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

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 Nuclear Bulge Disk Sun Halo Globular Clusters

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 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 Population II 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 known. 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 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 H II regions and OB stars 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 (T2=R3) 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

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 one 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=v/H H varies from 15-30 km/s/Mly (50-90km/s/Mpc) 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