Stellar Evolution:Evolution off the Main Sequence

Main Sequence Lifetimes

Most massive (O and B stars): millions of years

Stars like the Sun (G stars): billions of years

Low mass stars (K and M stars): a trillion years!

While on Main Sequence, stellar core has H -> He fusion, by p-p chain in stars like Sun or less massive. In more massive stars, “CNO cycle” becomes more important.

Units of Chapter 12

Leaving the Main Sequence

Evolution of a Sun-like Star

The Death of a Low-Mass Star

Evolution of Stars More Massive than the Sun

Supernova Explosions

Observing Stellar Evolution in Star Clusters

The Cycle of Stellar Evolution

Summary of Chapter 12

On the Main Sequence

During its stay on the main sequence, any fluctuations in a star’s condition are quickly restored; the star is in equilibrium.

Leaving the Main Sequence

Eventually, as hydrogen in the core is consumed, the star begins to leave the main sequence.

Its evolution from then on depends very much on the mass of the star:

Low-mass stars go quietly.

High-mass stars go out with a bang!

Evolution of a Sun-like Star

Even while on the main sequence, the composition of a star’s core is changing.

Evolution of a Sun-like Star

As the fuel in the core is used up, the core contracts; when it is used up the core begins to collapse.

Hydrogen begins to fuse outside the core.

Evolution of a Low-Mass Star(< 8 M

sun , focus on 1 M

sun case)

- All H converted to He in core.

- Core too cool for He burning. Contracts, Collapses, Heats up.

Red Giant

- Tremendous energy produced. Star must expand.

- Star now a "Red Giant". Diameter ~ 1 AU! Sun-like star (1 M

Sun ) out to Mercury

- Phase lasts ~ 109 years for 1 MSun

star.

- Example: Arcturus

- H burns in shell around core: "H-shell burning phase".

Red Giant Star on H-R Diagram

Eventually: Core Helium Fusion

- Core shrinks and heats up to 10,000,000 (108) K, helium can now burn into carbon.

"Triple-alpha process"

4He + 4He -> 8Be + energy8Be + 4He -> 12C + energy

- First occurs in a runaway process: "the helium flash". Energy from fusion goes into re-expanding and cooling the core. Takes only a few seconds! This slows fusion, so star gets dimmer again.

- Then stable He -> C burning. Still have H -> He shell burning surrounding it.

- Now star on "Horizontal Branch" of H-R diagram. Lasts ~108 years for 1 M

Sun star.

Core fusionHe -> C

Shell fusionH -> He

Horizontal branch star structure

More massive less massive

Evolution of a Sun-like Star

Stage 11: Back to the giant branch:

As the helium in the core fuses to carbon and oxygen, the core becomes hotter and hotter, and the helium burns faster and faster.

The star is now similar to its condition just as it left the main sequence, except now there are two shells.

Helium Runs out in Core, Back to the Giant Branch

-All He -> C in core. Not hot enough for C fusion.

- Core shrinks and heats up.

- Get new helium burning shell (inside H burning shell).

Red Supergiant

- High rate of burning, star expands, luminosity way up.

- Called ''Red Supergiant'' (or Asymptotic Giant Branch) phase.

- Only ~106 years for 1 MSun

star.

The Death of a Low-Mass Star

This graphic shows the entire evolution of a Sun-like star.

Such stars never become hot enough for fusion past oxygen to take place.

"Planetary Nebulae"

- Core continues to contract. Never gets hot enough for carbon fusion.

- Helium shell burning becomes unstable -> "helium shell flashes".

- Whole star pulsates more and more violently.

- Eventually, shells thrown off star altogether! 0.1 - 0.2 MSun

ejected.

- Shells appear as a nebula around star, called "Planetary Nebula" (awful, historical name, nothing to do with planets).

The Death of a Low-Mass Star

There is no more outward fusion pressure being generated in the core, which continues to contract.

Stage 12: The outer layers of the star expand to form a planetary nebula.

The Death of a Low-Mass Star

The star now has two parts:

• A small, extremely dense carbon core

• An envelope about the size of our solar system.

The envelope is called a planetary nebula, even though it has nothing to do with planets – early astronomers viewing the fuzzy envelope thought it resembled a planetary system.

Bipolar

Planetary nebulae

Clicker Question:

What is the Helium Flash?

A: Explosive onset of Helium fusing to make Carbon

B: A flash of light when Helium fissions to Hydrogren

C: Bright emission of light from Helium atoms in the Sun

D: Explosive onset of Hydrogen fusing to Helium

Clicker Question:

What is happening in the interior of a star that is on the main sequence on the Hertzsprung-Russell diagram?

A: Stars that have reached the main sequence have ceased nuclear "burning" and are simply cooling down by emitting radiation.

B: The star is slowly shrinking as it slides down the main sequence from top left to bottom right.

C: The star is generating energy by helium fusion, having stopped hydrogen "burning."

D: The star is generating internal energy by hydrogen fusion.

Clicker Question:

What causes the formation of bipolar planetary nebulae?

A: A progenitor star with a rapid rotation

B: A progenitor star in a dense environment

C: A progenitor star in a binary system

D: A progenitor star with strong magnetic fields

The Death of a Low-Mass Star

Stages 13 and 14: White and black dwarfs:

Once the nebula has gone, the remaining core is extremely dense and extremely hot, but quite small.

It is luminous only due to its high temperature.

White Dwarfs

- Dead core of low-mass star after Planetary Nebula thrown off.

- Mass: few tenths of a MSun

.

-Radius: about REarth

.

- Density: 106 g/cm3! (a cubic cm of it would weigh a ton on Earth).

- White dwarfs slowly cool to oblivion. No fusion.

The Death of a Low-Mass Star

The small star Sirius B is a white dwarf companion of the much larger and brighter Sirius A.

The Death of a Low-Mass Star

The Hubble Space Telescope has detected white dwarf stars in globular clusters.

Death of a 1 solar mass star

Bipolar

Planetary nebulae

Stellar Explosions: Nova

A nova is a star that flares up very suddenly and then returns slowly to its former luminosity.

Stellar Explosions

Novae

White dwarf inclose binary system

WD's tidal force stretches out companion, until parts of outer envelope spill onto WD. Surface gets hotter and denser. Eventually, a burst of fusion. Binary brightens by 10'000's! Some gas expelled into space. Whole cycle may repeat every few decades => recurrent novae.

The Death of a Low-Mass Star

Material falls onto the white dwarf from its main-sequence companion.

When enough material has accreted, fusion can reignite very suddenly, burning off the new material.

Material keeps being transferred to the white dwarf, and the process repeats.

Novae

Evolution of Stars > 8 MSun

Higher mass stars evolve more rapidly and fuse heavier elements.

Example: 20 MSun

star lives

"only" ~107 years.

Result is "onion" structure with many shells of fusion-produced elements. Heaviest element made is iron.

Eventual state of > 8 MSun

star

Fusion Reactions and Stellar Mass

In stars like the Sun or less massive, H -> Hemost efficient through proton-proton chain.

In higher mass stars, "CNO cycle" more efficient. Same net result: 4 protons -> He nucleusCarbon just a catalyst.

Need Tcenter

> 16 million K for CNO cycle to be

more efficient.

(mass) ->

Sun

Clicker Question:

In which phase of a star’s life is it converting He to Carbon?

A: main sequence

B: giant branch

C: horizontal branch

D: white dwarf

Star Clusters

Extremely useful for studying evolution, since all stars formed at same time and are at same distance from us.

Comparing with theory, can easily determine cluster age from H-R diagram.

Galactic or Open Cluster

Globular Cluster

Star Clusters

Two kinds:

1) Open Clusters

-Example: The Pleiades

-10's to 100's of stars

-Few pc across

-Loose grouping of stars

-Tend to be young (10's to 100's of millions of years, not billions, but there are exceptions)

2) Globular Clusters

- few x 10 5 or 10 6 stars

- size about 50 pc

- very tightly packed, roughly spherical shape

- billions of years old

Clusters are crucial for stellar evolution studies because:

1) All stars in a cluster formed at about same time (so all have same age)

2) All stars are at about the same distance

3) All stars have same chemical composition

Following the evolution of a cluster on the H-R diagram

T

Globular clusters formed 12-14 billion years ago. Useful info for studying the history of the Milky Way Galaxy.

Globular Cluster M80 and composite H-R diagram for similar-age clusters.

Schematic Picture of Cluster Evolution

Time 0. Cluster looks blue

Time: few million years.Cluster redder

Time: 10 billion years.Cluster looks red

Massive, hot, bright, blue, short-lived stars

Low-mass, cool, red, dim, long-lived stars

Clicker Question:

The age of a cluster can be found by:

A: Looking at its velocity through the galaxy.

B: Determining the turnoff point from the main sequence.

C: Counting the number of stars in the cluster

D: Determining how fast it is expanding

Clicker Question:

Why do globular clusters contain stars with fewer metals (heavy elements) compared to open clusters?

A: Open clusters have formed later in the evolution of the universe after considerably more processing

B: Metals are gradually destroyed in globular clusters.

C: Metals are blown out of globular clusters during supernova explosions

D: Metals spontaneously decay to lighter elements during the 10 billion year age of the globular cluster.

Death of a High-Mass Star

M > 8 MSun

Iron core

Iron fusion doesn't produce energy (actually requires energy) => core shrinks and heats up

Ejection speeds 1000's to 10,000's of km/sec!(see DEMO)

Remnant is a “neutron star” or “black hole”.

T ~ 1010 K, radiation disrupts nuclei, p + e => n + neutrino

Collapses until neutrons come into contact. Rebounds outward, violent shock ejects rest of star => A Core-collapse or Type II Supernova

Such supernovae occurroughly every 50 yearsin Milky Way.

Binding Energy per nucleon

Example Supernova: 1998bw

Cassiopeia A: Supernova Remnant

A Carbon-Detonation or “Type Ia” Supernova

Despite novae, mass continues to build up on White Dwarf.

If mass grows to 1.4 MSun

(the "Chandrasekhar limit"), gravity overwhelms the Pauli exclusion pressure supporting the WD, so it contracts and heats up.

This starts carbon fusion everywhere at once.

Tremendous energy makes star explode. No core remnant.

Supernova 1987A in the Large Magellanic Cloud

1994 1998

Expanding debris from star. Speed almost 3000 km/sec!

Light from supernova hitting ring of gas, probably a shell from earlier mass loss event.

SN 1987A is evolving fast!

A Young Supernova

SN 1993JRupen et al.

In 1000 years, the exploded debris might look something like this:

Crab Nebula: debris from a stellar explosion observed in 1054 AD.

Vela Nebula: debris from a stellar explosion in about 9000 BC.

Or in 10,000 years:

2 pc

50 pc

Remember, core collapse (Type II) and carbon-detonation (Type I) supernovae have very different origins

Supernova light curves

Making the Elements

Universe initially all H (p’s and e’s). Some He made soon after Big Bang before stars, galaxies formed. All the rest made in stars, and returned to ISM by supernovae.

Elements up to iron (56Fe, 26 p + 30 n in nucleus) produced by steady fusion (less abundant elements we didn’t discuss, like Cl, Na, made in reactions that aren’t important energy makers).

Solar System formed from such "enriched" gas 4.6 billion years ago. As Milky Way ages, the abundances of elements compared to H in gas and new stars are increasing due to fusion and supernovae.

Heavier elements (such as lead, gold, copper, silver, etc.) by "neutron capture" in core, even heavier ones (uranium, plutonium, etc.) in supernova itself.

Clicker Question:

What is the remnant left over from a Type Ia (carbon detonation) supernova?

A: a white dwarf + expanding shell

B: a neutron star + expanding shell

C: a black hole + expanding shell

D: no remnant, just the expanding shell

Clicker Question:

What is the heaviest element produced by steady fusion in the core of a massive star?

A: Hydrogen

B: Carbon

C: Iron

D: Uranium

Clicker Question:

All of the following atoms have a total of 4 nucleons (protons or neutrons). Which of the following has the smallest mass?

A: 4 hydrogen atoms

B: 2 deuterium atoms

C: 1 tritium atom and 1 hydrogen atom

D: 1 Helium atom

E: None of the above, they all have the same total mass

1. White Dwarf If initial star mass < 8 M

Sun or so

2. Neutron Star If initial mass > 8 M

Sun and < 25 M

Sun

3. Black Hole If initial mass > 25 M

Sun

Final States of a Star

No Event + PN

Supernova + ejecta

GRB + Hypernova + ejecta