Download HNRS 227 Lecture #2 Chapters 2 and 3

HNRS 227 Lecture #16, 17
Chapter 12
The Universe
presented by Prof. Geller
21, 26 October 2004
Key Points of Chapter 12 (including
material no longer in textbook)
The Night Sky
Historical View of Our Universe
geocentric model of the universe
• Ptolemaic Model
heliocentric model of the universe
• Copernican Model
Coordinate Systems – how do we find
something in the sky?
Local Horizon System
• altitude, azimuth
Celestial Coordinate System
• right ascension, declination
Key Points of Chapter 12 (including
material no longer in textbook)
The Structure of Stars
The Brightness of Stars
The Temperature of Stars
The Types of Stars
The Life Cycle of Stars
Galaxies
Hubble classification of galaxies
AGNs
Key Points of Chapters 12 (including
material no longer in textbook)
The Big Bang
Theories of creation of universe
Big Bang vs. Steady-state
The Curvature of the Space-Time
Continuum
The destiny of the universe
The density of matter in the universe
• Dark matter and dark energy
Some Fundamental Units in
Astronomy
Angular measure
1 degree = 60 minutes = 3600 seconds
Hour-angle measure
one hour is 15 degrees of arc
Light Year
distance traveled by light in a year
Almost 6 trillion miles
• Calculate it
Astronomical Unit (AU)
mean distance of Earth to Sun
Historical Perspective
Geocentric Model of the Universe
Earth at center
Ptolemaic model
• Ptolemy then church
Heliocentric Model of the Universe
Sun at center
Copernicus (some early Greeks before), Galileo,
Kepler, Tycho, Newton
The Night Sky
Finding an object in the sky
Relative to your location and time
Altitude – angular measure above horizon
Azimuth – angular measure for direction starting
at North and going eastward along the horizon
Independent of location on Earth
Right Ascension – hour angle from Vernal Equinox
Declination – angular measure above or below
celestial equator
The Stars in the Sky
The Brightest Stars
Common
Name
Sun
Sirius
Canopus
Rigil
Kentaurus
Arcturus
Vega
Capella
Scientific
Name
Sol
Alpha CMa
Alpha Car
Alpha Cen
Distance
(light years)
1.5 x 10-5
8.6
74
4.3
Apparent
Magnitude
-26.72
-1.46
-0.72
-0.27
Absolute
Magnitude
4.8
1.4
-2.5
4.4
Spectral
Type
G2V
A1Vm
A9II
G2V + K1V
Alpha Boo
Alpha Lyr
Alpha Aur
34
25
41
-0.04
0.03
0.08
0.2
0.6
0.4
Rigel
Procyon
Achernar
Betelgeuse
Hadar
Acrux
Beta Ori
Alpha CMi
Alpha Eri
Alpha Ori
Beta Cen
Alpha Cru
~1400
11.4
69
~1400
320
510
0.12
0.38
0.46
0.50 (var.)
0.61 (var.)
0.76
-8.1
2.6
-1.3
-7.2
-4.4
-4.6
Altair
Aldebaran
Antares
Spica
Pollux
Fomalhaut
Becrux
Deneb
Regulus
Adhara
Alpha Aql
Alpha Tau
Alpha Sco
Alpha Vir
Beta Gem
Alpha PsA
Beta Cru
Alpha Cyg
Alpha Leo
Epsilon
CMa
Alpha Gem
Gamma Cru
Lambda Sco
16
60
~520
220
40
22
460
1500
69
570
0.77
0.85
0.96
0.98
1.14
1.16
1.25
1.25
1.35
1.50
2.3
-0.3
-5.2
-3.2
0.7
2.0
-4.7
-7.2
-0.3
-4.8
K1.5IIIp
A0Va
G6III +
G2III
B81ae
F5IV-V
B3Vnp
M2Iab
B1III
B0.5Iv +
B1Vn
A7Vn
K5III
M1.5Iab
B1V
K0IIIb
A3Va
B0.5III
A2Ia
B7Vn
B2II
49
120
330
1.57
1.63 (var.)
1.63 (var.)
0.5
-1.2
-3.5
A1V + A2V
M3.5III
B1.5IV
Castor
Gacrux
Shaula
(var.)
(var.)
(var.)
(var.)
Interpreting the Table
Distance
In light years
Apparent Magnitude
Absolute Magnitude
Spectral Type (example for Sun which is G2V)
G is spectral class
2 is spectral sub-class
With spectral class leads to specific surface temperature
V is luminosity class
Giant, sub-giant or main sequence
• Main sequence is defined as hydrogen core fusion
Stellar Structure
Stellar Structure
Our Sun (and others)
Core
Radiation zone
Convection zone
Photosphere
A theoretical model of the Sun shows how
energy gets from its center to its surface
 Hydrogen fusion takes
place in a core extending
from the Sun’s center to
about 0.25 solar radius
 The core is surrounded by
a radiative zone extending
to about 0.71 solar radius
In this zone, energy travels
outward through radiative
diffusion
 The radiative zone is
surrounded by a rather
opaque convective zone of
gas at relatively low
temperature and pressure
In this zone, energy travels
outward primarily through
convection
Astronomers probe the solar interior
using the Sun’s own vibrations
Helioseismology is the
study of how the Sun
vibrates
These vibrations have
been used to infer
pressures, densities,
chemical
compositions, and
rotation rates within
the Sun
Neutrinos reveal information about the Sun’s
core—and have surprises of their own
Neutrinos emitted in
thermonuclear
reactions in the
Sun’s core have
been detected, but
in smaller numbers
than expected
Recent neutrino
experiments explain
why this is so
The photosphere is the lowest of three main
layers in the Sun’s atmosphere
 The Sun’s atmosphere
has three main layers:
the photosphere, the
chromosphere, and the
corona
 Everything below the
solar atmosphere is
called the solar interior
 The visible surface of the
Sun, the photosphere, is
the lowest layer in the
solar atmosphere
Convection in the photosphere
produces granules
Think:
How do
we
know?
The chromosphere is characterized
by spikes of rising gas
 Above the
photosphere is a layer
of less dense but
higher temperature
gases called the
chromosphere
 Spicules extend
upward from the
photosphere into the
chromosphere along
the boundaries of
supergranules
 The outermost
layer of the solar
atmosphere, the
corona, is made
of very hightemperature
gases at
extremely low
density
 The solar corona
blends into the
solar wind at
great distances
from the Sun
The corona ejects mass into space to
form the solar wind
Activity in the corona includes coronal
mass ejections and coronal holes
Apparent and Absolute
Magnitude
How bright something is in our sky
Apparent magnitude
How bright celestial object is compared to all
others
Absolute magnitude
Luminosity
Magnitude Scale
log scale
lower value brighter (x 2.5) than higher value
absolute versus apparent
absolute is magnitude at 10 parsecs
Astronomers often use the magnitude
scale to denote brightness
 The apparent magnitude
scale is an alternative
way to measure a star’s
apparent brightness
 The absolute magnitude
of a star is the apparent
magnitude it would have
if viewed from a distance
of 10 parsecs
From Wien’s Law: A star’s color depends on its
surface temperature
The spectra of stars reveal their chemical
compositions as well as surface temperatures
Stars are classified
into spectral types
(subdivisions of the
spectral classes O, B,
A, F, G, K, and M),
based on the major
patterns of spectral
lines in their spectra
Most brown dwarfs are in even cooler spectral
classes called L and T
Unlike true stars, brown dwarfs are too small to
sustain thermonuclear fusion
Relationship between a star’s luminosity,
radius, and surface temperature
Stars come in a wide variety of sizes
Finding Key Properties of Nearby Stars
Stellar Temperatures and
Classification
Temperature of stars
Wien’s Law
spectral classes based upon temperature
not linear scale
H-R Diagram
temperature versus absolute brightness
following the evolution of stars
Understanding the aging of stars requires both
observation and application of physical principles
Because stars shine by thermonuclear
reactions, they have a finite life span
The theory of stellar evolution (the life
cycle or aging of stars) describes how
stars form and change during their life
span
The Life Story of Stars
 Gravity squeezes
 Pressure forces resist
 Kinetic pressure of hot gases
 Repulsion from Pauli exclusion
principle for electrons - white dwarf
 Repulsion from Pauli exclusion
principle for neutrons - neutron star
 None equal to gravity - black hole
 Energy loss decreases
pressure
 Energy generation replaces
losses
 Star is “dead” when energy
generation stops
 White dwarf, neutron star, black hole
Luminosity
Surface
Gravity
Weight of outer layers
Gas
Pressure
Thermal
Energy
Center
The Spectral Measure of Stars Wien’s and Stefan-Boltzmann’s
Laws
The
HertzsprungRussell (HR)
Diagram
Interstellar gas and dust pervade the
galaxy
 Interstellar gas and dust, which
make up the interstellar medium,
are concentrated in the disk of the
Galaxy
 Clouds within the interstellar
medium are called nebulae
 Dark nebulae are so dense that
they are opaque
 They appear as dark blots against a
background of distant stars
 Emission nebulae, or H II regions,
are glowing, ionized clouds of gas
 Emission nebulae are powered by
ultraviolet light that they absorb
from nearby hot stars
 Reflection nebulae are produced
when starlight is reflected from
dust grains in the interstellar
medium, producing a characteristic
bluish glow
Interlude – Up in the Sky Tonight
Protostars form in cold, dark
nebulae
 Star formation begins in
dense, cold nebulae,
where gravitational
attraction causes a clump
of material to condense
into a protostar
 As a protostar grows by
the gravitational accretion
of gases, KelvinHelmholtz contraction
causes it to heat and
begin glowing
The more massive the protostar,
the more rapidly it evolves
Protostars evolve into main-sequence
stars
 A protostar’s relatively
low temperature and
high luminosity place it
in the upper right region
on an H-R diagram
 Further evolution of a
protostar causes it to
move toward the main
sequence on the H-R
diagram
 When its core
temperatures become
high enough to ignite
steady hydrogen
burning, it becomes a
main sequence star
Interlude - Humor
“OK stellar recruits, it’s time to learn what’s
really in store for you! I know that before you
signed up to be a massive star you read the
fancy brochures that talked about how brightly
you’d be shining and how you’d be visible from
halfway across the galaxy. But you mo-rons
must not have bothered to read the fine print
that said that you’d explode in seven million
years! And if you did read it then you’re even
stupider than you look. Seven million is not a
long time!”
– Eric Schulman [A Briefer History of Time]
Young star clusters give insight into star
formation and evolution
 Newborn stars may form
an open or galactic
cluster
 Stars are held together
in such a cluster by
gravity
 Occasionally a star
moving more rapidly
than average will escape,
or leave the cluster
 A stellar association is a
group of newborn stars
that are moving apart so
rapidly that their
gravitational attraction
for one another cannot
pull them into orbit
about one another
 Star-forming regions
appear when a giant
molecular cloud is
compressed
 This can be caused
by the cloud’s
passage through one
of the spiral arms of
our Galaxy, by a
supernova explosion,
or by other
mechanisms
Supernovae compress the interstellar
medium and can trigger star birth
A star’s lifetime on the main sequence is
proportional to its mass divided by its luminosity
 The duration of a star’s main sequence lifetime depends
on the amount of hydrogen in the star’s core and the
rate at which the hydrogen is consumed
 The more massive a star, the shorter is its mainsequence lifetime
The Sun has been a main-sequence star for about 4.56 billion years
and should remain one for about another 7 billion years
During a star’s main-sequence lifetime, the star expands somewhat and
undergoes a modest increase in luminosity
When core hydrogen fusion ceases, a mainsequence star becomes a red giant
Red Giants
 Core hydrogen fusion
ceases when the
hydrogen has been
exhausted in the core of
a main-sequence star
 This leaves a core of
nearly pure helium
surrounded by a shell
through which hydrogen
fusion works its way
outward in the star
 The core shrinks and
becomes hotter, while the
star’s outer layers expand
and cool
 The result is a red giant
star
Fusion of helium into carbon and oxygen begins at
the center of a red giant
 When the central temperature of a red giant reaches about 100 million K,
helium fusion begins in the core
 This process, also called the triple alpha process, converts helium to carbon
and oxygen
Planetary Nebulae – Death
of a Solar Mass Star
Planetary Nebula - NGC 7293
450 light years away in Aquarius
Planetary Nebula - NGC 7027
3000 light years away in Cygnus
Evolution from Giants to Dwarfs
Dwarf
Properties
Sirius A
Sirius B - WD
Property
Earth
Sirius B
Su n
Mass (Msun)
3x10
-6
0 .9 4
1 .0 0
0 .0 0 9
0 .0 0 8
1 .0 0
Luminosity (Lsun)
0 .0
0.0028
1 .0 0
Surface temperature (K)
287
27,000
5770
6
1 .4 1
Radius (Rsun)
3
Mean density (g/cm )
Central temp (K)
3
Central density (g/cm )
5 .5
2.8x10
4200
2.2x10
9 .6
3.3x10
7
7
7
1.6x10
160
Stellar Evolution by Mass
from the Main Sequence
Main sequence stars
Supergiants
Giants
Helium flash
C detonation
Heavy nuclei fusion
Supernovae
Planetary nebulae
Black holes
Ns
White dwarfs
100
40
10
4.0
Mass (MSun = 1)
1.0
0.4
0.1
A Massive Star (~25 Msun)
SN 1987A Outburst
Large Magellanic Cloud
February 23, 1987
Progenitor star was a blue
supergiant of about 20 Msun
Crab Nebula - 1054 A.D.
Neutron star
NASA JPL GENESIS Education/Public Outreach
Copyright © Periodic Table of the Elements, Los Alamos National Laboratories
© Periodic Table of the Elements
Los Alamos National Laboratories
There are 92 elements found in nature. They were all
produced BY THE STARS.
From Galaxies to Cosmology
Galaxies
our own Milky Way
different types
elliptical, spiral, barred spiral
Hubble’s Law
Cosmology
Hubble proved that the spiral nebulae
are far beyond the Milky Way
Edwin Hubble
used Cepheid
variables to show
that the “nebula”
were actually
immense star
systems far
beyond our Galaxy
Galaxies are classified according
to their appearance
Galaxies can be grouped into four
major categories: spirals, barred
spirals, ellipticals, and irregulars
The disks of spiral and barred spiral galaxies
are sites of active star formation
Elliptical galaxies are nearly devoid of
interstellar gas and dust, and so star
formation is severely inhibited
Irregular galaxies have ill-defined,
asymmetrical shapes
They are often found associated with other galaxies
Astronomers use various techniques
to determine the distances to remote
galaxies
Standard candles, such
as Cepheid variables
and the most luminous
supergiants, globular
clusters, H II regions,
and supernovae in a
galaxy, are used in
estimating intergalactic
distances
The Distance Ladder
 The Tully-Fisher relation, which correlates the width of the 21cm line of hydrogen in a spiral galaxy with its luminosity, can
also be used for determining distance
 A method that can be used for elliptical galaxies is the
fundamental plane, which relates the galaxy’s size to its surface
brightness distribution and to the motions of its stars
Recall the Doppler Shift
A change in measured frequency caused
by the motion of the observer or the
source
classical example of pitch of train coming
towards you and moving away
Hubble’s Law
The further away a galaxy is, the greater
its recessional velocity and the greater its
spectral red shift
Hubble’s Conculsion
From Hubble’s Law we can calculate a
time in the past when universe was a
point
Big bang occurred about 15 billion years
ago
big bang first proposed by George Gamow
based upon such evidence
big bang named by antagonist Fred Hoyle
who preferred the steady-state model
Big Bang Summary
Era
The Vacuum Era
Epochs
Main Event
Planck Epoch
Quantum
Inflationary Epoch fluctuation
Inflation
Time after bang
<10-43 sec.
<10-10 sec.
The Radiation Era Electroweak Epoch Formation of
Strong Epoch
leptons, bosons,
Decoupling
hydrogen, helium
and deuterium
The Matter Era
Galaxy Epoch
Galaxy formation
Stellar Epoch
Stellar birth
10-10 sec.
10-4 sec.
1 sec. - 1 month
The Degenerate
Dark Era
20-100 billion yrs.
100 billion - ????
Dead Star Epoch
Black Hole Epoch
Death of stars
Black holes
engulf?
1-2 billion years
2-15 billion years
Kepler’s Laws of Planetary Motion
Kepler’s First Law of Planetary Motion
planets orbit sun in an ellipse with sun at one
foci
Kepler’s Second Law of Planetary Motion
planets sweep out equal areas in equal times
travel faster when closer, slower when farther
Kepler’s Third Law of Planetary Motion
orbital period squared is proportional to
semi-major axis cubed
• P2 = a3
Planetary Observations
Planets formed at same time as Sun
Planetary and satellite/ring systems are
similar to remnants of dusty disks such as
that seen about stars being born
Planet composition dependent upon
where it formed in solar system
Other Planet Observations
Terrestrial planets are closer to sun
Mercury
Venus
Earth
Mars
Jovian planets furthest from sun
Jupiter
Saturn
Uranus
Neptune
Other Observations
Radioactive dating of solar system rocks
Earth ~ 4 billion years
Moon ~4.5 billion years
Meteorites ~4.6 billion years
Most orbital and rotation planes confined to
ecliptic plane with counterclockwise motion
Extensive satellite and rings around Jovians
Planets have more of the heavier elements than
the sun
A Linear View of Abundance
Linear Plot of Chemical Abundance
100000
90000
80000
Relative abundance
70000
60000
50000
40000
30000
20000
10000
0
H
He
C
N
O
Ne
Chemical Species
Mg
Si
Si
Fe
Log Abundance of Elements
Logarithmic Plot of Chemical Abundance of Elements
100000
Relative Abundance
10000
1000
100
10
1
H
He
C
N
O
Ne
Chemical Species
Mg
Si
Si
Fe
Planetary Summary
Major
Constituents
Mass
(Earth=1)
Density
(g/cm3)
Mercury
Venus
Earth
Mars
0.06
0.82
1.00
0.11
5.4
5.2
5.5
3.9
Jupiter
Saturn
318
95
1.3
0.7
H, He
H, He
Uranus
Neptune
14
17
1.3
1.7
Ices, H, He
Ices, H, He
Planet
Rock,
Rock,
Rock,
Rock,
Iron
Iron
Iron
Iron
Nebular Condensation
(protoplanet) Model
Most remnant heat from collapse retained
near center
After sun ignites, remaining dust reaches
an equilibrium temperature
Different densities of the planets are
explained by condensation temperatures
Nebular dust temperature increases to
center of nebula
Nebular Condensation Physics
Energy absorbed per unit area from sun =
energy emitted as thermal radiator
Solar Flux = Lum (Sun) / 4 x distance2
Flux emitted = constant x T4 [Stefan-Boltzmann]
Concluding from above yields
T = constant / distance0.5
Nebular Condensation
Chemistry
Molecule
H2
H2O
CH4
NH3
FeSO4
SiO4
Freezing Point Distance from
Center
>100 AU
10 K
>10 AU
273 K
>35 AU
35 K
>8 AU
190 K
>1 AU
700 K
>0.5 AU
1000 K