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.

NOTES: The Lives of StarsGestation, 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.

How do we find the mass of a star?

The mass--luminosity relation (a line in a logarithmic plot--main sequence stars only)

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

The cloud starts to contract. The cloud fragment is about 104 AU in size

A protostar has condensed in the middle. The protostar is about 1 AU in size; the whole picture is about 100 AU in size

Protostar evolution: different tracks for different mass.

A protostar has a womb of dust--it is an infrared black body.

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.

Infancy:--Jets of gas may heat the interstellar medium--Herbig Haro objects or YSOs (Young Stellar Objects). Bipolar outflow.

--Mass less than .08 M(sun) but larger than Jupiter: failed star or brown dwarf (large planet).

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.

Hot O and B stars form HII regions - Stromgren Spheres.

Chain Reaction Star FormationMassive star formation triggers nearby regions to become new star formation regions. Shockwaves from ionization and supernovae bunch up material to form stars.

'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.

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).

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.

Giant phase evolutionary trackvaries with mass.

Mass loss as Red Giant is as much as 10-6 msun/year!

The Red Giant contracts and Helium begins to burn in Helium flash with electron degeneracy holding up core in 1 Msun star.

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 thisis called the triple alpha process as well, even though more than threealpha particles are involved.

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 theinstability stripon H-R Diagram).

'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 bythe hot UV core.

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).

Overview heavy element nucleosynthesis

process conditions timescale site

s-process(n-capture, ...)

T~ 0.1 GKn~ 1-1000 yr, nn~107-8/cm3

102 yrand 105-6 yrs

Massive stars (weak)Low mass AGB stars (main)

r-process(n-capture, ...)

T~1-2 GKn ~ s, nn~1024 /cm3

< 1s Type II Supernovae ?Neutron Star Mergers ?

p-process((,n), ...)

T~2-3 GK ~1s Type II Supernovae

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 .

'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.

The Crab Nebula in Taurus is a supernova II remnant.It exploded almost a thousand years ago.

The Anasazi (native americans) recorded the event in Chaco canyon. (The Chinese read their manuscriptsat night in the light of a night-time sun.)

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.

The heating creates an explosion: Supernova Type I, if the white dwarf is destroyed, Nova if it is not.

Supernovae type Ia are standard candles--of same peak luminosity.Which means we automatically know their what?

White Dwarf: death state of low mass starsabout 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.

Remnant: End State: Supporting Pressure: < 1.3 MO White Dwarf Electron degeneracy 1.3 MO<M<3.0 MO Neutron Star Neutron > 3.0 MO Black Hole None

Chandrasekar Limit--white dwarfs form with remnant under 1.3 Msun.

Think very thick styrofoam coating on golf balls: styrofoam is like electron cloud of H atom, golf ball is like proton.

Put these styrofoam balls in a pile with electron cloudstouching and you have white dwarf material.

Squeeze the styrofoam into the golf ball and you haveAn analogy to neutron star matter.

It is this process backwards—inverse beta decay,or squeezing and electron into a proton to makea neutron.

Beta decay forward,Inverse beta decay backward.

A black hole is like putting the golf balls into the ultimate trash compactor the neutrons are squeezed intoa point—the black hole singularity.

. Neutron Star: --Electrons squeezed into protons make neutrons.

--Gravity sufficient to make neutrons 'touch'. --10's of km in diameter with some surface electrons.

Cons. of angular momentum--rapid spin, strong magnetic fields and synchotron radio radiation aselectrons are spun out along field lines.

As NS axis wobbles, the beams may be detected as pulsars,with a period of milliseconds to seconds. Not all neutron stars are pulsars.

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 Hewishmade an effective radio dish by stringingwires in a grape arbor.

The Crab Nebula has a pulsar in its core with period 1 sec.