the milky way and beyond galaxies and the larger scale structure of the cosmos
Post on 25-Dec-2015
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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
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
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
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
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
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!
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
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