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NOTES: The Lives of Stars Gestation, Birth, and Youth : 1. The womb : Stars are born in dense molecular clouds . --The interstellar medium must be dense enough so H atoms can collide and form H2 molecules. This also is

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slide1

NOTES: The Lives of Stars

Gestation, Birth, and Youth:

1. The womb: Stars are born in dense molecular clouds.

--The interstellar medium must be dense enough so H atoms

can collide and form H2 molecules.This also is

facilitated on dust--for other molecules as well. It increases

gravitation enough for stars to form in reasonable time.

--Different sized clumps form stars of differing mass.

--Disk with central sphere (protostar) formed. Gravity heats by

Helmholtz contraction. Disk forms solar system.

--Stability when gravity balances gas pressure (overlay).

(Fully developed fetus)

--Star draws a womb of dust around it. It glows in the IR.

2. Birth: A star is born when its cores temperature reaches

10 million K. This happens for masses > 0.08 M(Sun).

--the star blasts away its womb of dust and shines.

--T Tauri Stars: variable brightness (like contractions).

Low mass stars just about to move to the main sequence.

slide2

How do we find the mass of a star?

The mass--luminosity relation (a line in a logarithmic plot

--main sequence stars only)

slide3

The womb: Stars are born in dense molecular clouds.

--The interstellar medium must be dense enough so H atoms can collide and form H2 molecules.This also is

facilitated on dust--for other molecules as well. It increases gravitation enough for stars to form in reasonable time.

slide4

The cloud starts to contract.

The cloud fragment is about 104 AU in size

slide5

A protostar has condensed in the middle.

The protostar is about 1 AU in size;

the whole picture is about 100 AU in size

slide9

2. Birth: A star is born when its cores temperature reaches

10 million K. This happens for masses > 0.08 M(Sun).

--the star blasts away its womb of dust and shines.

--T Tauri Stars: variable brightness (like contractions),

low mass, strong magnetic fields, large sunspots.

slide10

Infancy:

--Jets of gas may heat the interstellar medium

--Herbig Haroobjects or YSOs (Young Stellar Objects).

Bipolar outflow.

slide11

--Mass less than .08 M(sun) but larger than Jupiter:

failed star or brown dwarf(large planet).

slide12

IONIZATION STATE OF ATOMS

Each state for a given element has a unique spectrum.

Number of electrons

removed:

H

He………

O

--------------------------------------------------------------------------

0 (neutral atom) HI HeI…….. OI

1 HII HeII……. OII

2 HeIII…… OIII

6 OVII

Number of electrons removed = roman numeral – 1.

slide14

Chain Reaction Star Formation

Massive star formation triggers nearby regions to become

new star formation regions. Shockwaves from ionization and

supernovae bunch up material to form stars.

slide15

'Working Years'--Main Sequence--H burning phase.

Lasts 9 billion years for the Sun. Moves very slightly up and

to the right in H-R diagram. As H in core is depleted, star

contracts slightly and Luminosity increases a little. He

has less gas pressure than H.

slide16

Midlife Crisis'--Red Giant Phase:

1. Stops burning H in the core, contracts, starts burning

H in a shell around the core (Shell H-Burning).

slide17

The heat expands the outer envelope of the star.

It moves way up in the H-R diagram for a 1 solar mass star,

stays at the same luminosity, but gets redder for a 5 Msun star.

slide18

Giant phase

evolutionary track

varies with mass.

Mass loss as Red Giant

is as much as

10-6 msun/year!

slide19

The Red Giant contracts and Helium begins to burn

in Helium flash with electron degeneracy holding up core

in 1 Msun star.

slide20

He continues to burn to C by triple alpha process.

In larger mass stars, alpha particles are added one by one,

creating elements with an even atomic number. Sometimes this

is called the triple alpha process as well, even though more than three

alpha particles are involved.

slide22

Shell He-burning. He and H rekindles around core. 1 Msun star expands to Red Giant again and 5 Msun redder and lower temp. (To right in H-R.) 5 Msun or more undergoes thermal pulsations (Cepheids and

RR Lyrae variables

--are on the

instability strip

on H-R Diagram).

slide23

'Retirement'

1. Stars starting with less than about 2 Msun finish burning

to carbon, become unstable as they burn H and He in a shell

and shuck off a shell of 10-20% of their mass, becoming a

planetary nebula, glowing because they are ionized by

the hot UV core.

slide24

2. Stars with more than 2 Msun burn to whatever

element is the largest possible for their temperature.

In very large stars (over 10 Msun), core burns to iron(Fe).

slide25

Overview heavy element nucleosynthesis

The s (slow) and r (rapid) process: elements heavier

than Fe are formed by addition of neutrons and then beta

decay (see overlay). The s process adds one neutron at a

time, the r process many at a time.

Ex. of s process: 114Cd + 1n --> 115Cd --> 115In + e- + ν .

slide26

'Death'of stars:

1. Supernova Type II: A star of over 2 solar masses

burns to all it can, collapses as supporting radiation

turns off, gets hot, produces neutrinos by combining

protons and electrons, and rebounds, and explodes.

Supernova 1987A in Large Magellanic

cloud detected by Ian Shelton--new star on plate.

slide27

The Crab Nebula in Taurus is a supernova II remnant.

It exploded almost a thousand years ago.

slide28

The Anasazi (native americans) recorded the event

in Chaco canyon. (The Chinese read their manuscripts

at night in the light of a night-time sun.)

slide29

3. Nova and Supernova Type Ia: A binary with a white

dwarf and a red giant creates an explosion. Mass

from the red giant is pulled onto the surface of the

white dwarf until it reaches 1.43 solar masses—critical mass.

slide31

The heating creates an explosion: Supernova Type I,

if the white dwarf is destroyed, Nova if it is not.

slide32

Supernovae type Ia are

standard candles--of same peak luminosity.

Which means we automatically know their what?

slide33

White Dwarf: death state of low mass stars

about earth-sized, held up by electron pressure.

Fusion has ceased. Hot at first on surface--20,000 K then cool to

black dwarf(a carbon cinder in space) in tens of billions of years.

slide34

Chandrasekar Limit--white dwarfs form

with remnant under 1.3 Msun.

slide35

Think very thick styrofoam coating on golf balls:

styrofoam is like electron cloud of H atom,

golf ball is like proton.

slide36

Put these styrofoam balls in a pile with electron clouds

touching and you have white dwarf material.

slide38

It is this process backwards—inverse beta decay,

or squeezing and electron into a proton to make

a neutron.

Beta decay forward,

Inverse beta decay backward.

slide39

A black hole is like putting the golf balls into

the ultimate trash compactor the neutrons are squeezed into

a point—the black hole singularity.

slide41

Cons. of angular momentum--rapid spin, strong

magnetic fields and synchotron radio radiation as

electrons are spun out along field lines.

slide42

As NS axis wobbles, the beams may be detected as pulsars,

with a period of milliseconds to seconds. Not all neutron stars

are pulsars.

slide43

Jocelyn Bell discovered the first pulsar in 1967.

It was at first thought to be a signal from an alien

civilization and had a period of about 1 s.

Her thesis advisor, Anthony Hewish

made an effective radio dish by stringing

wires in a grape arbor.