The Milky Way and Beyond

Galaxies and the Larger Scale Structure of the Cosmos

The Milky Way

Our Home Galaxy We live on the “fringes” 75% of the distance out from center Our solar systems makes one orbit of

Galactic center every 250 million years! Makes Galaxy difficult to describe due to

our perception

The Milky Way

Home to some 100 billion stars Believed to have a Spiral Structure This is inferred from various observations Interstellar dust thwarted early observers Advent of Radio and IR telescopes

improved model

Shapley’s work

His work disputed earlier work by Kapteyn Shapley said that the Milky Way was

larger than initially believed Kapteyn didn’t take into account the dust

present causing dimming Shapley took the dust into account but

didn’t think about dimming

Shapley’s work

Also, he didn’t realize there were TWO classes of variable stars RR Lyraes Cepheids

The two classes have different brightness and different periods for variablity

Using the Period-Luminosity relationship (Leavitt) he estimated distance

What Shape is the Milky Way?

Dust initially confused observers Thought we were in the center of the Galaxy Stars seemed to be equally distributed

Shapley demonstrated that the 100+ Globular Clusters didn’t orbit us

They orbit a point 30,000 ly distant

The Milky Way Galaxy Side View

Halo

Globular Clusters

SunDisk

Nuclear Bulge

Side View Structure

Disk- 100,000 ly across, 2000 ly thick Contains Spiral Arms

Nuclear Bulge- 20,000 ly across Contains Nucleus of Galaxy

Halo- 300,000 ly across Contains Orbiting Globular Clusters and Dark

Matter Each part has a different population of

Stars

Stellar Populations

Mass function- # of stars of each mass Observations tell us that a variety of

masses are made Observations tell us that star formation is

ongoing Baade grouped the stars according to

location and color

Stellar Populations

Blue disk stars = Population I Red bulge and halo stars = Population II Further study yields

Bulge- Aging Population I Stars and Pop II Disk- Young Population I Stars Halo- Old Population II stars

Stellar Populations

Population I lots of metals Young and blue Circular orbits

Population II metal poor Old and red Elliptical and tilted orbits

Stellar Populations

Not all stars (i.e. Sun) fit easily into either category

Subdivisions include extreme and intermediate Populations, and the “old disk” category

Open clusters contain Pop I Globular clusters contain Pop II

Galactic Motions

Globular Clusters orbit around the nucleus randomly

Bulge stars are “semi-random”

Galactic Motions

Sun and other disk material orbits nucleus of Galaxy in an orderly way

Experiences Differential rotation Observed in other Spiral Galaxies

“Rotation” occurs due to a Density Wave It is not a rigid motion of an “arm” Wind-up problem

Density Waves

Material in the wave is not fixed Material can move through the wave Not a material wave but a disruption wave Like a traffic jam behind a slow moving

vehicle Wave passes through ISM and triggers

star formation

Spiral Arm Structure

Number of Arms isn’t well know. All numbers between 2-10 have been

suggested Use Spiral Arms Tracers to map the arms

Molecular Clouds (Radio) H II regions (Optical) Cepheid Variables (Optical) OB Stars (Optical)

Variable Stars

Cepheid and RR Lyrae Variables Variable Stars Luminosity Varies in predictable ways

RR Lyrae vary over 0.5-1 day Cepheids vary 1-100 days Both on Instability Strip of HR diagram

Period-Luminosity Relation

Relationship of Period of Pulse and Luminosity of Star

Linear for Cepheids Constant for RR Lyraes Cepheid distances can then be

determined Used for large distances because they are

brighter

Period Luminosity

RR Lyrae

Found in Globular Clusters Shapley used observations to establish

distances to GC

Other Tracers

Molecular Clouds emit in Radio Use Doppler shift to map arm structure

H II regions and OB stars Luminosity is known Distance obtained from Inverse Square Law

Group objects by distance, spiral structure seen

Nucleus

Very Obscured Very crowded Sagittarius A- powerful radio source, x-ray

jets Million M Black Hole? Radio reveals two H arms shooting out

Nucleus

Jansky first looked into the heart of the Galaxy with Radio waves

Evidence of star formation ongoing with giant molecular clouds and HII regions

Cool hydrogen and a ring of molecule rich gas exist even closer to the center

As we approach the center, we use many “eyes” to see

The Heart of the Galaxy

Swarm of stars circle the center of the Galaxy

Millions packed into a cubic light year At the very center is a ring of dust and gas This surrounds a very small (10 AU) but

very powerful source This is the suspected black hole

In the Halo

If mass were condensed in the center of the Galaxy, rotation would obey Kepler’s 3rd law

More distant objects would orbit more slowly and we can calculate speeds

This relationship doesn’t hold true

Rotation Curve

Plotting speeds of objects based on distance from Galactic center

Appears that most of the mass is contained in the halo

Rotation Curve

Rotation Curve

Formation of the Galaxy

Similar to Star Formation Everything is on a much larger scale Halo objects form first

Globular Clusters Halo Stars

Disk and Nucleus collapse next Collapse generates star formation

A Universe of Galaxies

Normal Galaxies come in 3 types Spirals Ellipticals Irregulars

Each galaxy has a different morphology Also different stellar populations Classified on Hubble Tuning Fork Diagram

Hubble Tuning Fork Diagram

Spiral Galaxies S0 (Sa?)

Spiral Galaxies (Sb)

Spiral Galaxies (Sc)

Spiral Galaxies (Sc)

Elliptical Galaxies (E1)

Elliptical Galaxies (E2)

Elliptical Galaxies

Barred Spirals

Barred Spiral (w/ Star Formation)

Barred Spiral

Irregular Galaxies (LMC)

Comparisons

Ellipticals have a wide range of sizes Giants can contain trillions of stars Dwarfs contain millions of stars

Spirals are more consistent in size 100’s of Billions of stars

Irregulars smaller than Spirals 100 million to 10 billion stars Smaller Irregulars are more common

Comparisons

Spiral Galaxies contain a mix of stars Much ISM present

Ellipticals primarily contain old stars Very little ISM present

Irregulars contain many young stars Much ISM present

Causes of Shapes

Perhaps the circumstances of collapse determines galaxy type

Motion within pre-galactic gas cloud determines organization and star formation rate

Small motions=Spiral Large motions=Elliptical

Groups of Galaxies

Galaxies tend to cluster into groups Small Groups contain 10’s Large groups can contain 1000’s or millions

Our group = Local Group 20 or so galaxies

Virgo Cluster contains 2500+ galaxies Clusters cluster forming superclusters!

Distribution of Clusters

Superclusters and voids

Great Wall- large distant supercluster

Galactic Cannibalism

In clusters, galaxies can get trapped in a gravity war

The galaxies can merge, pass through one another, or get eaten by a larger on

Often such activities trigger large amounts of star formation

Could explain the presence of the giant galaxies in some clusters

Selection Effects

Spirals generally more luminous OB stars and H II regions

Easier to see than Irr and E’s Count more of them, under count Irr and E’s Bias data to Spirals (appear to be most

common) Irregulars and Ellipticals are much more

common

Galaxy Comparison

Level the playing field! Need to know distance to Galaxies No HR diagram for Galaxies

Brightness= closeness, smallness=farness

To find distance, use distance indicators

Distance Indicators

Aka Standard Candles Assumptions

Physics is Universal Stellar Evolution is Universal

Many methods use to find distance

Distance Indicators

Find a familiar object Star, H II region, SN, etc

Know the object’s luminosity Determine distance using Inverse Square

law for Light Example: Cephied Variables

Know L from P-L relation Find distance

Distance Indicators

Not every galaxy has Cepheid Other objects can be used Some work for nearby galaxies, others for

more distant objects Error in luminosity can cause error in

distance

Distance Indicators

Cepheids (near) OB stars (near) Novae (near) Globular Clusters

(mid) Planetary Nebula

(near) HII regions (mid)

Type I SN (Far) Tully-Fisher relation

(Far) Galaxy Luminosity

(Far)

What’s the Point?

Hubble determined there were other Galaxies (1924)

Determined the Universe was expanding Red-shift of light from Galaxies Red-shift= moving away Distant galaxies are receding faster! Leads to an important law…

Hubble’s Law

d=distance v=radial (recessional) velocity H=Hubble Constant

d=v/H H varies from 15-30 km/s/Mly (50-

90km/s/Mpc) H is a measure of age of Universe!

Hubble Constant Consternation!

Depending on what you use as a distance indicator, errors arise

Different groups give different values of H Gives a radically different value for age of

the Universe! Lower value =older universe Higher value= younger universe

Flat, Closed or Open

The ultimate fate of the universe determined by it’s “shape”

Shape is determined by mass Too much mass-Big Crunch, Oscillating

Universe (closed) Too little mass-never ending expansion

(open) Just right mass-Flat, expansion ceases, no

collapse