Transcript JimH This is Your Life - The Atlanta Astronomy Club

Kirchhoff's laws ,there are three types of
spectra: continuum, emission line, and
absorption line.
High pressure, high temperature gas
Low pressure, high temperature gas
Cool gas in front of
continuous spectra source
Hydrogen
Helium
Oxygen
Neon
Iron
Flux
Hydrogen
Continuum
Absorption Lines
4000
5000
6000
Wavelength
7000
Doppler effect
• similar in light and sound
–Waves compressed with source moving toward you
Sound pitch is higher, light wavelength is compressed (bluer)
v    0
 
c 0
0
–Waves stretched with source moving away from you
•Sound pitch is lower, light wavelength is longer (redder)
Red Shift
Inverse square of light
d=1
B=1
d=2
B=1/4
If two stars are similar
and one star is 3 times as
far away, as the other,
its intensity will be 1/9.
d=3
B=1/9
Stars are different colors, because they are
different temperatures
Spectral Classification
•Annie Cannon classified stars according to the strength of
the hydrogen absorption lines in the sequence A, B, C….P
•These spectral classes were changed to a
temperature-ordered sequence and some were
discarded, finally leaving :
Subclasses
K7
A5
O
35,000 K
B
A
F
G
K
Sun(G2)
•Oh, Be A Fine Girl (Guy) Kiss Me
M
3,000 K
The Spectral Sequence
O
B
Bluest
Hottest
50,000K
A
F
G
K
M
L
Reddest
Coolest
1300K
Spectral Sequence is a Temperature Sequence
O Stars
Hottest Stars: T>30,000 K; Strong
He+ lines; no H lines
B Stars
T = 11,000 - 30,000 K; Strong He
lines; very weak H lines
A Stars
T = 7500 - 11,000 K; Strongest H
lines, Weak Ca+ lines.
F Stars
T = 5900 - 7500 K; H grows weaker
Ca+ grows stronger, weak metals
begin to emerge.
G Stars
T = 5200 - 5900 K; Strong Ca+, Fe+
and other metals dominate,
K Stars
T = 3900 - 5200 K; Strong metal
lines, molecular bands begin to
appear
M Stars
4000 A
T = 2500 - 3900 K; strong molecular
absorption bands particularly of TiO
Solar Spectrum
7000 A
Quantum Mechanics
Electrons can
only orbit the
nucleus in
certain orbits.
n =1
First orbital: Ground State)
•Lowest energy orbit .
Up absorption
Hydrogen Spectrum
Hydrogen (1H) consists of:
•A single proton in the nucleus.
•A single electron orbiting the nucleus.
Down emission
Emission Lines: Balmer Lines
When an electron jumps from a higher to a lower
energy orbital, a single photon is emitted with
exactly the energy difference between orbitals. No
more, no less.
Absorption Lines: Balmer Lines
An electron absorbs a photon with exactly the
energy needed to jump from a lower to a higher
orbital. No more, no less.
Hydrogen lines absent in the hottest stars
because, photons ionize electrons.
They are also absent in the coolest stars
because, photons don’t have enough
energy to move the electrons from n=2
to higher energy levels.
No electrons, no lines.
In 1905, Danish astronomer Hertzsprung, and
American astronomer Russell, noticed that
the luminosity of stars decreased from
spectral type O to M.
To bring some order to the study of stars, they
organize them in the HR diagram.
H–R Diagram
Supergiants
Luminosity (Lsun)
106
104
102
Giants
1
102
104
40,000
White Dwarfs
20,000
10,000
5,000
Temperature (K)
2,500
As you move up the H-R diagram on the Main Sequence
from M to O, the stars get hotter and larger
Back to this is your life
Star
Formation
“All
we are is dust in the wind” Kansas
Protostars form in cold, Giant Molecular Clouds
(GMC) in Orion
•About 1000
GMCs are
known in our
galaxy
• These clouds
lie in the
spiral arms of
the galaxy
The Cone
Nebula
Examining
a Star
Forming
Region
Giant Molecular Clouds (GMC)
are mostly composed of molecular hydrogen.
Properties:
•Radius ~50 pc (~160 ly)
•Mass ~105 Msun
•Temperature: 10-30 K
•Also, small amounts of He,and others
Size of cloud – large, Compression area - small
GMC’s resist forming stars because of internal
pressure (kinetic energy) so, a cooler gas is
needed.
•A shockwave is needed to trigger formation,
and to compress the material .
Sources of Shockwaves:
1.Supernova explosions: Massive stars die
young .
2. Previous star formation can trigger more
formations
3. Spiral arms in galaxies like our Milky
Way:
Spirals arms are
probably rotating
shock waves.
View all images
An expanding supernova explosion ,
occurring about 15,000 years ago.
Gravity As the cloud is compressed,
cool blobs contract into
Contraction individual stars.
The blobs glow faintly
in radio or microwave
light.
As they heat up, blobs
glow in the infrared,
but they remain
hidden .
As protostar compresses:
Density increases
Temperature rises. Photospheres (~3000K)
Rotation increases as it shrinks in size.
What types of stars form ?
OB - Few
AFG - More
KM - Many, Many
Many of the cooler stars, spectral classes
G,K,M, become heavy gas-ejecting stars
called T-Tauri stars.
Stars blows away
their cocoon
Leave behind a T Tauri star
with an accretion disk and
a jet of hot gas.
A T-Tauri star can lose up to 50% of its mass
before settling down as a main sequence star.
False Color: Green = scattered starlight and red = emission from hot gas.
Motion of Herbig-Haro 34 in Orion
• You can actually see the knots, called Herbig-Haro
objects, in the jet move with time
•They can have wind velocities of 200-300 km/s.
This phase lasts about 10 million years.
Low-Mass Protostars
Collapse is slower for lower masses:
•1 Msun (solar Mass) ~30 Myr
•0.2 Msun ~1 Billion years
When core temperature ~ 10 Million K:
•Core ignites, P-P chain fusion begins
•Settles slowly onto the Main Sequence
•Has a rotating disk, from which planets
might form .
Actual Protoplanetary Disks
•The disks are 99%
gas and 1% dust.
•The dust shows as
a dark silhouette
against the glowing
gas of the nebula.
High-Mass Protostars
Collapse is very rapid: 30 solar mass
protostar collapses in ~30,000 years
When core Temperature >10 Million K:
Ignite first P-P Chain then CNO fusion
in the core.
nearstars
the stars
Clouds are blown away from the new
Protostars!
The Cocoons of proto-stars are
exposed when the surrounding
gas is blown away by winds and
radiation from nearby massive
stars.
The Main Sequence
Core temperature & pressure rise
•Collapse begins to slow down
Finally:
•Pressure=Gravity & collapse stops.
•Becomes a Zero-Age Main Sequence
•Star, (ZAMS).
•Pre-main sequence evolutionary tracks
Most everything about a star's life depends on its
MASS.
Meanwhile, back in the GMC, things are still
happening
The original stars are growing, especially O & B stars.
Stars Form in Clusters
Our own Sun is part of an open cluster that includes
Alpha Centauri and Barnard's star.
Gravitational interactions
will cause some stars to
eventually leave over time
Extreme :Minimum Mass: ~0.08 Msun
Below this mass, the core never gets hot
enough to ignite H fusion.
Star becomes a Brown Dwarf
Resemble "Super Jupiters"
Only about 100 are known
•Shine mostly in the infrared
Extreme :Maximum Mass: 60-100 Msun
The core of a very massive star gets so hot:
•Radiation pressure overcomes gravity,
•star becomes unstable & disrupts.
Upper mass limit is not well known.
Such stars are very rare.
Star spends 90% of their life on the MS
Main
Sequence
Stars on the Main Sequence, are in
Hydrostatic Equilibrium .
Gravity pulling inward wants to contract the star
Pressure pushing outward wants to make the star expand
The star neither
expands nor
contracts.
Core-Envelope Structure
Outer layers press down on the inner layers.
The deeper you go, the greater the pressure.
The star develops a :
• hot, dense, compact central CORE
•surrounded by a cooler, less dense, ENVELOPE
Core
•CORE
Envelope
Energy is transferred inside stars by:
Radiation (core)
Energy is carried by photons from core.
•Photons hit atoms and get scattered.
•Slowly stagger to the surface
•Takes ~1 Million years to reach the surface.
Convection (Envelope)
Energy carried from hotter
regions to cooler regions above by
the motions of the gas.
Everyday examples of convection are boiling water.
Energy in a Main-Sequence star is generated
by fusion of H into He
This process is performed in two ways
1. Proton-Proton (P-P) Chain: (Low mass stars)
•4
1H
into 1
4He. +
energy.
Efficient at low core Temperatures (TC<18M K)
2. CNO Cycle: (High mass stars)
•Carbon acts as a catalyst
•Efficient at high core Temperatures(TC>18MK)
More massive stars have the shorter life time
•O & B stars burn fuel like an airplane!
•M stars burn fuel like a compact car!
Every M dwarf ever created is still on the main sequence!!
Main Sequence Lifetimes
Spectral Type
Mass
(Solar masses)
Main sequence lifetime (million
years)
O5
B0
A0
40
16
3.3
1
10
500
F0
1.7
2700
2.7 BY
G0
K0
1.1
0.8
9000
14 000
9 BY
14 BY
M0
0.4
200 000
200BY
Death of Low Mass Star
“It’s the end of the world as we know it” . REM
The End-States for Low and High Mass Stars
Initial Stellar Mass
Final Core Mass
Final State
White
.08 - 8
0.5 - 1.4
8 - 30
1.4 - 3.0
Neutron Star
> 30
> 3.0
Black hole
dwarf
Evolution of Low-Mass Stars
Main Sequence Phase
Energy Source: H core fusion (P-P cycle)
Slowly builds up an inert He core
Lifetime:
•~10 Byr for a 1 Msun star( Sun)
•~10 Tyr for a 0.1 Msun star (red dwarf)
When all H in core converted to He
He core collapses and heats up
•High temperatures ignites H burning in a shell
Outer layer
expands and
cools
Star becomes a
Red Giant
Outside: Envelope ~ size of orbit of Venus
•The star gets brighter and redder, climbs up
the Giant Branch. (Takes 1 Byr)
At the top of the Red Giant Branch:
•Tcore reaches 100 Million K
He fusion begins in core
Fusion of three 4He nuclei into one 12C
nucleus.
*A secondary reaction forms Oxygen from
Carbon & Helium:
Helium Flash in the core.
Short period of fast burning, then.
star contracts, gets a little dimmer, but hotter
M
. oves onto the horizontal branch.
Horizontal Branch Phase
Structure:
•He-burning core
•H-burning shell
Build up of a C-O core, still too cool to
ignite Carbon
After 100 Myr, core runs out of He.
Inside:
•C-O core collapses and heats up
•He burning shell outside the C-O core
•H burning shell outside the He shell
Outside:
Star swells & cools
Climbs the Giant Branch again, slightly to
the left of the original Giant Branch .
Helium shell flash produces a new
powerful explosion, that pushes the outer
envelope outward.
Core and Envelope separate.
With weight of envelope gone, core never
reaches 600 million K, no Carbon fusion
Core contraction is stopped by electron
degeneracy.
A Planetary Nebula forms
Hot C-O core is exposed, moves to the left
Becomes a White Dwarf
Expanding envelope forms a ring nebula
around the White Dwarf core.
Ring is Ionized and heated by the hot
central core of WD.
Called planetary nebula because look like
a tiny planet in a small telescope.
•The nebula expands at the ~
35,000 to 70,000 miles/hour.
Expands away in ~ 10,000 yrs
Planetary Nebulae
Often asymmetric, possibly due to :
Stellar rotation
Magnetic fields
The Hour Glass Nebula
The Butterfly Nebula
White Dwarf Properties
Radii ~ 1000-5000 km (~ size of Earth!)
Temp. – from 100,000 to 2500 K.
So small, that they can only be seen if
close-by, or in a binary systems.
White Dwarf’s mass < than the
Chandrasekhar mass (1.4 Solar Masses).
•White Dwarf Properties
•The core is tightly packed
•One teaspoon weighs about 5 tons.
Shine by leftover heat, no fusion.
Fade slowly, becoming a "Black Dwarf“.
•Takes ~10 Tyr to cool off , so none exists yet.
The most famous W.D. is Sirius’ companion .
Sirius B
Temp. 25,000 K
Size: 92% Earth's diameter
Mass: 1.2 solar masses
The mass of a star, in the size of aSirius
planet.
B
But wait that’s not all!
About half the stars in the sky are
binaries.
What about Binary Stars with one
being a W.D. !
Mass could transfer
from the star
to the W.D.
White Dwarf in a binary system…..
I
White Dwarf
Evolving (dying) star
Roche Lobes
II
Evolving (dying) star
White Dwarf
Accretion Disk
III
Evolving (dying) star
Roche Lobe filled
A w.d. can take on material but , if
the w.d. exceeds 1.4 solar masses,
powerful explosions take place, and they
can repeat.
Type 1a
super
NOVA!!
Since the Type 1a supernova is always a white
dwarf they can be used to judge very great
distances (using the inverse square law).
Crab Nebula
Supernova
Remnant
Stellar Graveyard
High Mass Stars
The End-States for Low and High Mass Stars
Initial Stellar Mass
(Solar Mass)
Final Core Mass
Final State
1-8
0.5 - 1.4
White dwarf
1.4 - 3.0
Neutron
Star
> 3.0
Black
hole
8 - 30
> 30
Evolution of High Mass Stars
Massive stars go through about the
same internal changes as low mass
stars, except :
•massive stars evolve more
rapidly due to greater gravity.
•massive stars can produce
heavier elements
Evolution of High-Mass Stars
O & B Stars (M > 8 Msun): (The James Dean of stars )
•Burn Hot
•Live Fast
•Die Young
Main Sequence Phase:
•Burn H to He in core using the CNO cycle
•Build up a He core, like low-mass stars
•But this lasts for only ~ 10 Myr
After H core exhausted:
•Inert He core contracts & heats up
•H burning in a shell
•The Envelope expands and cools
•Envelope ~ size of orbit of Jupiter
Moves horizontally across the H-R
diagram, becoming a Red Super giant star
Takes about 1 Myr to cross the H-R diagram.
Core Temperature reaches 170 Million K
Helium Flash : Helium Ignites producing C & O
Star becomes a Blue Supergiant.
He runs out in the core:
•Inert C-O core collapses & heats up
•H & He burning shells expand
Star becomes a Red Supergiant again
C-O Core collapses until:
•Tcore > 600 Million K
•Ignites Carbon Burning in the
Core.
Carbon Burning:
12
2- C fuse to form : Mg, Ne and O
Carbon burning: 1000 years
Fusion now takes place rapidly
.
Neon burning: ~10 years
Oxygen burning: ~1 year
Silicon burning: ~1 day
Finally builds up an inert Iron core.
End of the road !
Core of a massive star at the end of
Silicon Burning:
Onion Skin
Collapse is final :Protons & electrons
form neutrons & neutrinos.
At
. the start of Iron Core collapse:
•Radius ~ 6000 km (~radius of earth)
•Density ~ 108 g/cc
•A second later!! , the properties are:
•Radius ~50 km
•Density ~1014 g/cc
•Collapse Speed ~0.25 c !
Material falling inwards is
stopped by neutron degeneracy
pressure .
This material rebounds, causing
the outer atmosphere, and shells,
to be blown off in a violent
explosion called a supernova.
Elements heavier than Lead are
produced in the explosion.
The supernova star will outshine all
the other stars in the galaxy
combined.
The Famous
Supernova
SN 1987A
type II Supernova
• The Crab Nebula.
• This nebula is the result of a supernova
that, exploded in 1054.
• The supernova was brighter than Venus for
weeks before fading from view.
•The nebula is
expanding at more
than 3 million miles
per hour.
Structure of a Neutron Star
•Diameter- 10 km in diameter
•3> Mass > 1.4 times that of our Sun.
•One teaspoonful would weigh a billion tons!
Rotation Rate:
1 to 100 rotations/sec
Inside a
Neutron Star
Pulsar
Magnetic axis is not aligned with the rotation axis.
Lighthouse Model:
Spinning magnetic
field generates a
a strong electric field.
We will see regular, sharp pulses of light (optical,
radio, X-ray) , if its pointed toward the earth.
The discovery of a pulsar in the crab nebula was the
key connecting pulsars and neutron stars.
Black Holes
We know of no mechanism to halt the collapse of
a compact object with mass > 3 Msun.
Relativity implies nothing can go faster than light.
As you travel faster, time slows down, you get more
massive and your length appears to get shorter.
The effect of gravity on light
Singularities
•If the core of a star collapses with more than 3
solar masses, electron degeneracy and neutron
degeneracy can’t stop the gravitational collapse.
•The star collapses to a radius of zero , with infinite
density and gravity—called a Singularity.
Position
Particle paths in a
collapsing star
singularity
Event horizon
Time
The Schwarzschild Black Hole
The simplest of all black holes.
A static, non-rotating mass.
The Schwarzschild Radius defines the Event
Horizon.
We have no way of finding
out what’s happening
inside the “Event horizon”
The Kerr Rotating Black Hole
The singularity of a Kerr Black Hole is in
infinitely thin ring around the center of the
hole.
The event horizon is surrounded by the
ergosphere, where nothing can remain at rest.
Here spacetime is being pulled around the
rotating black hole.
An object is moving fast enough, can enter the
ergosphere and fly out again. If the object stops in
the ergosphere, it must fall into the Black Hole.
General Relativity predicts Wormholes for Kerr
Black Holes, but Astrophysicists are skeptical.
It may be possible to avoid the
singularity.
•Various Black Holes
•Primordial – can be any size (created with Big
Bang).
•“Stellar mass” black holes – must be at least 3 Mo
– many examples are known
•Intermediate black holes – range from 100 to 1000
Mo - located in normal galaxies – many seen
•Massive black holes – about 106 Mo – such as in the
center of the Milky Way – many seen
•Supermassive black holes – about 109-10 Mo-
located in Active Galactic Nuclei, have jets – many seen
Candidate For Black Hole
Cygnus X-1 Binary Star w/ two objects:
•M=30 Msun primary ,
• M=7 Msun companion
Bright in X-rays.
–Far too massive to be a white
dwarf or neutron star.
–The simplest interpretation is :
– A 30 M star and a 7 M black
hole
Measured orbital
motion of HDE
226868.
Evidence for BH
800 light years
A disk of dust fueling
a massive black hole
in the centre of a
galaxy.
The speed of the gas
around the center
indicates that the
object at the centre
is 1.2 billion times
the mass of our Sun.
Signature of a Black Hole
Thanks to the following for allowing me to
use information from their web site :
Nick Stobel
Bill Keel
Richard Pogge
NASA