Everything%20you%20always%20wanted%20to%20know%20about%20stars

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Everything you always wanted to know about stars
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The Spectra of Stars
Inner, dense layers of a star produce a
continuous (black body) spectrum.
Cooler surface layers absorb light at specific
frequencies.

• Spectra of stars are absorption spectra.
• Spectrum provides temperature, chemical
  composition

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Spectral Classification of Stars (I)
Different types of stars show different
characteristic sets of absorption lines.
Temperature
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Mnemonics to remember the spectral sequence
Oh Oh Only
Be Boy, Bad
A An Astronomers
Fine F Forget
Girl/Guy Grade Generally
Kiss Kills Known
Me Me Mnemonics
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Stellar spectra
O
B
A
F
Surface temperature
G
K
M
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0
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We have learned how to determine a stars

• surface temperature

• chemical composition

Now we can determine its

• distance

• luminosity

• radius

• mass

and how all the different types of stars make up
the big family of stars.
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Distances to Stars
d in parsec (pc) p in arc seconds
__
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d
p
Trigonometric Parallax
Star appears slightly shifted from different
positions of Earth on its orbit
1 pc 3.26 LY
The farther away the star is (larger d), the
smaller the parallax angle p.
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The Trigonometric Parallax
Example Nearest star, ? Centauri, has a
parallax of p 0.76 arc seconds
d 1/p 1.3 pc 4.3 LY
With ground-based telescopes, we can measure
parallaxes p 0.02 arc sec gt d 50 pc
This method does not work for stars farther away
than about 50 pc (nearly 200 light-years).
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Intrinsic Brightness
The more distant a light source is, the fainter
it appears.
The same amount of light falls onto a smaller
area at distance 1 than at distance 2 gt smaller
apparent brightness.
Area increases as square of distance gt apparent
brightness decreases as inverse of distance
squared
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Intrinsic Brightness / Flux and Luminosity
The flux received from the light is proportional
to its intrinsic brightness or luminosity (L) and
inversely proportional to the square of the
distance (d)
L
__
F
d2
Star A
Star B
Earth
Both stars may appear equally bright, although
star A is intrinsically much brighter than star B.
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The Size (Radius) of a Star
We already know flux increases with surface
temperature ( T4) hotter stars are brighter.
But brightness also increases with size
Star B will be brighter than star A.
A
B
Absolute brightness is proportional to radius
squared, L R2.
Quantitatively L 4 ? R2 ? T4
Surface flux due to a blackbody spectrum
Surface area of the star
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Example
Polaris has just about the same spectral type
(and thus surface temperature) as our sun, but it
is 10,000 times brighter than our sun.
Thus, Polaris is 100 times larger than the sun.
This causes its luminosity to be 1002 10,000
times more than our suns.
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The Hertzsprung Russell Diagram
Most stars are found along the main sequence
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The Hertzsprung-Russell Diagram (II)
Same temperature, but much brighter than MS stars
? Must be much larger
Stars spend most of their active life time on the
Main Sequence.
? Giant Stars
Same temp., but fainter ? Dwarfs
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Radii of Stars in the Hertzsprung-Russell Diagram
Rigel
Betelgeuse
10,000 times the suns radius
Polaris
100 times the suns radius
Sun
As large as the sun
100 times smaller than the sun
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Luminosity Classes
Ia Bright Supergiants
Ia
Ib
Ib Supergiants
II
II Bright Giants
III
III Giants
IV Subgiants
IV
V
V Main-Sequence Stars
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Examples

• Our Sun G2 star on the main sequence G2V
• Polaris G2 star with supergiant luminosity G2Ib

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Masses of Stars in the Hertzsprung-Russell Diagram
Masses in units of solar masses
The higher a stars mass, the more luminous
(brighter) it is
High masses
L M3.5
High-mass stars have much shorter lives than
low-mass stars
Mass
tlife M-2.5
Low masses
Sun 10 billion yr.
10 Msun 30 million yr.
0.1 Msun 3 trillion yr.
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Surveys of Stars
Ideal situation
Determine properties of all stars within a
certain volume.
Problem
Fainter stars are hard to observe we might be
biased towards the more luminous stars.
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A Census of the Stars
Faint, red dwarfs (low mass) are the most common
stars.
Bright, hot, blue main-sequence stars (high-mass)
are very rare.
Giants and supergiants are extremely rare.
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Shocks Triggering Star Formation
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Henize 206 (infrared)
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The Contraction of a Protostar
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Protostellar Disks and Jets Herbig-Haro Objects
Disks of matter accreted onto the protostar
(accretion disks) often lead to the formation
of jets (directed outflows bipolar outflows)
Herbig-Haro objects
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Herbig-Haro 34 in Orion

• Jet along the axis visible as red
• Lobes at each end where jets run into surrounding
  gas clouds

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Motion of Herbig-Haro 34 in Orion

• Can actually see the knots in the jet move with
  time
• In time jets, UV photons, supernova, will disrupt
  the stellar nursery

Hubble Space Telescope Image
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The Source of Stellar Energy
Stars produce energy by nuclear fusion of
hydrogen into helium.
In the sun, this happens primarily through the
proton-proton (PP) chain
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The Deaths and End States of Stars
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The End of a Stars Life
When all the nuclear fuel in a star is used up,
gravity will win over pressure and the star will
die.
High-mass stars will die first, in a gigantic
explosion, called a supernova.
Less massive stars will die in less dramatic
events.
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Evolution off the Main Sequence Expansion into a
Red Giant
Hydrogen in the core completely converted into He
? Hydrogen burning (i.e. fusion of H into He)
ceases in the core.
H burning continues in a shell around the core.
He core H-burning shell produce more energy
than needed for pressure support
Expansion and cooling of the outer layers of the
star ? red giant
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Expansion onto the Giant Branch
Expansion and surface cooling during the phase of
an inactive He core and a H-burning shell
Sun will expand beyond Earths orbit!
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HR Diagram of a Star Cluster
High-mass stars evolved onto the giant branch
Turn-off point
Low-mass stars still on the main sequence
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Red Dwarfs
Recall
Stars with less than 0.4 solar masses are
completely convective.
Mass
? Hydrogen and helium remain well mixed
throughout the entire star.
? No phase of shell burning with expansion to
giant.
Star not hot enough to ignite He burning.
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Sunlike Stars
Sunlike stars ( 0.4 4 solar masses) develop a
helium core.
Mass
? Expansion to red giant during H burning shell
phase
? Ignition of He burning in the He core
? Formation of a degenerate C,O core
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White Dwarfs
Degenerate stellar remnant (C,O core)
Extremely dense 1 teaspoon of white dwarf
material mass 16 tons!!!
Chunk of white dwarf material the size of a beach
ball would outweigh an ocean liner!
white dwarfs Mass Msun Temp. 25,000
K Luminosity 0.01 Lsun
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Low luminosity high temperature gt White dwarfs
are found in the lower center/left of the H-R
diagram.
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The Chandrasekhar Limit
The more massive a white dwarf, the smaller it is.
? Pressure becomes larger, until electron
degeneracy pressure can no longer hold up against
gravity.
WDs with more than 1.4 solar masses can not
exist!
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The Final Breaths of Sun-Like Stars Planetary
Nebulae
Remnants of stars with 1 a few Msun
Radii R 0.2 - 3 light years
Expanding at 10 20 km/s (? Doppler shifts)
Less than 10,000 years old
Have nothing to do with planets!
The Helix Nebula
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The Formation of Planetary Nebulae
Two-stage process
Slow wind from a red giant blows away cool, outer
layers of the star
The Ring Nebula in Lyra
Fast wind from hot, inner layers of the star
overtakes the slow wind and excites it gt
planetary nebula
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The Fate of our Sunand the End of Earth

• Sun will expand to a red giant in 5 billion
  years
• Expands to Earths orbit
• Earth will then be incinerated!
• Sun may form a planetary nebula (but uncertain)
• Suns C,O core will become a white dwarf

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The Deaths of Massive Stars Supernovae
Final stages of fusion in high-mass stars (gt 8
Msun), leading to the formation of an iron core,
happen extremely rapidly Si burning lasts only
for 1 day.
Iron core ultimately collapses, triggering an
explosion that destroys the star Supernova
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The Crab NebulaSupernova from 1050 AD

• Can see expansion between 1973 and 2001
• Kitt Peak National Observatory Images

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The Famous Supernova of 1987 Supernova 1987A
Before
At maximum
Unusual type II supernova in the Large Magellanic
Cloud in Feb. 1987
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Type I and II Supernovae
Core collapse of a massive star type II supernova
If an accreting white dwarf exceeds the
Chandrasekhar mass limit, it collapses,
triggering a type Ia supernova.
Type I No hydrogen lines in the spectrum Type
II Hydrogen lines in the spectrum
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Neutron Stars
A supernova explosion of an M gt 8 Msun star blows
away its outer layers.
Pressure becomes so high that electrons and
protons combine to form stable neutrons
throughout the object.
The central core will collapse into a compact
object of a few Msun.
Typical size R 10 km
Mass M 1.4 3 Msun
Density ? 1014 g/cm3
? Piece of neutron star matter of the size of a
sugar cube has a mass of 100 million tons!!!
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Discovery of Pulsars
Angular momentum conservation
gt Collapsing stellar core spins up to periods of
a few milliseconds.
Magnetic fields are amplified up to B 109
1015 G.
(up to 1012 times the average magnetic field of
the sun)
gt Rapidly pulsed (optical and radio) emission
from some objects interpreted as spin period of
neutron stars
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The Crab Pulsar
Pulsar wind jets
Remnant of a supernova observed in A.D. 1054
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Black Holes
Just like white dwarfs (Chandrasekhar limit 1.4
Msun), there is a mass limit for neutron stars
Neutron stars can not exist with masses gt 3 Msun
We know of no mechanism to halt the collapse of a
compact object with gt 3 Msun.
It will collapse into a single point a
singularity
gt A black hole!
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Escape Velocity
Velocity needed to escape Earths gravity from
the surface vesc 11.6 km/s.
vesc
Now, gravitational force decreases with distance
( 1/d2) gt Starting out high above the surface
gt lower escape velocity.
vesc
If you could compress Earth to a smaller radius
gt higher escape velocity from the surface.
vesc
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The Schwarzschild Radius
gt There is a limiting radius where the escape
velocity reaches the speed of light, c
Vesc c
2GM
____
Rs
c2
G gravitational constant
M mass
Rs is called the Schwarzschild radius.
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Schwarzschild Radius and Event Horizon
No object can travel faster than the speed of
light
gt nothing (not even light) can escape from
inside the Schwarzschild radius

• We have no way of finding out whats happening
  inside the Schwarzschild radius.

• Event horizon

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Black Holes Have No Hair
Matter forming a black hole is losing almost all
of its properties.
black holes are completely determined by 3
quantities
mass
angular momentum
(electric charge)
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The Gravitational Field of a Black Hole
Gravitational Potential
Distance from central mass
The gravitational potential (and gravitational
attraction force) at the Schwarzschild radius of
a black hole becomes infinite.
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General Relativity Effects Near Black Holes
An astronaut descending down towards the event
horizon of the black hole will be stretched
vertically (tidal effects) and squeezed laterally.
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General Relativity Effects Near Black Holes (II)
Time dilation
Clocks starting at 1200 at each point. After 3
hours (for an observer far away from the black
hole)
Clocks closer to the black hole run more slowly.
Time dilation becomes infinite at the event
horizon.
Event horizon
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General Relativity Effects Near Black Holes (III)
gravitational redshift
All wavelengths of emissions from near the event
horizon are stretched (redshifted). ? Frequencies
are lowered.
Event horizon
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Observing Black Holes
No light can escape a black hole
gt Black holes can not be observed directly.
If an invisible compact object is part of a
binary, we can estimate its mass from the orbital
period and radial velocity.
Mass gt 3 Msun gt Black hole!