Main Sequence Membership

Astronomy 101

Main Sequence Membership For a star to be located on the Main Sequence in the H R diagram: must fuse Hydrogen into Helium in its core. must be in a state of Hydrosta)c Equilibrium. Relax either of these and the star can no longer remain on the Main Sequence.

The Main Sequence is a Mass Sequence. The locacon of a star along the M S is determined by its Mass. Low Mass Stars: Cooler & Fainter High Mass Stars: Ho9er & Brighter Follows from the Mass Luminosity RelaCon: Luminosity ~ Mass 3.5

10 6 High Mass Main Sequence Luminosity (L sun ) 10 4 10 2 1 10 2 10 4 Low Mass 40,000 20,000 10,000 5,000 2,500 Temperature (K)

Internal Structure Nuclear reaccon rates are very sensicve to core temperature: P P Chain: fusion rate ~ T 4 CNO Cycle: fusion rate ~ T 18! Leads to: Differences in internal structure. Division into Upper & Lower M S by mass.

Upper Main Sequence Upper Main Sequence stars: M > 1.2 M sun T Core > 18 Million K Generate Energy by the CNO Cycle Structure: ConvecCve Cores RadiaCve Envelopes

Upper Main Sequence Star Radiative Envelope Convective Core

Lower Main Sequence Lower Main Sequence stars: M < 1.2 M sun T Core < 18 Million K Generate Energy by the Proton Proton Chain Structure: RadiaCve Cores ConvecCve Envelopes

Lower Main Sequence Star Convective Envelope Radiative Core

The Lowest Mass Stars For 0.25 < M * < 0.08 M sun : Generate energy by the P P Chain Fully ConvecCve Interiors: ConvecCve Core and ConvecCve Envelope Reddest Main Sequence Stars

Red Main Sequence Star Convective Envelope Convective Core

Structure along the Main Sequence

Main Sequence LifeCme How long a star can burn H to He depends on: Amount of H available = MASS How Fast it burns H to He = LUMINOSITY Recall: LifeCme = Mass Luminosity Mass Luminosity RelaConship: Luminosity ~ Mass 3.5

Main Sequence LifeCme Therefore: LifeCme ~ 1 / M 2.5 The higher the mass, the shorter its life. Examples: Sun: ~ 10 Billion Years 30 M sun O star: ~ 2 Million years 0.1 M sun M star: ~ 3 Trillion years

Consequences: If you see an O or B dwarf star, it must be young as they only live for a few Million years. You can t tell how old an M dwarf is because their lives can be so long. The Sun is ~ 5 Billion years old, so it will last only for ~ 5 Billion years longer.

Structure & Mixing Upper & Lower M S Stars: Core & Envelope are separate. No mixing of nuclear fusion products between the deep core and the envelope. Surface composicon is constant over lifecme. Red Main Sequence Stars: Fully mixed: core & envelope are conveccve. Enhances surface helium composicon?

Structure & Mixing Upper & Lower M S Stars: Core & Envelope are separate. No mixing of nuclear fusion products between the deep core and the envelope. Surface composicon is constant over lifecme. Red Main Sequence Stars: Fully mixed: core & envelope are conveccve. Enhances surface helium composicon?

Main Sequence Phase Energy Source: H fusion in the core What happens to the He created by H fusion? Too cool to ignite He fusion Slowly build up an inert He core LifeCme: ~10 Gyr for a 1 M sun star (e.g., Sun) ~10 Tyr for a 0.1 M sun star (red dwarf)

Hydrogen ExhausCon Inside: He core collapses & starts to heat up. H burning zone shoved into a shell. Collapsing core heats the H shell above it, driving the fusion faster. More fusion, more heacng, so Pressure > Gravity Outside: Envelope expands and cools Star gets brighter and redder. Becomes a Red Giant Star

Red Giant Star Inert He Core H Burning Shell Cool, Extended Envelope

Climbing the Red Giant Branch 10 6 Luminosity (L sun ) 10 4 10 2 1 10-2 H-core exhaustion Red Giant Branch 10-4 40,000 20,000 10,000 5,000 2,500 Temperature (K)

Climbing the Red Giant Branch Takes ~1 Gyr to climb the Red Giant Branch He core contraccng & heacng, but no fusion H burning to He in a shell around the core Huge, puffy envelope ~ size of orbit of Venus Top of the Red Giant Branch: T core reaches 100 Million K Ignite He burning in the core in a flash.

Helium Flash Triple α Process: Fusion of 3 4 He nuclei into 1 12 C (Carbon): Secondary reaction with 12 C makes 16 O (Oxygen):

Leaving the Giant Branch Inside: Primary energy from He burning core. AddiConal energy from an H burning shell. Outside: Gets ho9er and bluer. Star shrinks in radius, gelng fainter. Moves onto the Horizontal Branch

Horizontal Branch Star He Burning Core H Burning Shell Envelope

Horizontal Branch 10 6 Helium Flash Luminosity (L sun ) 10 4 10 2 1 10-2 Horizontal Branch H-core exhaustion Red Giant Branch 10-4 40,000 20,000 10,000 5,000 2,500 Temperature (K)

Horizontal Branch Phase Structure: He burning core H burning shell Triple α Process is inefficient, can only last for ~100 Myr. Build up a C O core, but too cool to ignite Carbon fusion

AsymptoCc Giant Branch Aner 100 Myr, core runs out of He C O core collapses and heats up He burning shell H burning shell Star swells and cools Climbs the Giant Branch again, but at higher T Asympto)c Giant Branch Star

AsymptoCc Giant Branch Star H Burning Shell Inert C-O Core He Burning Shell Cool, Extended Envelope

The AsymptoCc Giant Branch 10 6 Asymptotic Giant Branch Luminosity (L sun ) 10 4 10 2 1 10-2 Horizontal Branch H-core exhaustion Red Giant Branch 10-4 40,000 20,000 10,000 5,000 2,500 Temperature (K)

The InstabiliCes of Old Age He burning is very temperature sensicve: Consequences: Triple α fusion rate ~ T 40! Small changes in T lead to Large changes in fusion energy output Star experiences huge Thermal Pulses that destabilize the outer envelope.

Core Envelope SeparaCon Rapid Process: takes ~10 5 years Outer envelope gets slowly ejected (fast wind) C O core concnues to contract: with weight of envelope taken off, heats up less never reaches Carbon ignicon temperature of 600 Million K Core and Envelope go their separate ways.

Planetary Nebula Phase Expanding envelope forms a ring nebula around the contraccng C O core. Ionized and heated by the hot central core. Expands away to nothing in ~10 4 years. Planetary Nebula Hot C O core is exposed, moves to the len on the H R Diagram

Planetary Nebula Phase C-O Core Envelope Ejection 10 6 Luminosity (L sun ) 10 4 10 2 1 10-2 10-4 White Dwarf 40,000 20,000 10,000 5,000 2,500 Temperature (K)

Core Collapse to White Dwarf ContracCng C O core becomes so dense that a new gas law takes over. Degenerate Electron Gas: Pressure becomes independent of Temperature P grows rapidly & soon counteracts Gravity Collapse halts when R ~ 0.01 R sun (~ R earth ) White Dwarf Star

Summary: Main Sequence stars burn H into He in their cores. The Main Sequence is a Mass Sequence. Lower M S: p p chain, radiacve cores & conveccve envelopes Upper M S: CNO cycle, conveccve cores & radiacve envelopes Larger Mass = Shorter LifeCme

Summary: Stage: Main Sequence Red Giant Horizontal Branch AsymptoCc Giant White Dwarf Energy Source: H Burning Core H Burning Shell He Core + H Shell He Shell + H Shell None!

QuesCons What happens when main sequence stars run out of fuel? The Sun will end its main sequence lifecme in ~5 billion years; then what? What are red giants? What are white dwarfs?