Chapter 5: Cosmic foundations for origins of life - stars

Stellar evolution: forming the elements for biolmolecules and planets….

Stars are fusion reactors that convert lighter elements into heavier ones, liberating energy (from E=mc^2)

They therefore continuously evolve as their fuels are used up. H burns to He, He burns to C, etc…

Stellar end-states: white dwarfs, neutron stars, or black holes.

In all of these cases, significant fraction of stellar mass, ejected into interstellar medium

Planets, and biomolecules made out of these materials

Energy liberated when light atomic nuclei undergo fusion! (eg. Proton-proton reaction)

• Two protons colliding at high enough speed, undergo fusion.

• Products: a deuteron (heavy water), a positron (positively charged electron), and a neutrino (very weakly interacting particle) + energy release:

• Special Relativity: Energy release per fusion is proportional to mass difference between products and reactants

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Energy production in the Sun’s core – the proton-proton chain

Net result of p-p burn:

For each Helium-4 nucleus produced:

- Consume 4 protons

- Liberate energy and 2 neutrinos

- Neutrinos arise from weak interaction.

eg. they arise during conversion of a proton to a neutron in building a deuteron

Nuclear reactions yield predictable neutrino fluxes from the Sun that directly reflect reaction rates

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Stellar temperatures: Stars have

different colours Corresponds to

different temperatures from black body curves

Note *huge* (and harmful) UV fluxes produced by massive stars

(life possible on planets around them?)

Spectral Classification of Stars – Consequence ofstellar temperatures:

- Stellar spectra can be divided into “spectral classes” of stars – O,B,A,F,G,K,M

- Atomic theory: this represents a sequence of decreasing temperature - hot stars are more completely ionized than cool stars so see fewer absorption features.

- The Sun is a G2 star.

Hertzsprung-Russell Diagram: Plotting L vs. T

Luminosity L and temperature T of a star are independent physical properties of a star.

-Temperature correlates with colour of a star (hot is blue, cool is red). L varies by factor of 100 million!

-Plot L of a star vs. its colour on a diagram: find that these are correlated with one another. Known as “colour-magnitude diagram”.

- Most stars occur along “main-sequence”, where they burn hydrogen.

H-R Diagrams (L vs. T) of Nearest, and brightest stars

Stars within 5pc of Sun 100 brightest stars in the sky

STELLAR RADII:

Range from 0.08 of the Sun, to 630 times the Sun’s radius (Betelgeuse)

Giants: radii of 10 – 100 solar radius

(Mira is Red Giant)

Supergiants: up to 1000 solar radii

Main Sequence: Stars confined to well defined band from top left (high T, high L), to bottom right (low T, low L).

•Temperature range over main sequence: 3,000K (M type) – 30,000 K (O type); 1 decade in temperature

• Range in luminosities over 8 decades!

- partly explained by black body relation;

• At top end – stars are hot and large: blue supergiants

• At bottom end – stars are cool and small: red dwarfs

• O and B stars extremely rare: one in 10,000

Stars spend most of their life on main-sequence burning hydrogen

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Off the main sequence: Red giants (upper right of H-R diagram:

high L, low T); and white dwarfs (lower left: low L, high T).

Red giants burn hydrogen in a shell White dwarfs hard to detect – very faint Sun will go through red-giant phase and

end up as a cooling white dwarf. Red giant will swell to orbit beyond

Earth… consequences for life!

Main sequence is a mass sequence: ie stellar mass determines stellar properties

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After 10 billion years, core of solar mass star uses up H, and consists of He. Fusion ceases at centre of core, and it begins to contract. Star leaves main sequence.

Structure of Red Giant star – furious hydrogen burning occurs in a shell gradually moving out through unburned material. Non-burning He ash accumulates in core.

Red Giant Branch:

Subgiant Branch: Stage 7 – Stage 8;

-H burns in a shell, He ash accumulates in core.

Red Giant Branch: Stage 8 – stage 9:

- Outer layers of star so cool that convection throughout star occurs – so ascend a vertical track

Tip of Giant Branch:

-Radius is 100 solar radii (size of Mercury’s orbit)

- He core is 1/1000 size of star - few times larger than the Earth. 25% of stellar mass locked up in core

- 10,000 times the luminosity of the Sun.

- Core density, about 100 million kg/ cubic metre.

- Envelope density, about 1/1000 kg/ cubic metre

At stage 9 – tip of Giant Branch – central temperatures are

100 million K, at densities of kg/cubic metre, conditions allow ignition of helium “ash” accumulating in stellar core:

• Beryllium – 8 highly unstable. Decays very quickly into 2 alpha particles again - about ! SLIGHT CHANGE IN STRENGTH OF NUCLEAR FORCE AND THIS REACTION IS IMPOSSIBLE!

• Resonant interaction between Be and alpha particle allow second reaction above to occur - carbon is the “ash”

Helium fusion: the Triple-Alpha Process fine tuning!

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Horizontal Branch – Helium “Main-Sequence”

Helium Flash: Explosive onset of He burning at tip of Red Giant Branch (RGB). (stage 9)

• He burning core (stage 10) known as Horizontal Branch.

Ascending the Asymptotic Giant Branch – the

Accumulation of Carbon When He in core of star on Horizontal Branch is used up – He shell burn commences – star moves off Horizontal Branch.

• Now have 2 burning shells, H, and deeper in, He – with Carbon “ash” accumulating in core

• Star moves up “asymptotic giant branch” increasing in size and luminosity. Carbon core continues to contract

• Horizontal Branch (stage 10): He core burn – and H shell burn. The “Main-Sequence” for He burning.

• Asymptotic Giant Branch (AGB) stage 10 – stage 11: Shell burning for both He and H. Carbon ash accumulates in core.

•Produces much larger red star – Red Supergiant [500 solar radii – swallows Mars!, surface temperature 4000 K, central T 250 million K.

5 billion years into the future – the fate of the Sun

Planetary nebula – NGC 3132.

- ejection of envelope of star leaving a degenerate stellar core (white dwarf).

- White dwarf

- Outer edge of envelope

Evolution of Massive StarsStars more massive than 8 solar masses lead to supernova explosions

• High mass stars move almost horizontally (rather than vertically) in post main-sequence evolution:

- luminosity of star stays fairly constant but radius increases,reducing surface temperature

• High mass stars fuse carbon, oxygen, and other elements

For massive stars (more than 8 solar masses) – series of burning shells – ash of burn above it igniting producing ash beneath it.

• Creates an “onion-like” series of burning layers…. at bottom of which is iron ash.

• Carbon burns for 1000 yr, oxygen for a yr, silicon for a week.

• Iron core grows for less than a day!

Iron is nature’s most stable element• Small nuclei liberate energy by fusion

• Elements more massive than iron liberate energy by fission into smaller nuclei.

• IRON DOES NOT BURN!

• Degenerate iron core is end-state of nuclear fusion in interior.

Carbon burning: (a) occurs at T= 600 million K, while (b) occurs at 200 million K

The route to iron:

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Oxygen Fusion:

Silicon Burning: Building up to Nickel (T = 3 billion K !)

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Nickel-56 quickly decays via cobalt-56 into stable iron-56

Synthesizing Elements Beyond Iron

Occurs by “neutron capture” to iron (which just changes the isotope), followed by radioactive decay into stable element: eg.

Neutron capture occurs during supernova explosion (high density and temperature) – either by “rapid” (r) or “slow” (s) process

Elements produced during explosion much rarer because time available to produce them is so short

eCoFe

FenFe

FenFe

FenFe

5959

5958

5857

5756

Nucleosynthesis in stars: explains abundances of the elements

Sharp drop in abundance as go to higher atomic number – reflects increasing Coulomb barrier to fusion

Peaks and troughs in distribution – reflect stable “closed shell” nuclei, etc.

Supernova remnant: the Crab nebula (supernova seen by Chinese astronomers in 1054 A.D.)

Canadian Galactic Plane Survey (CGPS): the interstellar medium… stirred by supernova explosions and stellar winds….Map of atomic hydrogen.

[Midplane of Milky Way - near constellation Perseus]