region•Fast star formation•Recent supernovae remnants•Hot gas•Fast motion

•Density of stars is very high here•Intense radio sources can penetrate the gas and dust

The Nucleus in Radio•Close in, we see streamers of gas apparently flowing in•At the heart is an intense radio source called Sagittarius A*

What is that thing in the Nucleus?•Copious quantities of X-rays close in•Stars orbit this source very quickly

•Kepler’s Laws tell us mass

X-ray image

•It appears to be a 4 million solar mass black hole

The Monster in the Middle•Radio waves can’t come from black hole itself•Gas from nearby attracted by gravity•Accelerates to near light speed•Friction creates heat/X-rays/etc.•More efficient than any other power source

•The disk•The bulge•The nucleus•The halo

•Globular clusters

•Black hole

•4.0 million MSun

New Discovery?

•Gamma Rays coming from pair of lobes •Outburst of energy from nucleus?•Was our Galaxy formerly active?

The Halo•The disk•The bulge•The nucleus•The halo

•Globular clusters

•Roughly spherical shape•At least 2 size of disk•Stars:

•Old stars (about 10 Gyr)•Orbits well out of plane

•Globular Clusters•Up to a million stars each•Oldest stars (up to about 13 Gyr)

•Little gas or dust

Total Mass in the GalaxyObject Mass (MSun )Disk Stars 60 billionDisk Gas ~10 billiionBulge 20 billionHalo Stars 1 billionNucleus 4 million

•Total of a few hundred billion stars•Total mass about 100 billion Sun•Stars mostly concentrated near center•Treat gravity as if it all comes from a point source in the center•Use Kepler’s Thirdlaw: MP2 = a3 If all the mass is in the center, how would

we expect the velocity of the stars to change as we move away from the center?A) Fastest for stars near the centerB) Fastest for stars far outC) Roughly same speed for all

22

d a Mv

t P a

•Measure rotational velocities using Doppler shift of 21 cm line•Plot vs. distance from center of galaxy

Where is the Mass?

Dark Matter•85% of mass is not concentrated in the center•It does not glow, it is dark•We do not know what it is•We know a lot of things it isn’t:

•Living Stars•Gas•Dust

•Could it beMACHOs?

What is the dark matter?A) Neutron StarsB) Black HolesC) White DwarfsD) Planets without starsE) Something else

Massive Compact Halo Objects

•The disk•The bulge•The nucleus•The halo

•Globular clusters

MACHOs

MACHO: We see:

How to catch a MACHO•All of these objects are dark•But they do have gravity!•Einstein says they can bend light•They can magnify a distant star

MACHOsNeutron StarsBlack Holes

White DwarfsPlanets without stars

MACHO’s: What the Data Tells Us•MACHO’s do exist•Substantial fraction of stars

•But not the majority

•There are not enough of them to account for the dark matter•They are probably mostly white dwarfs

•The disk•The bulge•The nucleus•The halo

•Globular clusters

•Most likely dark matter is some weird new particle