Final Flashcards
(101 cards)
1
Q
What are stars?
A
gas spheres where the force of gravity is balanced by thermal pressure
2
Q
hydrostatic equilibrium
A
force of gravity is balanced by thermal pressure
3
Q
What causes star pressure?
A
energy produced inside the star that is trying to escape
4
Q
Stars are in a stable equilibrium
What happens when they are squeezed?
A
they get hotter and pressure increases
5
Q
what is the relation between outward pressure force and weight of layers above
A
exactly balance
6
Q
what changes as you go deeper in star
A
higher pressure to balance the weight above
higher temperature
7
Q
conduction
A
direct contact between cool and hot materials. Not important in normal stars
8
Q
radiation
A
EM waves carry the energy
9
Q
convection
A
physical motion of hot material carrying the energy to cooler regions
10
Q
what heat transfer do most stars use
A
radiation and convection
11
Q
where is the radiative zone in a star with approximately 1 solar mass or less
A
the middle 2/3
12
Q
where is the convection zone in a star with approximately 1 solar mass or less
A
the top 1/3
13
Q
solar convection causes …
A
granulation or convection cells
hot rising gas is brighter than cooler, sinking gas
size of texas
14
Q
sunspots
A
cooler regions of the photosphere
still bright just dimmer than rest of star
caused by magnetic field - related north and south parts
larger than earth
vary over 11 years
magnetic polarity switches every other cycle
start in higher latitudes evolve to lower latitudes
15
Q
sun rotation
A
faster at equator than near the poles
differential rotation might be responsible for magnetic activity of the sun
faster at equator
16
Q
Babcock model/solar cycle
A
magnetic fields are trapped by ionized gas and pulled along by differential rotation
After 11 years, the magnetic field pattern becomes so complex that the field structure is re-arranged
New magnetic field structure is similar to the original one, but reversed
New 11-year cycle starts with reversed magnetic-field orientation
17
Q
Maunder Minimum
A
quite phase in the fluctuation of sun spot numbers
18
Q
How can chromosphere in stars be seen?
A
as week emission lines in spectra
19
Q
Stellar spectra
A
images show total flux in wavelength span
spectra show flux as a function of wavelength
20
Q
Spectrum: Plot of Flux versus wavelength
A
1880s to 1980s - spectra recorded on photographic plates
modern spectra are recorded digitally and represented as plot of intensity vs wavelength
21
Q
Temperature from Blackbody Radiation
A
Stars emit energy with a distribution close to a black body
BB spectrum peak is a clue to the star’s temperature
Dust in galaxy scatters blue light more than red - makes stars look redder
22
Q
Harvard Star Classification origin
A
1890 photographic spectra of thousands of stars obtained at Harvard - classified based on Balmer line strengths
Classified by computers at Harvard
the physical cause of the line strength changes was not understood
23
Q
Star Classifications from balmer lines
A
A-type - strongest Balmer hydrogen lines
B-type - next strongest, C,D,E
O-type - weakest H
24
Q
Cause of Balmer(Hydrogen Lines)
A
balmer hydrogen absorption can only happen when an electron is in the second level
A collision with another atom can knock the ground-state electron to level 2
But if enough energy is absorbed the electron an be unbound from the atom(ionized)
25
Talmer Thermometer
Balmer lines strength is sensitive to temperature
Only a narrow range of temperature can keep electrons in the second level
Low temperatures the atom collisions don't have enough energy to raise the electron to level 2
at high temperatures the atom collisions are so energetic that the electron is ionized and lost to the atom
Most hydrogen atoms are ionized - weak balmer lines
Almost all hydrogen atoms in the ground state so few transitions from n=2 so weak Balmer lines
26
Stellar spectra from surface temperature
O
B
A
F
G
K
M
O is hottest and M is coolest
Oh Boy An F Grade Kills Me
L and T are new brown dwarf classifications
lower additional numbers are colder
27
Measuring star's surface temperature
Comparing line strengths or blackbody peak
28
Energy output of stars
1850: distances to stars determined and found that there is a wide range of energy output from stars with the Sun being in the middle
29
Typical star energy
4x10^26 Joules/sec = 4x10^26 Watts
30
Types of energy
chemical energy - oxidation, burning of coal
Gravitational energy - lowering potential energy, water flowing down hill and drives turbines
Nuclear energy - building of light elements, fusion energy difference between four hydrogen atoms ad one helium atom is converted to energy and released in fusion
31
Star lifetime
energy consumption rate = fuel amount/lifetime
32
Fission
induced by neutrons hitting a heavy nucleus like uranium
Chain reaction also possible if neutrons are produced in the collision
not the source of energy from stars
33
Fusion
combining light into heavy
fusion can produce much more energy/gram than Fission
Strong Force binds nuclei, but protons are positively charged so repel each other
Strong force will take over if protons are close enough
Need high temperatures to overcome electric field repulsion
Creates isotopes where the number of protons determine chemical properties
34
Proton-Proton Chain
Turns hydrogen into helium
Happens at 10 million degrees
4H to He
2H to D
D+H to 3He
3He+3He to 4He +2H
Gamma rays deposit energy in the star
Positrons(antimatter of electron) created positron+e-=energy
Neutrino made but directly escapes and doesn't interact with matter much
35
Neutrinos
very low mass particles that do not interact strongly with matter but are produced in fusion reactions
proves fusion in the sun
have mass of 10^-37kg smaller than electron
36
The solar neutrino Problem
neutrinos come in 3 flavors related to fundamental particles - electron, muon, and tau
Muons and tau not detected explains the 1/3 of expected neutrinos
37
Intrinsic Brightness(Luminosity) vs apparent brightness(Flux)
the more distant a light source is the fainter it appears - a source could be close and intrinsically faint or distance and luminous
need distance to determine
38
Radii of Stars
100 times smaller than the sun to 10,000 times bigger
Main sequence is the similar to the sun going slightly smaller and slightly bigger
39
luminosity
consumption of fuel
the intrinsic energy output from the star
equals absolute magnitude
found from distance and apparent magnitude
true amount of energy emitted per second by the star
40
Hertzsprung-Russell diagram
plot of luminosity vs temperature
tells about physics of stars and how they evolve
41
Apparent Magnitudes
the measure of the energy flux (joules/s/m^2) coming from a star as viewed from the Earth
the flux is the energy received at the earth per second per square area
42
Inverse square law flux
luminosity/(4pi distance^2)
for an isotropic source of energy the energy spreads out evenly over a sphere of radius r. so the energy per area or flux falls off as 1/4pir^2
43
Parsec
angle shift in arcseconds for 1 AU motion
44
Parallax
geometric measurement of distance
45
Binary stars
more then 50% in Milky way
pairs or multiple systems of stars which orbit their common center of mass
measure and understand orbital motion can estimate stellar masses
46
Types of Binary Stars
ability to see two stars depends on separation, distance and brightness ratio
Visual binary: both stars are visible
Spectroscopic binary: orbit detected by radial velocity variations
Eclipsing binary: as stars orbit one star gets in front of the other and the total apparent brightness changes
47
Visual Binaries
both stars can be seen directly and their separation and relative motion can be followed directly
The period of most visual binaries are long>to years since the separation has to be large to see both stars
Orbiting about a common center of mass
Can be great distances away limited to orbital period of >10 million years
48
Measuring the Mass of Binary stars
sum of the masses of the two stars equals their separation cubed divided by their orbital period squared
48
Center of mass in binary stars
balance point of the system
the more unequal the masses are the more it shifts toward the more massive star
49
Eclipsing Binaries
usually inclination angle of binary systems is unknown - uncertainty in mass estimates except for eclipsing binaries
if the orbital plane is close to the line-of-sight, the stars may get in each other's way blocking some of the light
This is a way to measure the size of stars
50
Primary/secondary Eclipse
The deeper eclipse is called the primary
Flat bottom suggests total eclipse
Primary eclipse occurs when the hotter star is blocked by the cooler star - black body flux/area
51
Binary stars to get stellar masses
get luminosity from the parallax and apparent brightness
only for stars on the main sequence
52
Masses of stars in the H-R Diagram
Main sequence only
Cool, low luminosity stars have small masses
Hot, bright stars have high masses
Sun - in the middle
53
Time on Main Sequence
High mass stars have short lifetimes, billions of years
3 trillion to 30 million
54
Stellar evolution on the main sequence
start by fusing H to he
get more luminous with time on MS - core full of He
Region of fusion expands to the edges of core
Hydrogen in the core completely converted into He
Hydrogen burning stops in core
H burning continues in a shell around the core
He Core and H burning shell produce more energy then needed for pressure support
Expansion and cooling of the outer layers of the star - red giant
Expansion and surface cooling during the phase of an inactive He core and H burning shell - moves only Giant Branch
55
Globular Clusters/Star clusters
millions of stars
gravitationally bound
Formed at the same time - use as a clock mass of stars that have left the Main sequence give age
Identify turnoff from main sequence
the higher the turnoff point the younger the cluster is
56
Distance Modulus
apparent magnitude - absolute magnitude
m-M
57
Helium Fusion
helium nuclei can fuse to build heavier elements
Helium ignition at the tip of the red giant branch - when the pressure and temperature are high enough
58
Red Giant Evolution
helium burning
H burning shell keeps dumping He onto the core
He core gets denser and hotter until the next stage of nuclear burning can begin in the ocre
He fusion through the Triple-Alpha Process - makes Carbon and Oxygen
59
Stellar evolution of sunlike stars
Hydrogen exhausted in core - star increase in size and surface cools so moves up giant branch
Inert Helium core with hydrogen fusion in a shell around the core - core shrinks and heats up
Helium Flash - helium begins fusing in core at the tip of the giant branch
Star shrinks and surface gets hotter as it evolves to horizontal branch - helium core burning
Helium core burning to carbon on horizontal branch or helium main sequence or red clump
Helium exhausted in core - energy from shell burning of hydrogen and helium - moves up asymptotic giant branch
Stars more massive than the sun will ignite carbon in core however 1 solar mass stars do not get hot enough to fuse carbon
60
Remnants of sunlike stars
white dwarfs
sunlike stars build up a carbon-oxygen core which does not ignite carbon fusion
He burning shell keeps dumping C and O onto the core and it shrinks and the matter becomes degenerate
Forms white dwarf
Low luminosity, start out hot and slowly cool
Found in the lower left corner of the H-R diagram
Initial star mass <8 solar masses
Very dense
pressure support in white dwarf is due to the fact that electrons can not be packed close together - own quantum states
61
Planetary Nebula
formation of a white dwarf - very hot core of the remaining star after the atmosphere has drifted off
only form from low mass stars - less than 8 solar masses
Two stage process: slow wind from a red giant blows away cool, outer layers of the star, fast wind from hot, inner layers of the star overtake the slow wind and excite it
gas ejection is slow 10 km/s to 20 km/s
age is based on angular size and distance/radial velocity
gas is ionized by the hot stellar core, so it glows as electrons recombine
gentle release of outer layers leave a dense white dwarf
62
Electron Degeneracy
Pauli Exclusion principle - no more than one electron can occupy a quantum state
Electrons can not be squeezed into a small space
The pressure balances gravity - degeneracy pressure
62
Escape Velocity
how fast you need to go to completely escape the gravity of an object
Depends on distance from center
63
White Dwarf escape velocity
2% the speed of light
6500 km/s
64
Radii of White Dwarf stars
Normal stars radius increases with mass
Radii of white dwarfs decrease with mas
Radius is 0 for a WD of 1.4 Solar mass
65
Chandrasekhar limit
WD radius is 0 for 1.4 solar mass
degenerate electrons are forced to very high energy states in stars with mass near 1.4 solar mass so they lose their pressure
if the star gains mass from another star it will restart fusion and cause a thermonuclear supernova - type la supernova
66
Massive star evolution
massive stars burn fuel fast so have a short lifetime
Don't move far
Can hit high temperatures in core to fuse beyond He
67
Fusion Into Heavier Elements
H-He He-C C-Ne/O Ne/O-Si Si-Fe
core turns to iron with heavier elements towards the center
68
Core Collapse and bounce supernova
if Iron Core is greater than 1.4 M sun electron degeneracy can't support it
Shrinks from 5000 km to 10km
Other layers fall and then hit the core and bounce
Ejected at 10000 km/s
69
Two types of supernova
Type la - thermonuclear fusion in white dwarf
Type II, Ib, Ic - core collapse in massive star
light curves are similar, very bright
70
Color-magnitude diagram
B-V color is a measure of temperature
Blue hot stars have small B-V index
Red cool stars have large B-V index
71
Neutron Stars
Iron core collapses down to densities so high that electrons and protons combine to make neutrons and neutrinos - produces a huge burst of neutrinos
The neutron star is stopped from collapsing by neutron degeneracy - same as electron degeneracy but at higher densities
Radius of a neutron star is about 10 kilometers
Mass of the sun
Same density as the nucleus of an atom
72
Shock Waves
explosions can push the ambient medium faster than the local sound speed
Trying to move faster than the sound speed causes a pressure discontinuity
The bounce produces a shock wave that tries to drive the outer layers off
73
Remnants of Supernova
Type la supernovae(thermonuclear fusion) synthesizes lots of iron
Core collapse SN makes some iron, but also heavier elements silver, gold, uranium
Give off remnants moving into the interstellar medium ionizing the gas
strong magnetic fields near the new neutron star
Large amounts of energy in light and heat
Shock waves sweep up interstellar gas and slow it down - see for 100000 years
They are galactic fountains that spread heavy elements around
Can stop star formation
74
Spectra of Supernovae
Type II - Hydrogen
Type I - No Hydrogen
Type Ia - silicon
Type Ib - Helium
Type Ic - Iron
75
Supernova classification by spectrum
core collapse of massive stars - II(stong H), Ib (strong He), Ic(weak He)
Thermonuclear explosion of white dwarfs - Ia(strong Si)
76
Supernova 1987A
in Large Magellanic Cloud
nearest supernova in 300 years, but outside Milky Way
visible to the unaided eye at 2nd magnitude
Type II supernova(hydrogen in spectrum)
Had rings years after explosion cam from mass lost from the progenitor star
one bright inner ring and two fainter outer rings - hourglass shape similar to planetary nebulae
They are gas ejected by the progenitor star before explosion
The Gas ionized by the explosion and is now recombining
77
Properties of Neutron stars
R=10km
M=1.4-3Msun
Density=10^14g/cm^3
conserve angular momentum in collapse so are rapidly spinning
Spin can generate strong magnetic fields plus the magnetic fields are trapped in the core so get amplified in the collapse
Rapidly rotating magnet drives electric current
Beam of energy sent out the magnetic poles - might not be aligned with the spin axis - this is a pulsar
78
Discovery of Pulsars
1967:While looking for rapid variations in radio sources Hewish and Bell detected a regular signal with a period of about 1Hz
Signal moved across the sky at the sidereal rate
It was a rapidly spinning, magnetic neutron star that produces a regular fast signal
79
Synchrotron Radiation
electrons spiral in magnetic field emits this
80
Do all core-collapse SN make pulsars
NO
81
Gamma-Ray Bursts discovery
Discovered by the Defense Department's Vela satellite in the 1960s while secretly spying on the USSR
About once per day a flash of gamma-rays is seen on the sky. Bursts last up to a few minutes
Until 1997 their origins were unknown
Found to have X-ray/optical counterparts called afterglows - allow distance to be determined
Origin is Cosmological
82
Gamma rays
the most energetic light
Gamma-rays are the electromagnetic radiation with the shortest wavelengths, highest frequencies, and most energy - a million times shorter wavelength than visible light
83
Two types of gamma ray bursts
Short duration - less than 2 sec, come from jets from neutron star mergers, also generate bursts of gravitational radiation
Long bursts - 2 sec to minutes, very powerful supernova that is pointed at us, turns into black hole after
84
The Schwarzschild Radius
Limiting radius where the escape velocity reaches the speed of light
Nothing can escape from inside
Even horizon
85
Black holes
have been known in binary star systems
identified by mass transfer from a normal star
can merge - come from massive stellar binaries
86
Special relativity
1905: motions of objects with no acceleration
Postulates: 1.time must be treated as a 4th dimension on the same footing as space
2. inertial frames are reference frames moving in straight lines with no acceleration
3. The speed of light is a constant as seen from all inertial reference frames. All laws of nature are seen from same inertial frames
simultaneous events are not simultaneous in all reference frames
87
General Relativity
1915: accelerations and a new theory of gravity as a curvature of space-time
Acceleration is a curving line in the space-time diagram
Space and time cannot be separated
gravity is a warp of spacetime
Space gets stretched and time slows near masses
Light must follow the shortest path in space-time so it curves
Postulates: 1. Accelerations due to gravity or due to motion are equivalent
2. Inertial frame near a massive object is accelerating
acceleration towards mass
clocks run more slowly the stronger the gravity
light leaving mass is redshifted
88
Einstein's revelations
Einstein realized that Newton was wrong about some things
1905 - realized there is no absolute frame of reference, all motion is relative
89
Time Dilation
moving clocks run slower
very small for everyday velocities
Only significant when velocity is close to the speed of light
clocks run more slowly in strong gravitational fields
90
Gravitational lensing
massive objects like galaxies can bend light so much that multiple images can form
when the source and lende are in good alignment see an Einstein Ring needs enough mass
91
Black hole
compressing a neutron star so that its escape velocity at the surface is greater than the speed of light
light will be bent around the black hole so observer will see whole sky behind black hole
92
Falling into a black hole
gravity will pull apart anything because of this increasing strength - tidal disruption
Crossing event horizon is smooth just lose communication with outside
an observing ship will see the ship entering going slower and slower all communication will redshift and get endlessly long
93
Hawking's Radiation
virtual particles will cross the event horizon while its anti-particle won't and will escape
The escaping radiation has a blackbody spectrum - temperature, it increases as mass decreases
94
Evaporating Black Holes
supermassive black holes at galaxies centers
black holes can evaporate if they release more mass and radiation that in acquires
95
Milky Way
galactic disk - flattened distribution of stars and interstellar material 200-400 billion stars
stellar halo - the spherical distribution of Population II stars that are found mainly in globular clusters - population II stars must have formed before many generations of star formation
bulge - roughly spherical distribution of stars and dust about 5 kpc in radius, stars tightly packed together
There is dark matter
96
Galactic Black Hole in middle of galaxy
ours is less active than others they usually have a lot of energy coming out of them
97
Types of galaxies
spirals - well defined spiral arms, 70% of galaxies
barred spirals - visual bar seen running from one end to another of a spiral
ellipticals - football shaped, 15% of galaxies
irregulars - don't fit in any category, no easy geometry
98
Distance to galaxies
65 million light years
1 billion pc
99
Hubble's law
linear relationship between recession velocity and distance
every galaxy is moving away from us
universe has beginning
distance comes straight from velocity
