PART 1 CLUES TO THE FORMATION AND EVOLUTION OF THE MILKY WAY. This scenario is based upon observations of the motions , ages , metalicities of stars as a function of their locations in the galaxy. There are very clear trends in these observables .
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CLUES TO THE FORMATION AND EVOLUTION OF THE MILKY WAY
This scenario is based upon observations of the motions, ages, metalicities of stars as a function of their locations in the galaxy.
There are very clear trends in these observables.
The motions of disk stars are in a plane.
The motion of Halo stars and clusters are elliptical about the center.
Constructing a Model of the Galaxy and its Formation.
Two Populations of Stars: Pop I and Pop II.
Pop I stars are younger, formed as a later generation, and have higher metal content.
Pop II stars are older, formed in first generation, and have lower metal content.
Pop I stars are located in the galaxy disk where stars are continually forming.
Pop II stars are located in the bulge, halo, and globular clusters.
Location Spiral Arms Disk Bulge Halo
Metals (%) 3 1.6 0.8 <0.8
Orbit shape circular slight moderate highly
elliptical elliptical elliptical
Average Age 100 million 0.2-10 billion 2-10 billion 10-14 billion
(yrs) and younger
Heavy Element build up in the Galaxy
Pop II stars are formed first when the Galaxy is contracting from the intergalactic medium.
They reside in the halo and bulge (elliptical orbits) and since they formed first, they are made of primarily hydrogen and helium (light elements).
As the galaxy flattened, 2nd and 3rd generation stars were formed (Pop I stars). These formed from gas that was enriched by Pop II stars! So, Pop I stars have heavier elements in them.
Stars are very efficient at making carbon, nitrogen, oxygen, and iron (in the massive stars).
And now you know the story...
Note the colors; the globular clusters and bulge are reddish (cool low mass stars that are old) whereas the disk stars are bothblue (hot high mass that are young) and reddish.
SPRIAL ARMS: STRUCTURE AND STAR
Dude, Galactic radio is the ultimate!
How do we measure their locations?
How do they form?
How are they sustained over many rotations?
Hydrogen in the Radio Band! 21-cm Radiation
The best way to structure in the Galaxtic Disk is to observe the hyperfine structure of the hydrogen atom.
The magnetic fields lines of the spinning electron and proton interact. When the fields are aligned magnetic energy is tense; when fields are opposite magnetic energy is relaxed.
Energy Difference is very low; the light emitted is in the radio band
High energy (excited) state.
Low energy (ground) state.
When the electron flips from the excited state to the ground state, energy is released.
The energy is carried off by a photon (light) with a wavelength of 21-cm (radio).
Using radio waves works well because radio waves pass right through gas and dust clouds (whereas visible light does not).
RADIO PHOTONS DO NOT SUFFER INTERSTELLAR EXTINCTION
Thus, we can see right through the interstellar medium and measure the locations of all the clouds, which all have lots of neutral hydrogen.
NOTE HIGH LEVEL OF STRUCTURE!
CAUTION: this requires a model of the galactic rotation to work… and that is subject to some uncertainty and assumptions.
Location of Hydrogen Clouds
Spiral Arms are Traced out by O and B Stars
When one maps out the location of nearby O and B stars, we find that they align along the spiral arms of the Galaxy. Objects that lie in the spiral arms with which we can map them out are called spiral tracers.
All tracers are very young objects.
This would imply that stars and star clusters are formed right smack in the middle of spiral arms. As they age they must migrate out of the arms.
This beautiful scene brings out a clear view of the spiral arms and gas clouds in the foreground galaxy. Note that the spiral arms are bright with O and B stars. Note also the great amount of gas and dust in the galaxy disk.
A “model” of Star Formation in Spiral Arms
This is called density wave theory. Basically, the spiral arms are congested areas. As gas complexes collide with the spiral arms the shock waves, etc. triggers stars formation. Then the relative life times of high mass and low mass stars goes into effect…
The reason this theory is needed is because the spiral arms would dissipate in about 1 billion years.
Complex Structure of Spiral Arms
Much structure is seen in spiral galaxies that is not easily explained by the density wave theory.