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Chapter 24 The Galaxy The Milky Way Galaxy = The Galaxy Phenomenon

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Chapter 24 The Galaxy The Milky Way Galaxy = The Galaxy Phenomenon Powered By Docstoc
					Chapter 24 The Galaxy
The Milky Way Galaxy = The Galaxy
Phenomenon of the Milky Way in the Sky
Milky Way = the band of faint white light that encircles the entire inside of the
celestial sphere (it is brighter in the direction of the summer night sky)
Significance of this observation about the structure of the Galaxy: we live in a flat
disk shaped galaxy and we are not at the center of it

Structure of the Galaxy
I. Flat wheel shaped disk 100,000 LY in diameter with two parts
    o Thin disk: 1,000 LY
    o Thick disk: 3,000 LY in thickness
In these disks lie the
    o Open star clusters
    o Young O and B associations
    o GMC’s
    o Cold H I regions
    o Hot H II regions
II. Spherical concentric halo a few times that diameter, contains old globular star
clusters
III. Central bulge 24,000 LY in diameter, contains older stars spaced very close
together
Old planetary nebulas are found in all parts

Sun’s Location in the Galaxy
In the disk, about 26,000 LY (0.6 of the way out) from the center
Harlow Shapley, using the RR Lyrae variable stars in the globular star clusters, first
determined this

Spiral Structure of the Galaxy’s Disk
At optical wavelengths dust prevents a complete view of the disk, we can only “see”
about 1,000 to 3,000 LY into the disk
Optically we do see bright blue O and B spectral class stars along the spiral arms
near us (just as they are seen along spiral arms of other galaxies)
Must use radio observations of cold H I clouds in the disk to map out the spiral
structure
These clouds emit 21 cm radio waves

Stellar Populations
“Metallicity” – astronomers put the elements into only three groups: hydrogen,
helium, and “everything else”
“Metals” refers to “everything else” and includes some elements that are gases
Population I – stars with relatively large percent of metals (and objects related to
these kinds of stars). Our sun and other objects from spiral arms
   o Open clusters
   o Young stars
   o Cepheid variables
   o Associations
Population II – stars with very low percent of metals (considered “1st generation
stars”)
   o Globular star clusters
   o Dim, red main sequence stars
   o RR Lyrae variable stars

Rotation of the Galaxy
The motion of the Sun with respect to distant globular star clusters has been
measured
Doppler effect in their spectra gives a roughly circular orbit with a velocity of about
220 km/sec (toward Cygnus)
The Sun’s galactic orbit has a circumference of 2(26,000 LY)
To travel this distance at 220 km/sec requires about 225 million years
Mass of the Galaxy – use of Newton’s revision to Kepler’s 3rd law

(m1 + m2) p2 = a3

p = 225 million years and a = 26,000 LY (converted to AU), about 100 billion solar
masses
If the typical mass per star is ½ solar mass then the number of stars in the Galaxy
is at least twice this: 200 billion stars (there may be more mass than this!)

Dark Matter in (and around) the Galaxy
The disk of the Galaxy contains lots of interstellar dust, but this is NOT dark matter
Dark matter = material that is not emitting any kind of radiation, invisible at all
wavelengths
The presence of this dark matter has been deduced by studying the orbital motion
of various objects in the Galaxy
A “galactic rotation curve” shows that the velocity of objects does NOT fall off with
increasing distance from center, but stays “flat”
90 percent of the mass of the Galaxy may be this “dark matter”
Could be:
    o Black holes
    o Dim red stars
    o Brown dwarfs
    o Neutrinos or other exotic particles
    o Weakly Interacting Massive Particle (WIMPs)
Real reason has not been discovered

Nucleus of the Galaxy
The strongest radio source in Sagittarius is called “Sagittarius A”
It lies about 24,000 LY beyond the stars that make the pattern of Sagittarius
The VLA = Very Large Array radio telescope system in New Mexico has resolved this
into components as small as 3 AU across.
The part known as “Sagittarius A*” lies at the center of the Galaxy and is believed
to contain a super massive black hole. The diameter of the event horizon of a black
hole with 1 million solar masses would be 6 million km (3.5 million miles)
As matter gradually falls toward the SMBH an accretion disk has formed and is
responsible for the strong radio signals

The Origin and Evolution of the Galaxy
The age of the universe is estimated to be about 13.7 billion years and the age of
the Galaxy is somewhat less
Soon after the formation of the universe, from the “Big Bang”, only energy existed.
Later, matter began to condense from the energy
After more than 400,000 years galaxies such as ours began to form from the huge
collections of (mainly) hydrogen atoms
The huge collections of gas began a gravitational contraction to form the parts of
the Galaxy we know now:
    o Halo is the oldest part in the senses that stars and globular star clusters of
       stars are still around that formed early
    o Disk formed due to the rotation of the proto galaxy. Both young and old stars
       are found in the disk and it is the only part where stars are still forming

Chapter 25 Galaxies
The True Nature of the “Nebulae”
Catalogs of nebulas:
   o Messier (French comet hunter)
   o New General Catalog (William Herschel)
The Great Spiral Nebula in Andromeda:
   o M31 in the Messier Catalogue
   o NGC 224 in the NGC

Hubble’s Distance to M31
He used Cepheid variable stars and the “period luminosity relationship”
It is now the “the Great Spiral Galaxy in Andromeda”, 2,500,000 LY (770 kpc) away

Classification of Galaxies
Based on apparent shape seen in sky:
Elliptical (E)
    o Range: circular (E0) to elongated (E7)
    o Contain little gas/dust, no current star formation
Normal Spiral (S)
    o Range: large nucleus and tightly wound spiral arms to small nucleus and
        loosely wound arms (Sa > Sb > Sc)
Barred Spiral (SB)
    o Range: SBa > SBc like normal spirals but arms emerge from a “bar” in the
        center
Irregular (Irr)
    o No symmetry, have gas/dust, young/old stars

Properties of Galaxies
Amount of rotation of original proto galaxy may be the main reason for different
galaxy types
   o Little or no rotation > elliptical
   o Rotation > spiral
The irregular galaxies tend to be the smallest, the dimmest, and ones with fewer
stars
The spirals tend to be large, but the ellipticals are the largest of all
The ellipticals also have the greatest range in size, brightness, and number of stars
(can be giant or dwarf, nothing in between)
The ellipticals have little or no observable gas or dust and thus no new star
formation, unlike spirals and irregulars
Most of the apparently bright galaxies are of the spiral type, but if we include faint
dwarf ellipticals and faint irregulars, spirals are actually in the minority
The most numerous class of the galaxy is the irregular

Methods of Determining Distances
Measurements of the properties of galaxies depends on knowledge of their
distances
Astronomers use a variety of methods that generally build upon one another
The fundamental distance determination method is triangulation or surveying,
which uses the parallax of nearby stars (d = 1/p)
For greater distances other methods are used. One of these can be called the
“Standard Light Bulb” method (formerly “Standard Candle”)
Basically this means finding some way to estimate the true brightness of an object
and then calculate how far away it has to be to look that dim

Examples
I. Variable stars (RR Lyrae, Cepheid)
    o These distance indicators are easily recognized by their colors, their periods,
       and amplitudes of light variation. RR Lyrae stars all have M = 0.5 while the
       Cepheids obey the Period-Luminosity relation.
II. Novas, supernovas
    o These eruptive variables occur in our Galaxy and in nearby galaxies where
       we can estimate their maximum brightness. When one occurs in a more
       distant galaxy, we assume it has the same M as the ones closer to us

The “Law of the Redshifts”
Slipher discovered that 38 of 41 bright “spiral nebulas” (galaxies) had redshifts in
their spectra (i.e. they are moving away from us)
Edwin Hubble and Milton Humason continued this work by obtaining spectra of
more galaxies
They found that the fainter and presumably more distant galaxies always had
greater redshifts in their spectra (“Law of the Redshifts”)
Let z = “redshift”
                             z = [WL(observed) – WL(rest)]
                                        WL(rest)
Assuming z is due to the Doppler effect
                           v = c x [WL(observed) – WL(rest)]
                                        WL(rest)
                           v = c x z = cz (velocity in km/sec)
The Hubble Law
Hubble and Humason also measured the distances (using Cepheids) to the galaxies
they got redshifts for and found a linear relationship by converting z to v and
graphed vs. the distances they had measured.
The slope of the line is called H (for Hubble and Humason)
H = v / d = the Hubble “ratio”, Hubble Constant, measured in km/sec per Mpc
(or km/sec per MLY)
Thus v = H x d = Hd “Hubble Law”
Assume H = 25 km/sec per MLY, and then solve for the distance, d
d = v/H = cz / H = 300,000 z / 25
d = 12,000 z (in MLY)

Example: find d if a galaxy has z = 0.05
d = (300,000 x 0.05) / 25 = 12,000 x 0.05 = 600 MLY

Notes: use of the Hubble Law to measure distances is the biggest “meterstick”
Can use this when no objects of any kind can be resolved in a galaxy, when only
the spectrum can be obtained
The discovery of the Hubble Law also means that the universe is expanding

Chapter 26 Quasars and Active Galaxies
Active galaxies = those that have bright nuclei associated with great energy
production or energetic outflows of matter (most emit strong radio waves)
(Radio galaxies = active galaxies with more radio output than light output)
   o Seyfert galaxies = spiral galaxies with bright, star-like nuclei, and strong
       emission lines in their spectra (NGC 1566)
   o Double lobe = 2 regions of strong radio emission occur on either side of a
       peculiar galaxy (peculiar means the galaxy does not it well into the Hubble
       classification scheme (Cygnus A, Centaurus A)
   o Jet = a stream of material projects from the center of a galaxy. The stream
       is emitting radio waves, sometimes light and other wavelengths, like x rays
       (elliptical M 87)
The observations of these active galaxies imply that some unknown object exists at
the center of an active galaxy

The “Look Back” Principle
The farther you look into space the earlier back in time you see
The reason for this is the finite speed of light
300,000 km/sec (or 1 LY / year)
We see the Sun not as it is now, but as it was about 8 minutes ago
M 31, at a distance of 2.5 million LY, as it was about 2.5 million years ago
We see a galaxy with z = 0.05 not as it is now, but as it was 600 million years ago

Quasars
A new kind of object was discovered in the early 1960’s based on radio emission
Optical photos showed “star like” dots
Since stars are not strong radio sources, these were called quasars (from quasi
stellar radio sources)
3C 48 was the first quasar discovered
Spectra taken to learn about them only added to the puzzle
Later, M. Schmidt (1963) explained that these spectra had emission lines with large
redshifts (the quasars are moving away very rapidly and are very far away)
They must have a great luminosity to be seen at distances of billions of LY
Some quasars very in times as short as a few days
This implies that the region responsible for the great luminosity must be about a
few light days across: only about 10 times the size of our solar system!
To explain quasars we must use the “look back” principle: if the quasars are billions
of LY from us, we must be seeing objects as they were billions of years ago
Quasars must be the hyperactive centers of very distant, thus young galaxies

The Power Source of Quasars
Thousands of quasars have now been discovered, but only 1% are strong radio
emitters (thus “QSO’s”)
They are all very luminous and all show big redshifts in their spectra
The accretion disks of supermassive black holes are believed to be the power
source
No radiation can be emitted from the black hole itself!
The accretion disk funnels energy out along the two poles of the magnetic field, at
right angles to the disk. This produces two strong jets of particles and energy
If we see one of the jets nearly face on, it is perceived as a quasar
If we see the jets at other angles, we perceive active galaxies with jets or double
lobed radio emission

Gravitational Lenses
Because quasars produce very bright and very narrow beams of light, they can be
used to test Einstein’s General Relativity.
Space should be curbed in the vicinity of dense concentrations of matter
Light from a distant quasar could be deflected as it passes by a massive galaxy or
cluster of galaxies billions of LY away
In 1979 the first “double quasar” was discovered. It is actually light from one
quasar that has been split into two beams and deflected to appear as two
The name is “0957+561” based on its position in the sky
Einstein “rings”, “crosses”, and “arcs” have also been detected, confirming his
prediction of the curvature of space in the vicinity of compact mass

Chapter 27 Evolution and Distribution of Galaxies
Observations of Distant Galaxies
Spectra, colors, and shapes
Spectra show “metals” in the most remote galaxies we can now access
Thus galaxies formed within the first billion years (maybe within a few hundred
million years) of the universe
Do galaxies develop or evolve? Nearby galaxies tend not to except for collisions
But they must have, early in the history of the universe. To see such galaxies we
must look far back into space (and early back in time)
We can now do this with the HST and big new ground-based telescopes like Keck I
and II
For distant galaxies we study their color (blue=young, red=old), and shape
(spiral=young, elliptical=old)
Long exposure (deep) images obtained through large telescopes show that distant
galaxies do not fit into Hubble’s classification scheme
They are also generally smaller than nearby, modern galaxies
Thus early galaxies evolved or developed with time, primarily by interacting
(colliding) with each other

The Interaction of Modern Galaxies
Currently, galaxies don’t seem to evolve much unless they collide
When galaxies collide, stars don’t
Reason: separation between galaxies is tens of times their diameters; separation
between stars is 10,000,000 of times their diameters
In merger two roughly equal galaxies combine. In cannibalism a larger galaxy
devours a smaller one.

The Distribution of Galaxies in Space
Various studies of the distribution of galaxies in space indicate that the universe is
isotropic and homogenous
Thus, “the universe must look the same to any observer on very large scales”. This
is known as the cosmological principle

Congregation Levels of Galaxies
Name         Number of Galaxies           Size (MLY)
Group        10s                          3 million LY
Cluster      100s to 1,000s               10 – 20 million LY
Supercluster 10,000s                      150 – 300 million LY

Examples:
The “Local Group” (we are in it)
The Virgo cluster d=50 MLY
The Coma cluster d=250 – 300 MLY
The “Local Supercluster” (we are in it)

Note: “regular” clusters of galaxies are symmetrical and contain mostly ellipticals
The “irregular” clusters of galaxies are not symmetrical and have some spirals
“Rich” and “poor” refer to clusters with many and few galaxies, respectively

Large Scale Structure of the Universe
Detailed studies in certain directions reveal a “sponge – like” distribution of
superclusters of galaxies
The superclusters of galaxies are like the material of a sponge that surrounds
empty pockets or bubbles
The empty pockets in the universe are called voids
Voids are typically 150 MLY across and superclusters can appear strung out to 600
MLY

The Hubble Ultra – Deep Field
Formation of the Structure in the Universe
Forming voids and filaments
The large scale structures are believed to result from processed that occurred when
the universe was only a few hundred thousand years old
Although the density of the early universe must have been very smooth, the slight
irregularities led to the formation of proto-galaxies (pancakes of gas)
These proto-galaxies joined to make small galaxies. These in turn congregated to
make larger galaxies and clusters of galaxies

A Universe of (Mostly) Dark Matter
In the neighborhood
Our flat galactic rotation curve implies the presence of dark matter

Around galaxies
Studies of rotation rates of other spirals lead to mass estimates. Flat rotation
curves imply dark matter for most spirals, as in our galaxy

In clusters of galaxies
Galaxies should fly away from each other, but don’t. Must be mass we don’t see.

Mass-to-light ratio
In units of solar values, the Sun = 1, but for groups, clusters, and superclusters of
galaxies the values are in the 100s indicating that most matter in them is not
radiating light. Dark matter is NOT dust.

What is the dark matter?
Dark matter may be 10 times more abundant than luminous matter in most
galaxies. We still don’t know what it is.
The experiment to look for “ MACHOs” (massive astrophysical compact halo
objects) in the halo of the galaxy by their gravitational lensing properties has found
far too few to explain the dark matter

Chapter 28 The Big Bang
Cosmology = the study of the origin, structure, and evolution and fate of the
universe (if any)
“The universe” (everything that there is!) includes stars, comets, planets, galaxies,
etc. and space itself

Note: “string theory” postulates the existence of additional dimensions and other
possible universes but we shall consider only what we can currently see

Observational Cosmology
Olber’s paradox: if the universe is infinite, with stars uniformly distributed within it
and is static, “why is the night sky dark?”

The expanding universe
One profound implication of Hubble’s result is that the universe is observed to be
expanding
In an expanding universe, the velocity at which a galaxy recedes from us increases
with increasing distance
Are the more-distant galaxies flying through space at ever-greater speeds?
No! Space itself is expanding, carrying clusters and superclusters of galaxies in it.
The galaxies themselves do not get larger.

Important implications of an expanding universe:

I. There is no center
All observers in the universe see the same result! All other galaxies seem to recede
from any given galaxy. Thus there is NOT a center to the universe (it is a
meaningless concept… or is it?)

II. There is no edge (whether finite or not)

III. Gravity tries to slow down the expansion
Studies of distant Type Ia supernovas indicate they are fainter than expected
This can be explained by an acceleration of the expansion rate
Since the cause of such acceleration is unknown, the generic term “dark energy”
has been coined for the name

IV. There must have been a beginning for time and space! (= the “Big Bang”)
We must also realize that it is not appropriate to use the doppler effect to describe
redshift
Superclusters are not moving through space, rather space expands and separates
the superclusters
This is the cosmological redshift = space expands. Fortunately this can still be
described by an equation like the doppler equation

                                    The “Big Bang”
Era                 Time          Temp (K)    Remarks
?                   0             ?           “Big Bang” initiated expansion
Supergravity        10-43 s       1032        Planck time only radiation existed
Inflation           10-32 s       1027        Rapid expansion, 1050 x bigger
                                    10
Particle            1s            10          Protons, quarks, electrons, radiation
Nucleo              180-300 s     108         Temp low enough for fusion, H and He
Radiation/matter    4x105 years   3000        Universe transparent to radiation
Matter              2x108 years   100         Galaxies formed
Current             13.7 BY       2.725       Now

Observations Supporting the “Big Bang”
I. Quasar census: there are no quasars locally, now; but many far away, long ago.
This means the universe is changing as the “Big Bang” theory predicts
II. Cosmic microwave background radiation = “CBR” was discovered in 1965 by
Penzias and Wilson using a (microwave) radio telescope. This radiation comes to us
from all directions in space equally and all the time. The “CBR” is very strong
evidence of the “Big Bang” event
   o Point a radio telescope in a random direction
   o Look out into space as far as possible
   o The earliest back in time that we can see is when the universe cooled to
      about 3000 K at age = 400,000
   o The peak emission from that spot in the universe was then in the visible
   o BUT the Hubble Law states that if radiation comes from great distances, it
      will be redshifted to infrared and radio (microwave) wavelengths
The COBE (cosmic background explorer) satellite in 1990 measured the CBR and
showed a perfect fit to the theory (T = 2.725 K)
Results from the WMAP (Wilkinson microwave anisotropy probe) indicate that the
age of the universe if 13.7 BY (plus/minus 0.2 BY). The time at which the universe
became transparent to radiation has been pinned down to 379,000 years
(plus/minus 8,000 years)

Theoretical Cosmology
Cosmological principle = universe must look the same to any observer on a very
large scale
Perfect cosmological principle = universe must look the same to an observer on a
very large scale for all time
This led in the 1950s to the steady state theory = the universe has always existed
and will always exist
Evidence of evolution of the universe and the discovery of CBR are inconsistent with
this theory

“Big Bang” Theory (Plus Inflation)
This is the current best explanation that astronomers have developed for the origin
and evolution of the universe
Inflation had to be added to the BB to explain the “flatness” problem
If the universe if 13.7 BY old, then two superclusters in opposite directions are 27.5
BLY apart, but have similar properties
Must have formed close together, then sudden drastic expansion (inflation)
occurred briefly, THEN normal expansion resumed
During the inflation era space ballooned up by a factor of 1050 in a fraction of a
second
This is easily faster than light but that is okay because nothing was moving through
space that fast

Future of the BB Universe
Depends on which version we live in:
   o Closed: universe stops expanding after a finite time, then a contracting
      phase occurs (the “Big Crunch” or oscillation)
   o Open: universe still expanding after infinite time, eventually everything will
      get dark and cold everywhere
   o Flat: universe coasts to a halt after infinite time, this is a unique version of
      the BB
There are three possible geometries for a BB universe
    o Spherical space: two lasers come together (closed)
    o Flat space: parallel beams remain parallel forever (flat)
    o Hyperbolical space: two beams spread apart (open)
Which version is it?
Measure mass of all things seen in universe, divide by the volume
If the average density is:
> Critical density >> closed universe
< Critical density >> open universe
= Critical density >> flat universe

Originally we calculated <, which implies an open universe
Discovery of dark matter increases the average density to nearly the critical density
We think we are in a flat universe

Relativistic Evolving BB Models of the Universe
(Assumes H = 25 km/s per MLY, critical density = 2x10-29 gm/cm3)

Version      Age (BY)           Curvature     Average Density           Extent
Closed       t<9                +             > critical                finite
Open         9<t<14             -             < critical                infinite
Flat         t=14               0             =critical                 infinite

Measures of luminous mass indicate an average density less than critical (open
universe), BUT including dark matter pushes the average density near the critical
density (flat universe)
In order to visualize the 3 possible curvatures of the universe, consider these two-
dimensional analogs

Resolutions of Olbers’ Paradox
If any of the underlying assumptions can be shown false, then there is no paradox

I. The paradox implicitly assumes a static universe, but we now have evidence that
the universe is expanding
Thus, light from distant stars can be redshifted to invisibility in the infrared and
radio part of the spectrum

II. The paradox assumed an infinitely old universe in which light from infinitely far
away has had time to get here
BB says universe began 13.7 BY ago, so light from states say, 400 BLY away hasn’t
had time to get here yet
This resolves the paradox

The distance of 13.7 BLY is called the radius of the “cosmic horizon”
It is equal to the distance in LY given by the age of the universe in years
The “observable universe”, on the other hand, is how far we can see with current
technology. This has increased dramatically since about 1600!

Chapter 29 Life in the Universe
Life = carbon based, water based, i.e. “life as we know it”
This would seem to require: planets like the Earth, orbiting stars like the Sun
The Doppler effect has discovered more than 300 Jovian-like planets orbiting
nearby stars
None of these are terrestrial type planets yet!

Probability of Extra-Terrestrial Life
Drake equation (an estimate of the current number, Ncc, of intelligent,
communicative civilizations in the galaxy)
Ncc is the product of several factors:
The first two are astronomical in nature and are reasonably well known
The next several are biological in scope and not so well known
The last factor is the most uncertain: the longevity of civilizations
It is the most important factor and the one for which we have only one known case
(ours!)

Optimistic Ncc = (1)(0.5)(0.1)(1)(1)(0.5)(40x106) = 1 million!
Pessimistic Ncc = (1)(0.5)(0.1)(0.1)(0.1)(0.1)(4x103) = 1!

				
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