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

Hydrogen

Continuum

Absorption Lines

Doppler effect

0

0

0

c

v

–Waves compressed with source moving toward you Sound pitch is higher, light wavelength is compressed (bluer)

• similar in light and sound

–Waves stretched with source moving away from you

•Sound pitch is lower, light wavelength is longer (redder)

Red Shift

If two stars are similar and one star is 3 times as far away, as the other, its intensity will be 1/9.

Inverse square of light

d=1

d=2

d=3

B=1

B=1/9

B=1/4

Stars are different colors, because they are different temperatures

Spectral Classification

O B A F G K M35,000 K 3,000 K

Sun(G2)

A5 K7

•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 :

•Oh, Be A Fine Girl (Guy) Kiss Me

Subclasses

The Spectral Sequence

Bluest Reddest

Spectral Sequence is a Temperature Sequence

Hottest Coolest50,000K 1300K

O B A F G K M L

O Stars

B Stars

T = 11,000 - 30,000 K; Strong He lines; very weak H lines

A Stars

Hottest Stars: T>30,000 K; Strong He+ lines; no H lines

T = 7500 - 11,000 K; Strongest H lines, Weak Ca+ lines.

F StarsT = 5900 - 7500 K; H grows weaker Ca+ grows stronger, weak metals begin to emerge.

G StarsT = 5200 - 5900 K; Strong Ca+, Fe+ and other metals dominate,

K StarsT = 3900 - 5200 K; Strong metal lines, molecular bands begin to appear

M StarsT = 2500 - 3900 K; strong molecular absorption bands particularly of TiO

Solar Spectrum4000 A 7000 A

Quantum Mechanics

Electrons can only orbit the nucleus in certain orbits.

n =1 First orbital: Ground State)

•Lowest energy orbit .

Up absorptionDown emission Hydrogen Spectrum

Hydrogen (1H) consists of:

•A single proton in the nucleus.

•A single electron orbiting the nucleus.

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

40,000 20,000 10,000 5,000 2,500

106

104

102

1

102

104

Temperature (K)

Lu

min

osi

ty (

Ls

un)

White Dwarfs

Giants

Supergiants

Main Sequence

As you move up the H-R diagram on the Main Sequence from M to O, the stars get hotter and larger

Star Formation “All we are is dust in the wind” -

Kansas

Back to this is your life

(GMC) in Orion

•About 1000 GMCs are known in our galaxy

• These clouds lie in the spiral arms of the galaxy

Protostars form in cold, Giant Molecular Clouds

The Cone Nebula

Examining a Star

FormingRegion

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

•A shockwave is needed to trigger formation, and to compress the material .

GMC’s resist forming stars because of internal pressure (kinetic energy) so, a cooler gas is needed.

Sources of Shockwaves:

1.Supernova explosions: Massive stars die young .

2. Previous star formation can trigger more formations

Spirals arms are probably rotating shock waves.

3. Spiral arms in galaxies like our Milky Way:

                                                                        

              

View all images

An expanding supernova explosion , occurring about 15,000 years ago.

Gravity Contraction

As they heat up, blobs glow in the infrared, but they remain hidden .

As the cloud is compressed, cool blobs contract into individual stars.

The blobs glow faintly in radio or microwave light.

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.

False Color: Green = scattered starlight and red = emission from hot gas.

A T-Tauri star can lose up to 50% of its mass before settling down as a main sequence star.

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.

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 .

Collapse is slower for lower masses:

•1 Msun (solar Mass) ~30 Myr

•0.2 Msun ~1 Billion years

Low-Mass Protostars

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.

When core Temperature >10 Million K: Ignite first P-P Chain then CNO fusion in the core.

High-Mass Protostars

Collapse is very rapid: 30 solar mass protostar collapses in ~30,000 years

near the starsClouds are blown away from the new stars

Protostars!The Cocoons of proto-stars are exposed when the surrounding gas is blown away by winds and radiation from nearby massive stars.

Finally: •Pressure=Gravity & collapse stops.

•Becomes a Zero-Age Main Sequence •Star, (ZAMS).

The Main Sequence

Core temperature & pressure rise

•Collapse begins to slow down

•Pre-main sequence evolutionary tracksMost 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

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.

Extreme :Maximum Mass: 60-100 Msun

Main Sequence

Star spends 90% of their life on the MS

The star neither expands nor contracts.

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

                                                  

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)

Main Sequence LifetimesSpectral Type Mass

(Solar masses)Main sequence lifetime (million

years)

O5 40 1

B0 16 10

A0 3.3 500

F0 1.7 2700 2.7 BY

G0 1.1 9000 9 BY

K0 0.8 14 000 14 BY

M0 0.4 200 000 200BY

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!!

“It’s the end of the world as we know it” . REM

Death of Low Mass Star

The End-States for Low and High Mass Stars

Initial Stellar Mass Final Core Mass Final State

.08 - 8 0.5 - 1.4 White dwarf

8 - 30 1.4 - 3.0 Neutron Star

> 30 > 3.0 Black hole

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)

Outer layer expands and

cools

Star becomes a

Red Giant

He core collapses and heats up

•High temperatures ignites H burning in a shell

When all H in core converted to He

Outside:

•                                                                                        

•The star gets brighter and redder, climbs up the Giant Branch. (Takes 1 Byr)

Envelope ~ size of orbit of Venus

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 .Moves onto the horizontal branch.

Structure:

•He-burning core

•H-burning shell

Build up of a C-O core, still too cool to ignite Carbon

Horizontal Branch Phase

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 .

                                                                                        

With weight of envelope gone, core never reaches 600 million K, no Carbon fusion

Core contraction is stopped by electron degeneracy.

Helium shell flash produces a new powerful explosion, that pushes the outer envelope outward.

Core and Envelope separate.

A Planetary Nebula forms

                                                                                   

Hot C-O core is exposed, moves to the left

Becomes a White Dwarf

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.

Expanding envelope forms a ring nebula around the White Dwarf core.Ring is Ionized and heated by the hot central core of WD.

Expands away in ~ 10,000 yrs

Planetary NebulaeOften asymmetric, possibly due to :Stellar rotationMagnetic 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 White Dwarf’s mass < than the

Chandrasekhar massChandrasekhar mass (1.4 Solar Masses(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.

Sirius B Temp. 25,000 KSize: 92% Earth's diameterMass: 1.2 solar masses

Sirius B

The most famous W.D. is Sirius’ companion .

The mass of a star, in the size of a planet.

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.

But wait that’s not all!

White Dwarf in a binary system…..

White DwarfEvolving (dying) star

Roche Lobes

Evolving (dying) starWhite Dwarf

Accretion Disk

Roche Lobe filled

Evolving (dying) star

I

II

III

Type 1a

super NOVA!!

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.

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 NebulaSupernovaRemnantStellar 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

8 - 30 1.4 - 3.0Neutron

Star

> 30 > 3.0Black hole

•massive stars evolve more rapidly due to greater gravity.•massive stars can produce heavier elements

Evolution of High Mass Stars

Massive stars go through about the same internal changes as low mass stars, except :

Evolution of High-Mass StarsO & 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.

Star becomes a Blue Supergiant.

Core Temperature reaches 170 Million KHelium Flash : Helium Ignites producing C & O

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:

2- 12C 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.

The supernova star will outshine all the other stars in the galaxy combined.

Elements heavier than Lead are produced in the explosion.

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.

Inside a Neutron Star

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

We will see regular, sharp pulses of light (optical, radio, X-ray) , if its pointed toward the earth.

Lighthouse Model:

field generates a

Spinning magnetic

a strong electric field.

PulsarMagnetic axis is not aligned with the rotation axis.

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.

The effect of gravity on light

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.

Singularities

Position

Time

singularity

Event horizon

Particle paths in a collapsing star

•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.

The Schwarzschild Black HoleThe 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 HoleThe 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.

It may be possible to avoid the singularity.

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.

•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

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.800 light years

A disk of dust fueling a massive black hole in the centre of a galaxy.

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