Evolution of Stars Population III: Population II: Population I:

Evolution of Stars 1. 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

Color-Magnitude Diagram for NGC 6819 Models are for ages of t= 2, 2.5, 3.2, 4, 5 x 10 9 yrs. 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

Age of the Universe

Messier 3 Age of the Universe

Giants, Supergiants, & WDs

Giants, Supergiants, & WDs What happens when a star runs out of Hydrogen in its core? Core contracts & gets hotter, which puffs up the envelope

Evolution off the Main Sequence: Expansion into a Red Giant Hydrogen in the core completely converted into He: Hydrogen burning (i.e. fusion of H into He) ceases in the core. H burning continues in a shell around the core. He Core + H-burning shell produce more energy than needed for pressure support Slide Expansion and cooling of the outer layers of the star Red Giant

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 4 He + 4 He 8 Be + γ 8 Be + 4 He 12 C + 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 ~ R 2 T 4 ~ 100-1000 times Slide

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)

Log10 ( 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]

Log10 ( L / L ) Log10 T [K]

Post Main Sequence Tracks

Post Main Sequence Tracks 5 solar mass star

When He in the Core Runs Out

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

When He in the Core Runs Out Shell-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.

What is left?? A stellar remnant: white dwarf, composed mainly of carbon and oxygen Slide

1850: a strange star was discovered Detecting the presence of an unseen companion, Sirius B by its gravitational influence on the primary star, Sirius A. Wobbling motion of Sirius A Slide

Slide

White Dwarfs Degenerate stellar remnant (C,O core) Extremely dense: 1 teaspoon of WD material: mass 16 tons!!! Chunk of WD material the size of a beach ball would outweigh an ocean liner! White Dwarfs: Mass ~ M sun Temp. ~ 25,000 K Luminosity ~ 0.01 L sun Slide

Sun s Evolution into White Dwarf

Sun s Evolution into White Dwarf Low-Mass Star Maintains Gravitational Equilibrium What balances gravity? Electron Degeneracy!

White Dwarf

White Dwarf Pauli 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 Limit Mass-Radius relation for White Dwarfs: More mass means smaller radius!

Chandrasekhar Limit Mass-Radius relation for White Dwarfs: More mass means smaller radius! but only up to the Chandrasekhar mass... then neutron degeneracy takes over

Log10 ( L / L ) Post-main sequence of higher mass stars. Log10 T [K]

Nucleosynthesis

Nucleosynthesis Creating heavy elements (stars of at least 8 M sun )

Nucleosynthesis Creating heavy elements (stars of at least 8 M sun ) Hydrogen: 10 Myr Helium: 1 Myr Carbon: 1000 years Neon: 10 years Oxygen: 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 Magnitude varies from 3.5 to 9. Time (Period is 332 days)

Pulsating Stars In 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

Pulsating Stars 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). Cepheid Periodmagnitude Relation Henrietta Swan Leavitt (1868-1921) Leavitt observed that brighter Cepheids have longer periods

Large (left) and Small (right) Magellanic Clouds

Period-luminosity relation Since 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 Since all cepheids in SMC are at the same distance from us, the same relationship should be between their periods and absolute magnitudes! Average magnitude M <V> -2-3 -4-5 -6-7 Slide 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Period (log P)

Pulsating Stars Sandage & Tammann, 1968, ApJ, 151, 531 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

Cepheid Stars

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

Pulsating Stars

Cepheid Stars