Download UNIT 4 - Galaxies XIV. The Milky Way A. Structure

UNIT 4 - Galaxies
XIV. The Milky Way
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galaxy - a huge collection of millions or billions of stars, gas, and dust, isolated in space
and held together by its own gravity
A. Structure
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our sun and solar system belong to a galaxy called the Milky Way within the
flattened, circular region called the galactic disk
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seen in the night sky as a band of light
called the Milky Way by ancients - looked like a trail of milk spilled in the
sky by a goddess nursing her baby
light is the glow of billions of stars of our galaxy
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the center of our galaxy contains older stars in a broader region called the galactic bulge
spherical ball of faint stars surrounds our galaxy - called galactic halo
William Herschel (18th C) estimated the size and shape of our galaxy based on counting stars
thought the galaxy was flattened and disk shaped with the sun at the center
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Herschel did not know of the intervening gas and dust that blocks our view of stars
within the Milky Way
hides the true shape of our galaxy - allows us to see only so far
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to know the true shape of the Milky Way, the distance to very distant stars must be found
astronomers use variable stars to find the distance to distant parts of the Milky way and
other galaxies
variable stars change in luminosity over
time
some pulsate regularly with a characteristic
light curve - intrinsic variables
M31 - Andromeda Galaxy
M110
M32
Structure
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intrinsic variable stars represent a post main
sequence stage of evolution where a star undergoes a
period of instability
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temperature and radius of the star vary in a regular
way when radiation is unable to radiate away from the
interior of the star
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RR Lyrae variables - low mass post main sequence stars
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Cepheid's variables can be seen for millions of light years
their period of variability is related to their luminosity
comparing the star's luminosity with its apparent
brightness will give an estimate of its distance by the
inverse square law:
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Cepheid variables - high mass post main sequence stars
apparent brightness = luminosity
distance2
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Harlow Shapley measured the distance of RR Lyrae stars in globular clusters
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found these clusters to be many thousands of light years away
when measuring the distance and direction of these clusters, the sun was found not to be at the
center of all the globular clusters, but in the direction of Sagittarius
Structure
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globular clusters are contained within the halo of the galaxy
this maps out the true galactic center of the Milky Way and its size
ten times larger than previously thought
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the sun is not at the center of the Milky Way, but rather approximately 2/3 away from the center
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the Milky Way appears as a giant pinwheel (spiral) approximately 100,000 light
years across
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spiral arms contain gas and dust and is the site for star formation within the galaxy
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the disk is 1,000 - 3,000 light years thick
young stars (population I stars; have heavier elements) and gas are confined close to
the galactic plane where they form
older stars (population II stars; contain only hydrogen and helium) drift further out
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the galactic halo contains no gas and dust
no new stars form there
all stars formed there formed during the
formation of the Milky Way (at least 10 billion
years ago)
central bulge is football shaped and about
10,000 light years thick
outer part contains old stars (gas poor)
inner part has both old and young stars (gas rich)
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Milky Way rotates (like a pinwheel) once in about 225 million years (sun's distance from center)
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rotation is differential - the Milky Way rotates faster closer to the center and slower further out
also observed in other spiral galaxies
halo stars and globular cluster orbit the center of the galaxy in randomly in all directions
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Structure
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Interstellar gas and dust prevent observations of stars to great distances within the galaxy
radio telescopes (long wavelengths) can "see" through the dust and allow astronomers to map the distribution of
gas and dust in the galaxy - Milky Way has a spiral structure
Spiral arms contain gas and dust and is the site for star formation within the galaxy
appears bright and blue due to the blue color of high mass stars
do not live long so they are concentrated within the spiral arms
the galactic bulge is densely populated with stars
approximately 1 million times more dense than the region about the sun
at the galactic center (strong radio source called Sagittarius A) lies a supermassive
black hole
based on observations of a rotating ring of material over a few light years across
B. Mass
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to measure the mass of the galaxy, the orbital motion of stars at the outermost regions must be
measured
objects (stars) furthest from the center of the galaxy should orbit around the center slower than objects
closer to the center
as you get further from the center, there should be less material (stars, gas)
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at a distance of approximately 50,000 light years from the center, the orbital speed of stars and gas
increases slightly
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this implies that there is more mass outside the visible part of the
galaxy than can be accounted for
the visible part of the galaxy is only a small portion of the true
extent of the galaxy
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the visible portion of the galaxy is surrounded by a dark halo which consists of dark matter
cannot be seen
only its gravitational effects can be observed
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may consist of black dwarfs, brown dwarfs, black holes (all know as MACHOs - MAssive Compact Halo
Objects)
exotic massive subatomic particles that do not interact with ordinary matter called WIMPs (Weakly
Interacting Massive Particles) that were produced in the formation of the universe
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A map of the galaxy cluster
CL0024+1654: dark matter
appears as a halo in blue,
while visible matter is in red
XV. Normal Galaxies
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Galaxies do not all look alike
they are not all like spiral galaxies as the Milky Way
Edwin Hubble in 1926 categorized galaxies based on their appearance
called Hubble Classification Scheme
still used today
there are four basic types of galaxies:
Spirals
Barred Spirals
Ellipticals
Irregular
Barred Spiral
Spiral
A. Spiral Galaxies
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the Milky Way and its neighbor the
Andromeda Galaxy are examples of
Spiral Galaxies
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all contain a galactic disk (with spiral
arms), a central galactic bulge, and a faint
halo of old stars
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spiral galaxies are denoted by the letter S and subdivided by their central bulge
size: a, b, or c
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Sa galaxies have the largest central bulges and tightly wrapped spiral arms
Sc galaxies have the smallest central bulges with loose, poorly defined spiral arms
Elliptical
Irregular
B. Barred Spiral Galaxies
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Barred Spiral Galaxies have an elongated bar shaped
concentration of matter through the center of the galaxy
spiral arms project from the end of the bars rather than the
central bulge
barred spirals are denoted by the letters SB and subdivided into
categories a, b, and c (just like spirals - based on size of the central
bulge)
C. Elliptical Galaxies
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Elliptical Galaxies have no spiral arms and no flattened galactic disk
many are egg shaped
stellar density increases towards the center
elliptical galaxies are denoted by the letter E and subdivided by
how elliptical (egg shaped) they are
the most circular are E0, the most elongated are E7
there is a large range of sizes of elliptical galaxies from giant
ellipticals (contain trillions of stars) to dwarf ellipticals (contain a
few million stars)
dwarf ellipticals are far more numerous than giant ellipticals
elliptical galaxies lack spiral arms
contain little gas and thus have old, red, low-mass stars
intermediate between ellipticals and spirals are galaxies with a flat
disk, but no gas and no spiral arms
denoted S0 or SB0 (if bar present)
D. Irregular Galaxies
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Irregular Galaxies have no set geometric shape
they usually have much gas and dust and young blue stars
smallest irregular galaxies are called Dwarf Irregulars - most common
irregular type of galaxy
Small Megellanic Cloud
Large Megellanic Cloud
E. Hubble Classification Scheme
F. Distribution of Galaxies in Space
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to know where galaxies are in the universe, the distance to them must first be
known
Standard Candles - easily seen objects with luminosities that are know
comparing an objects apparent brightness to its luminosity gives the objects
distance (and the galaxy it resides in)
include Type 1 supernova, Cepheid Variables, and by observing the rotation
speed of galaxies (rotation speed gives the mass of the galaxy)
1. Galaxy Clusters
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galaxies tend to congregate in galaxy clusters
the Milky Way is part of a cluster called the Local Group
includes: 45 galaxies (Milky Way, Megellanic Clouds, Andromeda
Galaxy)- all gravitationally bound together
Local Group's diameter is approximately 6.5 million light years across
Virgo Cluster - next close galaxy
cluster – approximately 60 million
light years from the Milky Way
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contains 2,500 galaxies all gravitation bound together in a
space 10 million light years wide
2. Superclusters
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galaxy clusters tend to congregate into clusters
themselves forming huge superclusters
the Local Group, the Virgo Cluster and several other
clusters form a huge supercluster of galaxies centered
near the Virgo Cluster called the Local Supercluster
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the Local Supercluster contains several tens of
thousands of galaxies 150 million light years across
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beyond the Local Supercluster are countless other
superclusters of galaxies
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Newton's laws of gravity allow astronomers to measure the mass of a galaxy
masses of spiral galaxies are determined by the rotation speed of the spiral arms
measure the doppler shift of spectral lines for the galaxy
when measuring the mass of superclusters of galaxies, there appears to be much more mass within the cluster than
can be accounted for by the light emitted by the galaxies
this is dark matter
90% of the universe is composed of dark matter
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G. Masses of Galaxies
H. Formation and Evolution of Galaxies
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Astronomers do not know for certain how galaxies evolve into the categories within the Hubble
classification scheme
galaxies grow by the merging of smaller galaxies
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galaxy mergers are supported by computer
simulations of the early universe and by
observations of galaxies at great distances that
show these galaxies are smaller in size, blueish in
color (due to star formation from mergers) and
less regular in appearance
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galaxy collisions are common - galactic cannibalism
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some collisions between galaxies can cause distortions
leading to the formation of spiral arms where none existed
before
takes several hundred million years
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ex.: Hubble Deep Field
Evolution can account for the Hubble classification
sequence:
Spiral galaxies grow from the merger of small galaxies with
a larger spiral
the larger spiral retains its shape but grows larger
ex. Milky Way - shows evidence of past mergers
Elliptical galaxies result from the merger of galaxies of
similar sizes
these mergers destroy the spiral structure of the galaxies
the result after star formation has stopped is an elliptical
galaxy
observations of superclusters support this
giant ellipticals appear towards the center of superclusters
I. Hubble's Law
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Edwin Hubble's observations of the spectra of galaxies show that all galaxies (except a few
nearby galaxy systems) are moving away from the Milky Way in all directions
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galaxies furthest from us are moving faster away from us than galaxies that are closer
they are redshifted more
the greater the distance, the greater the red shift
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Hubble's Law - the rate at which a galaxy
recedes is directly proportional to its
distance from us:
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the entire universe is expanding
galaxies are getting further away from us
and each other
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the redshift caused by this expansion is called the cosmological redshift
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used as a distance measuring tool
has implications for the past and future evolution of the universe
Hubble's Constant (H0) - the value of the slope of the line which gives
the relation between recessional velocity and distance:
recessional velocity = H0 x distance
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the Hubble Constant specifies the rate of expansion of the universe
distance to an object is found by measuring the recessional velocity and dividing by
the Hubble constant
furthest objects are 13 billion years old
Hubble's Law
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using Hubble's Law, the distribution of galaxies and galaxy
clusters in the universe is not random
clusters of galaxies lie within filaments (surface of "bubbles")
which surround empty regions (voids) in space - much like suds
on soapy water
largest superclusters lie in regions where several bubbles meet - ex.
Virgo Supercluster
Great Wall - large scale filament structure - one of the largest known
structures in the universe
XVI. Active Galaxies and Quasars
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galaxies that are much brighter than normal are
known active galaxies
emit most of their radiation at long wavelengths
(radio waves) as opposed to normal galaxies (visible
wavelengths)
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there are three types of active galaxies: Seyfert Galaxies, Radio Galaxies, and Quasars
active galaxies look like normal galaxies but emit enormous amounts of radio/infrared
radiation
A. Seyfert Galaxies
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Seyfert Galaxies have properties between normal galaxies
and the most violent active galaxies
represent an evolutionary link between these extremes
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have large redshifts - very distant
in visible light, they look like normal spirals
NGC 4261
radio
Seyfert Galaxies
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spectral analysis of Seyfert galaxies indicates very rapid rotation near the galactic nucleus
energy emitted by Seyferts varies over time (can double or halve within a fraction of a year)
does not happen in normal galaxies
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this rapid change in brightness means the source of energy is very small in size
change in brightness occurs in less than a year so the size of the object must be less than one light year
across - very small for the amount of energy released
B. Radio Galaxies
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have many of the same characteristics as Seyfert galaxies
emit radiation from much larger areas (than central nucleus as in Seyfert galaxies)
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often the radio energy is emitted in two large extended regions called radio lobes - rounded clouds of gas
lobes are aligned with the center of the galaxy as material is ejected
most radio galaxies are associated with elliptical galaxies
C. Quasars
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radio sources with star-like visible objects - quasi-stellar
radio sources (quasars)
unusual spectrum - lines are redshifted
cannot be stars - redshift indicates that these objects are
very distant (from the early universe)
their great distance implies these are the brightest objects
known in the universe
a quasars energy output varies irregularly
the region of energy production must be very small
recent observations suggest that most, if not all,
Quasars reside in a host galaxy
D. Central Engine of Active Galaxies
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all active galaxies have:
- high luminosities
- energy emission is non-stellar - cannot be accounted for by just stars
- energy output is variable
- often have jets of material (explosive activity)
- their spectra show rapid internal rotation within the energy producing region
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the huge energy produced must come from a supermassive compact object (supermassive
black holes) at the center of the nucleus where gas or other material is accreted onto it
(billions of times more massive than the sun)
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1 billion solar mass black hole would have a diameter of only 20 AU
gas within the accretion disk would be heated to billions of degrees
observations from the Hubble Space Telescope support this theory - supermassive black
hole with an accretion disk
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some quasars exhibit gravitational lensing - the
gravitational field of a foreground galaxy (galaxy cluster)
bends the light of a of a distant galaxy or quasar into
multiple images
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can determine the masses of the galaxy (galaxy clusters)
including dark matter
Central Engine of Active Galaxies
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large luminosity of quasars is the result of there being more gas (fuel) available in the early universe
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quasars spend a relatively short period of time in this
highly luminous phase
represents an early phase in galaxy evolution
early formed galaxies may have merged and conditions existed (plentiful gas) to form supermassive black
holes and thus highly luminous quasars the result
as the fuel diminished, the quasars dimmed and the galaxy is visible as Seyfert and radio galaxies
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as fuel supply diminished further, central core became inactive and Seyfert spirals became normal spirals
and radio galaxies normal ellipticals