THE EVOLUTION OF THE GALAXY
Several problems face astromers from trying to figure out the structure of the Galaxy and from our location in it.  We reside in the disk of the Galaxy which is composed of several major spiral arms.  The nuclear bulge is the central regions of the Galaxy including the nucleus.  The sperical halo encircles the nuclear bulge and the disk.  Interstellar dust blocks the inner part of the galaxy from the view of optical astronomers, but radio astronomers can pick up radio emissions from clouds of gas that probably mark spiral arms.

One method used to find the sun's speed of orbit around the galaxy comes from the motions of globular clusters.  They seem to orbit the Galaxy in random orbits, with roughy spherical distribution around the nucleus.  With respect to the nucleus, the motion of the clusters is zero.  By a study of the radial velocities of the globular clusters by Doppler shifts enables us to find the sun's motion with respect to the system of globulars.  So, the sun's motion relative to the system of globulars is its rotational motion with respect to the galaxies center (about 220 km/s).  The sun's distance to the galactic center cannot be measured directly but can be measured indirectly.  By finding the distances of stars such as RR Lyrae variables that are above or below the galatic plain we find that the sun is about 30,000 ly from the center.  More globular clusters are visible in near the center of the galaxy than in other directions, giving us our relative position.

Knowing the sun's velocity and distance, Kepler's third law can be applied to deduce the mass of the Galaxy.  The major part of the mass is not in the galactic center, much of it beyond the sun's orbit (total mass is about 1012).  The period-luminosity relationship for cepheid variables, is the relation between the average luminosity and the time period over which the luminosity varies; the greater the luminosity, the longer the pulsational period.  By comparing the observed flux to the luminosity estimated from the period, the inverse- square law for light can be used to find the distance to the cepheid.  This helps determine the distances of the Galaxy's spiral arms and are known as spiral tracers-O and M type stars that give the spiral arms their bluish color and distinct structure.  Optical maps of the galaxy can only exist out to 15,000 ly because instellar dust hindered astronomers view.  Optical observations have revealed, however, at least three major arm segments spaced about 7000 lys apart.  The Galaxy appears to have a spiral structure with much irregularity in the general pattern, which may consist of two or four spiral arms.  The 21cm line from hydrogen (when an eletron switches its charge from parallel to opposing) tells radio astronomers the spiral tracers from giant molecular clouds.  The 21-cm radiation from H I clouds arrives at the earth Doppler shifted to different wavelengths because of the different velocities of the hydrogen gas clouds.  These differences in velocities come mostly from the rotation of the Galaxy.  Because of galactic rotation, the clouds along the line of sight have different radial velocities, different Doppler shifts, and signals recieved at slightly different wavelengths.  These observations show four spiral arms overall.

The spiral arms of the galaxy are a result of spiral waves of higher density moving through the Galaxy's disk.  The waves produce the young stars that are the signposts of the spiral arms.  Spiral arms always contain objects of the same kind, but not the same objects because the density wave births new spiral tracers from the interstellar medium.  Individual objects revolve at the speed determined by gravitational forces appropriate for their distances, but the wave pattern rotates with a constant angular speed and does not wind up.

The center of the galaxy is possibly the location of a supermassive black hole.  This hypothesis arises from radio and infrared observations that show rapid rotational motions near the Galaxy's core-the rotational velocities increase closer to the core as shown by their Doppler shifts.  The rotational velocities are so high that a huge concentration of mass is needed to hold it together.

Globular clusters outline the halo around the Galaxy.  They form a spherical distribution around teh Galaxy's center.  Some elliptical orbits bring them out to extreme distances of 300,000 light years from the nucleus, and travel at speeds of up to 100 km/s, diving them into and out of the galactic disk.  Most of the halo's gas is ionized hydrogen (H II) and may be an important factor in the density-wave model of the Galaxy.
The halo marks the fossil remains of the Galaxy's birth and exhibit the motions of the cloud from which they were formed.  A cloud roughly twice the size of the halo formed the proto-Galaxy.  The clouds self-gravity pulls it together, with its central regions getting denser faster than its outer parts.  Turbulent swirls of different sizes form, break up, and die away.  Eventually they become dense enough to contain sufficient mass to hold themselves together and become globular clusters.  The rest of the gas fell slowly into a disk and the conservation of angular momentum requires that it spin faster around its rotatinal axis as it contracts.  The kinetic energy of the cloud slowly decreases, as gas clouds collide and heat is radiated away.  The disk rapidly flattens, density increases and more stars form.  Finally the remaining gas and dust settle into the narrow layer that currently exists, with spiral arms driven by density waves.  Massive stars manufactured heavy elements and flung them back into the cloud in supernova explosions when the Galaxy was young and thus each new star born in sucession had a richer content of heavy elements.
 

THE UNIVERSE OF GALAXIES
Elliptical galaxies exhibit no spiral structure but show an elliptical shape.  Very little gas or dust appears in elliptical galaxies, and OB stars are almost absent.  They usually have an overall reddish color.
Spiral galaxies display obvious spiral structure, usually with two, but sometimes more, spiral arms.  One type of spiral has a dominant bar through the nucleus called barred (S).  Those that show no bar are called normal (S).  These two types are subdivided into catagories a, b, and c according to how tightly the spiral arms wind around the nucleus (a is the tightest, c the most open) and the size of the nucleus compared to the disk (a is the largest, c is the smallest).  Lenicular galaxies have a spiral disk of a spiral but no arms.
Irregular galaxies are those that are devoid of spiral structure or symetry but were resolvable to distinct patches of stars.  They are dominated by OB stars and are without dust clouds.

Physical properties of these galaxies are determined by optical observations of spiral structure and by luminosity class (just like stars I-V).  34% of galaxies are spirals, 12% are elliptical, and 54% are irregulars.  But to know the physical properties of stars we must first know the essential element of distance.
The essential technique of judging the distances of galaxies are the criteria that brightness means nearness and smallness means farness.  Galaxies with the smallest angular size tend to be the most distant and faint galaxies tend ot be far away.
General technique to find the distances to galaxies:
1) Establish the of distances in our Galaxy, starting with the solar system.  Paralax measurments are used: heliocentric and spectroscopic paralaxes; cepheid variable period-luminosity relationship.
2) Use identifiable objects within galaxies whose luminosities we know.  Compare their fluxes with their luminosities to infer their distances.
3) Starting with close galaxies, apply the period-luminosity relationship to cepheids in other galaxies.  Cepheids can be used as standards to approximately 15 Mly.
4) To go beyond the 15 Mly limit requires standards whose visiability is greater than that of cepheids, such as supernovas.
5) At distances greater than 100 Mly, individual objects cannot be seen, so the luminosities of the galaxies themselves must be used.
Once the distance to a galaxy is learned, its actual size can be found with a measurment of its angular size.  Dwarf ellipticals and small irregulars tend to be the smallest galaxies-300 to 3000 ly in diameter.  Giant ellipticals can be up to 200,000 ly in size.   The largest galaxies are the supergiant ellipticals, with radii of up to 3 million ly.
To find a galaxies mass is difficult because much of their material does not visable light.  The most widely used methods for finding mass are rotation curves and binary galaxies.  The rotation curve method can only be used for spiral galaxies only.  The rotation curve comes from the orbital motions of, described by Newton's laws, that arise from the distribution of mass within a galaxy.  So if a galaxy's rotation curve is observed, and make a model of its mass distribution, its total mass can be worked out.  The rotation curve is observed by directing a spectroscope's slit across the galaxy and measuring the Doppler shift at a number of points from the center out to the edge.  Spiral galaxies have masses as high as several trillion solar masses.  The Doppler shift is again used for binary galaxies.  Applying Newton's form of Kepler's third law to the sytem, the masses can be known if the distance, the angular size of the orbit, the period, and the position of the center of mass are also known.
If a galaxy's distance and flux then the luminosity can also be figured out using the inverse-square law for light.  Galactic dust from the observed galaxy and our own, and a galaxy's tilt do not permit a perfectly accurate measure of luminosity.
Mass-luminosity ratio(M/L)- divide the total mass of a galaxy by its luminosity.  It is an indication to the average energy output per unit solar mass from the galaxy.
The colors of galaxies can be measured by comparing the flux at two different wavelengths.  The color of a galaxy depends upon the predominant stellar type in its mixture of stars.  Ellipticals tend to be redder than spirals and spirals redder than irregular galaxies, implying that ellipticals are the oldest and irregulars the youngest.
The overall shape of a galaxy depends on its spin, its angular momentum.  The more spin a galaxy has the flatter its shape.

Hubbles Law-the farther away a  galaxy is from us, the greater its radial velocity of recession will be.  The number that relates to the recessional speed and the distance is Hubble's constast, H.  A comparison spectrum made at the same time as the galaxies spectrum affords a direct measurement of the shift in some prominent spectoral lines.  The redshift indicates the radial velocity of a galaxy.  The Hubble law equation (v=Hd)  v is radial velocity in km per second, H is the Hubble constant and d is the distance in millions of light years.  The procedure is reversed to find distances from redshift (d=v/H).

The cluster of galaxies to which the Milky Way Galaxy belongs is called the Local Group.  Most galaxy clusters and superclusters appear to be flattened with 98% empty space.  It may be true that all galaxies belong to clusters.  The M/L ratio can be used to estimate the amount of dark matter in clusters.  This mysterious matter may reside in the intergalactic medium.  Ionized hydrogen (H II) is probably the most likely candidate because intergalac material does not have a high density, ionized hydrogen would take a very long time to find an electron and recombine.  Many clusters of galaxies are also known to emit x-rays that might come from hot, ionized gas.  The dark matter may also neutrinos and control the structures of clusters and superclusters.

COSMIC VIOLENCE
The nucleus of our Galaxy shows evidence for violent activity in the form of radio emissions.  The region Sgr A has a size of less than 13 AU and is surrounded by clumps of ionized gas that move at speeds of about 100 km/s.  This region exhibts a jetlike structure above and below the galactic plane from the center of activity.  Some of the emission comes from the sychrotron process, so high-energy electrons, moving at speeds close to that of light, spiral in huge magnetic field, subsist in the nucleus.  Galaxies with a nucleus more active than ours display an energy output at a much higher level of violence.  The key clues are 1) emission over a wide range of wavelengths (usually nonthermal), 2) radio output concentrated in  a small space, 3) most of the energy coming out in the infrared, 4) variability in the emission, and 5) strong radio polarization with jetlike sturctures, all arising from the nucleus.
 The faster electons travel in a magnetic field, the more energetic (shorter the wavelength, higher the frequency) the radiation they emit, they lose energy and slow down.  So to keep a sychrotron source powered up, there must be a supply of electrons moving close to the speed of light.  Synchrotron emission turns out to power most the cosmic violence that follows.
 An "active" galaxies spectrum does not look like that of a collection of stars.  It is mostly nonthermal and has infrared, radio, ultraviolet, and x-ray outputs greater than those in the optical.  Activity originates in the nucleus of active galaxies called active galactic nuclei (AGNs).  The peak of emissions from these galaxies is in the radio part of the spetrum instead of the optical like in "normal" galaxies.  Radio galaxies are the largest class of active glaxies and have strong radio emission.  Extended radio sources spread far beyond the optically visable galaxy-sometimes into two giant lobes of emission up to a million light years in extent, symmetrically balanced on opposite sides of the nucleus.  A galaxy may show a compact source at its nucleus and also have the extended lobes; it usually then has radio jets connecting the nucleus to the extended radio emission.
Cygnus A is one of the strongest radio sources in the sky and one of the first to be discovered.  It has two giant lobes set in opposite sides of the optical galaxy.  Each lobe has a diameter of about 55,000 ly.  A needlelike jet extends from the nucleus to one of the lobes.  Centaurus A is a strong radio source with radio jets.  It is a supergiant elliptical galaxy, bisected by an irregular dust lane.  M 87 is a giant elliptical galaxy that has an optically visable jet firing out of its core.  Observed at wide radio wavelengths, the spectra of these galaxies is nonthermal.  This property points to sychorotron process generation of emission from the jets.  The nucleus provides the high energy electrons which are expelled from either as a farily constant beam of particles or as a sequence of ionized blobs that are thrown out along a magnetic field.  The ionized stream carries magnetic field lines within it, and these help to channel the flows outward.  The channel is leaky, which renders the jet's emissions visible.  As the galaxy travels through space, it leave behind a radio-visable trail.  The radio jets are aligned with the lobes, and suggests that high speed electrons are channeled by magnetic fields from the nucleus into the medium  around the galaxy where htey pile up and form a lobe.
Seyfert galaxies are active galaxies that show unusual, broad emission lines.  Seyferts are almost always spiral compared to the elliptical radio galaxies.  Some of their features are: extremely small and bright nuclei; the nuclei have spectra that show emission lines not usually seen in the spectra of spiral galaxies-these bright lines come from ionized gas; the emission lines are very wide-an indicater of violent activity; many Seyferts have compact, low-luminosity radio sources within their nuclei.
Quasars (quasi-stellar objects) have large radio redshifts of the emission lines of the visible spectrum.  If the redshifts are interpreted as coming from the expansion of the universe, the light from quasars must come from distances that are billions of ly distant.  They are the youngest and most distant distant objects known.  Some of their properties are: starlike appearance with a large redshift; broad emission lines in the spectrum with absorption lines sometimes present (usually the redshift of the absorbtion lines is less than that of the emission lines; often variable luminosity; in those that are radio sources, aligned, double-lobed structure.  Quasars emit an immense amount of energy from a small region of space in the center of the quasars (100X the energy output of the MWG in regions that are only light years in diameter.  The most developed quasar models have supermassive black holes at the center of quasars.  Stellar material forms an accretion disk and radiates as it spirals down into the black hole.

COSMIC HISTORY
Cosmology is the study of the nature and evolution of the universe.
Assumptions of cosmology:
1) The univeriasity of physical laws ("Law of Univerisality").
2) The cosmos is homogeneous-matter and radiation are spread out uniformally.
3) The universe is isotropic-cosmological principle-no direction or place in space can be distinguished from any other by any experiment or observation.
Observable universe is all that can be seen with telescopes.
Physical universe includes all observable matter as well as objects that we detect by the laws of physics.
What do we know?
-the universe evolves, changes with time
-matter in the universe is grouped, elementary particles make up protons, neutons, and electrons, which make up atoms.  Atoms make up gases and dust particles which form all matter in the universe
-the universe is expanding, the observed range is 15 to 30 km/s/Mly
Big Bang Model is the standard model of an expanding universe based on Einstein's general theory of relativity.  It is the best theory known to explain cosmological assumptions.
Cosmic blackbody microwave radiation-comes from all directions in space, its spectral shape is like a blackbody, its spectrum peaks at centimeter to millimeter wavelengths.  The radiation allows astronomers to glimpse the raw, young universe and conclude that the initial universe was homogeneous and isotropic.  The present temperature of the radiation (2.7 K) and the isotropy of the radiation set severe limits on the thermal history of the universe.  The radiations pressence establishes an important marker for galaxy formation, because until the radiation and matter stopped their interation, matter could not form large clumps. only after the ionized gas had recombined cound matter form clumps that eventually become stars and galaxies.  The radiation provides reference for measuring the motion of the Galaxy and the Local Group.
The primeval fireball-the hot beginning of the universe:
1) Temp 1012K, Time 10-6s: photons create pairs of particles and antiparticles.  The heavy particle era-photons have enough energy to create even the most massive elementary particles.
2) Temp 1013 to 6x109K, Time 10-6 to 6s: annihilation of heavy particles with their antiparticles.  The remaining photons lack the energy to create new heavy particles.  The light particle era-light particles like electrons can be made because less energy is required for their production from photons.  Protons and eletrons interact to create neutrons.  The radiation era also begins when neutrons decay into protons an electrons.
3) Temp 109K, Time 6 to 300s: remaining neutrons and protrons react to form nuclei of simple elements.  The net result is 25% helium compared to the rest of the matter.
4) Temp 3000K, Time 1 million years: temperature drops to deionize  matter.  Recombination-nuclei begin to capture electrons to form neutral atoms.  The neutral matter becomes transparent to radiation and light breaks through.  The radiation expands with the universe, and as it does it cools dowm to 2.7K as we detect today.  The matter era begins and material begins to clump together.

Galaxy formation: How did an originally very smooth universe become so clumpy?
A dense patch of gas in th early universe will have a high internal pressure because the radiation adds to the pressure force and pushes the patch apart.  When radiation pressure no longer resists gravity, small distrubances can condense out of the gas, along with disturbances of greater mass.  One million years after th Big Bang, galaxy formation began in the universe, called decoupling.  The pressure waves would form two kinds:
Isothermal: Mass=105 Temperature does not change
Adiabatic: Mass=1012 Energy is not transfered from the wave to its surroundings and visa versa

Some problems with this model include the fact that the disturbances in the early universe would grow very slowly, too slow to form the universe that we know today.
Grand Unification Theory (GUTs) are physical theories that attempt to unite the elementary particles and the forces of nature (gravitation, electromagnetism) as te actions of one particle and one force.  These theories are used to explain the dicrepancies in the Big Bang model.  GUTs has simplified our knowledge of elementary particles into two classes: quarks-an elementary particle that makes up other particles like protrons and neutrons.
lepton-an elementary particle that makes up light particles like electrons.
The final "freezing" of the universe takes place at one second when electromagnetic and weak forces split.  Quarks combine to form particles with then react in the nucleosynthesis era.  At 10-35s the electoweak force seperated from the strong force and an enormous amount of energy was released.  This marks the inflation era when the universe grew in size by many powers of ten.  Ripples, beginning with the Big Bang may account for the clumping of material into what are now galaxies.

Inflationary Universe Model-a modification of the Big Bang model in which the universe undergoes an early, brief interval of rapid expansion.  It copes with two major problems in the Big Bang model.
The flatness problem-the fact that the geometry of the universe is flat and remains flat as the universe evolves.  When the universe inflated, curved regions would have to become flat, just as the curved part of the surface of a balloon getter flatter as it expands.
The horizon problem-the problem that arises from the rapid expansion of the early universe, when different regions could not communicate.  The universe expanded so fast that it surpassed the horizon distance-the maximum distance that light can travel in some epoch of the universe.  The inflationary universe model solves this problem by shrinking the region of rapid expansion.
The dark matter problem is still inconslusive.  It could determine if the universe is open and will expand forever or if it is closes and will one day close in on itself in a cosmic crunch.

The present galaxies formed by two methods:
1) Gas fragmentation-created spiral and irregular galaxies.
-began as 1012 solar masses that slowly cooled and delayed star formation because thermal energy took a long time to radiate out.
-gas flattened out due to conservation of angular momentum and fragmented into 1-2 dozen pieces which became galaxies.
2)Hierarchical clustering-created star clusters and elliptical galaxies
-began as 105-108 solar masses that cooled quickly so star formed before it could flatten out into a disk.
-many collisions created much "cannibalizatio" where stars and star clusters built up into larger ellipticals
Small clusters of galaxies consist mostly of spirals and irregulars because they all formed from the same cloud; eg. The Local Group.  Large clusters are dominated by large elliptical clusters which through hierarchical clustering created immensely dense galaxies;  eg. The Virgo Cluster.