Evolution of Stars1. Formed from gas/dust cloud collapse from gravity

2. Fuse H to He on the Main Sequence. Then evolve off Main-sequence as they burn He and successive elements.

3.When nuclear fusion ceases, the stars either shed their atmospheres through winds or explosions. They return their “metal-polluted” gas to the galaxy. These heavy elements are then processed again and again in successive generations of stars.

Universe began 13.7 billion years ago. At that time, only Hydrogen and Helium were present (trace amounts of Li, B, Be). At this point the metal mass fraction of the Universe was Z=0.

Successive generations of stars have slowly increased the metal fraction in the Universe. As we study distant stars we find they have different metal mass fractions (the mass fraction when they were formed).

Population III: stars with Z ~ 0.

Population II: stars with some metals, Z~0.0001 to 0.001

Population I: stars with metals like the Sun, Z~0.01 to 0.03

We can construct theoretical HR (color-magnitude) diagrams for stellar populations as a function of the cluster age. The model for a fixed time is an isochrone. This lets us determine the age of the star cluster and study stellar evolution (are our models correct ?!)

Kalirai et al. 2001, AJ, 122, 266

Color-Magnitude Diagram for NGC 6819

Models are for ages of t= 2, 2.5, 3.2, 4, 5 x 109 yrs.

Age of the Universe

Age of the Universe

Messier 3

Age of the Universe

Messier 3

Giants, Supergiants, & WDs

What happens when a star runs out of Hydrogen in its core?

Core contracts & gets hotter, which puffs up the envelope

Giants, Supergiants, & WDs

What happens when a star runs out of Hydrogen in its core?

Core contracts & gets hotter, which puffs up the envelope

Giants, Supergiants, & WDs

Slide

Evolution off the Main Sequence: Expansion into a Red Giant

Hydrogen in the core completely converted into He:

H burning continues in a shell around the core.

He Core + H-burning shell produce more energy than needed for pressure support

Expansion and cooling of the outer layers of the star → Red Giant

→“Hydrogen burning” (i.e. fusion of H into He) ceases in the core.

Slide

Red Giant Evolution

4 H → He

He

H-burning shell keeps dumping He onto the

core.

He-core gets denser and hotter until the next

stage of nuclear burning can begin in the

core:

He fusion through the

“Triple-Alpha Process”

4He + 4He → 8Be + γ8Be + 4He → 12C + γ

Slide

Outer layers expand due to radiation pressure from a hot core • Surface temperature drops by a factor of ~ 2• The radius increases by a factor of ~ 100• Luminosity increases ~ R2 T4 ~ 100-1000 times

Our Sun’s Evolution

Our Sun’s Evolution

Post Main Sequence: Solar Mass

Post Main Sequence: Solar Mass

How our Sun evolves off the Main Sequence to end up as a White Dwarf (degenerate Carbon-Oxygen core)

Post Main Sequence: Solar Mass

How our Sun evolves off the Main Sequence to end up as a White Dwarf (degenerate Carbon-Oxygen core)

Log10 T [K]

Log 1

0 (

L / L

⊙)

Evolutionary Stages:

#1-2. (Zero-Age) Main Sequence

#3-5. Core-contraction, H-shell burning, supergiant branch

#6-7. He core burning, Red-Giant Branch.

#8-10. Core-contraction, Carbon burning.

Log10 T [K]

Log 1

0 (

L / L

⊙)

Log10 T [K]

Log 1

0 (

L / L

⊙)

Post Main Sequence Tracks

Post Main Sequence Tracks

5 solar mass star

Post Main Sequence Tracks

5 solar mass star

Post Main Sequence Tracks

5 solar mass star

When He in the Core Runs Out

When He in the Core Runs OutShell-burning = Hydrogen and Helium burning shells(Carbon-Oxygen ash core)

When He in the Core Runs OutShell-burning = Hydrogen and Helium burning shells(Carbon-Oxygen ash core)

Pulsations due to double-shell burning eventually blow off the star’s outer layers to form a planetary nebula.

Slide

What is left??

A stellar remnant: white dwarf, composed mainly of carbon and oxygen

Slide

Detecting the presence of an unseen companion, Sirius B by its gravitational influence on the primary star, Sirius A.

Wobbling motion of Sirius A

1850: a strange star was discovered

Slide

Slide

White DwarfsDegenerate stellar remnant (C,O core)

Extremely dense:1 teaspoon of WD material: mass ≈ 16 tons!!!

White Dwarfs:

Mass ~ Msun

Temp. ~ 25,000 K

Luminosity ~ 0.01 Lsun

Chunk of WD material the size of a beach ball would outweigh an ocean liner!

Sun’s Evolution into White Dwarf

Sun’s Evolution into White Dwarf

Low-Mass Star Maintains Gravitational EquilibriumWhat balances gravity? Electron Degeneracy!

Sun’s Evolution into White Dwarf

Low-Mass Star Maintains Gravitational EquilibriumWhat balances gravity? Electron Degeneracy!

White Dwarf

White DwarfPauli Exclusion Principle:No two electrons can be at the same place at the same time with the same energy.At high density, all the low energy states are occupied, leaving only high energy (high pressure) states.

Results in Degenerate Electron Gas: Pressure is independent of temperature Compression does NOT lead to heating

Works for stellar cores up to 1.4 solar masses.

Chandrasekhar Limit

Chandrasekhar LimitMass-Radius relation for White Dwarfs:More mass means smaller radius!

Chandrasekhar LimitMass-Radius relation for White Dwarfs:More mass means smaller radius!

but only up to the Chandrasekhar mass... then neutron degeneracy takes over

Log10 T [K]

Log 1

0 (

L / L

⊙)

Post-main sequence of higher mass stars.

Nucleosynthesis

NucleosynthesisCreating heavy elements (stars of at least 8 Msun)

NucleosynthesisCreating heavy elements (stars of at least 8 Msun)

Hydrogen: 10 MyrHelium: 1 Myr

Carbon: 1000 yearsNeon: 10 yearsOxygen: 1 year

Silicon: 1 day

Pulsating Stars

In 1596, David Fabricius (1564-1617, Lutheran Pastor) observed the star o Ceti (constellation Cetus). Over several months the

brightness of this star faded until it was not visible. After several more months the star returned to full brightness, he named this

star “Mira” (for miraculous).

Mira has an 11-month cycle.

Mira is a long-period variable star.

Pulsating Stars

Time (Period is 332 days)

Mag

nitu

de v

arie

s fr

om 3

.5 t

o 9.

Pulsating StarsIn 1784 John Goodricke (1764-1786) found that δ Cephei varied

with a period of 5 days, 8 hours, and 48 minutes.

This is a short-period variable star. Stars like it are called “classical cepheids”

Stebbins 1908, ApJ, 27, 188

Slide

Delta Cephei

By 2005, there are 40,000 cataloging pulsating stars.

More than 5% were discovered by Henrietta Swan Leavitt (1868-1921), who studied photographs of stars on different nights.

She discovered 2400 classical Cepheid stars with periods of 1-50 days, most of them in the Small Magellanic Cloud (a small galaxy orbiting the Milky Way Galaxy).

Pulsating Stars

Henrietta Swan Leavitt (1868-1921)

Leavitt observed that brighter Cepheids have longer periods

Cepheid Period-magnitude Relation

Large (left) and Small (right) Magellanic Clouds

Slide

Period-luminosity relationSince all cepheids in SMC are at the same distance from us, the same relationship should be between their periods and absolute magnitudes!

Slide

Period-luminosity relation A

vera

ge m

agni

tude

M<V

>

Period (log P)0.4 2.01.40.80.6 1.0 1.2 1.6 1.8

-2-7

-5-6

-4-3

Since all cepheids in SMC are at the same distance from us, the same relationship should be between their periods and absolute magnitudes!

Nearest Cepheid is Polaris (The North Star). In 1913 Ejnar Hertzsprung measured a parallax distance to this star of 200 pc. That allowed astronomers to place the

Cepheids on a Luminosity-Period relation:

〈MV〉= -2.81 log10 Pd - 1.43 or log10(〈L〉/ L⊙) = 1.15 log10 Pd + 2.47

Pulsating Stars

Sandage & Tammann, 1968, ApJ, 151, 531

Cepheid Stars

Cepheid StarsPeriod-Luminosity Relation: M = a + b log P

Cepheid StarsPeriod-Luminosity Relation: M = a + b log P

Cepheid StarsPeriod-Luminosity Relation: M = a + b log P

Cepheid StarsPeriod-Luminosity Relation: M = a + b log P

Pulsating Stars

Cepheid Stars