STAR LIFE & DEATH

Life on the Main Sequence

Life on the Main Sequence Stable fusion: hydrogen  helium Accumulation of helium in core  Steady increase in luminosity 90% of star’s life spent on main sequence More mass  shorter MS lifetime

Main Sequence structure depends on mass . . . pgs. 276

luminosity increases with age Low-mass stars: luminosity increases with age zero-age main sequence Luminosity Sun Temperature

Change in composition of 1 solar mass star. Fusion ceases when core converted to helium – star now leaves main sequence.

Sun: ~ 10 billion years

Star Death I: Low Mass Stars (M < 8M)

Surface cools, core contracts & heats, radius expands. ‘Evolutionary tracks’ Red Giant p. 277

Sun as a red giant Vigorous H  He fusion in shell drives envelope outward. Inert helium core (shrinking) p. 277

Red Giant: Aldebaran T = 3500 K L = 370 L R = 50 R M  3 M 

* Core temp  100 million K: Helium fusion begins Another Helium Beryllium Gamma Ray Helium Carbon Gamma Ray

In addition . . . 12C + 4He  16O + gamma ray

On the HR diagram . . . Supergiant Core He exhaustion He ignition Horizontal branch

Helium-burning, Horizontal Branch star p. 279

Supergiant Star Helium-fusing shell Contracting carbon-helium core Hydrogen-burning shell

* Supergiants lose mass: > Stellar winds > ‘Flashes’ in helium-burning shell Ring Nebula Old stellar core Planetary Nebula Ejected stellar envelope p. 281

p. 281

Hourglass Nebula Old stellar core shrinking to White Dwarf state.

The whole story . . . p. 280

Star Death II: High Mass Stars (M > 8M)

High temp., rapid fusion on CNO Cycle Again . . . hydrogen fusion ceases when core converted to helium – star now leaves main sequence.

Multiple core fusion stages are possible. p. 283 core re-ignition core exhaustion p. 283

Core Fusion Core Temp Duration For a 25 M star: Core Fusion Core Temp Duration H fusion 40 million K 7 million yr He fusion 200 million K 500,000 yr Carbon fusion 600 million K 600 yr Neon fusion 1.2 billion K 1 yr Oxygen fusion 1.5 billion K 6 mos Silicon fusion 2.7 billion K 1 day results in Iron

As fusion ceases . . . ‘Onion Skin’ p. 283

Fusion ceases when iron is produced . . .

SUPERNOVA! Iron core contracts, heats Nuclei disintegrate Protons absorb electrons: proton + electron  neutron + neutrino Core stiffens, bounces back slightly Core bounce + neutrino flow ejects envelope: SUPERNOVA!

Elements heavier than iron created in blast.

Before Supernova 1987A After

SN 1987A in 1999 SN ejecta Stuff ejected before SN.

SN blast wave reaches inner ring

proton + electron  neutron + neutrino SN 1987A (deep underground) Neutrino arrival

SN probably occur ~ once per 100 yrs in our galaxy.

Crab Nebula Supernova Remnant 600 mi/s (Exploded 1054 AD) Pulsar (rotating neutron star) Visible in broad daylight for 23 days in July, 1054!

".. In the 1st year of the period Chih-ho, the 5th moon, the day chi-ch'ou, a guest star appeared south-east of Tien-Kuan [Zeta Tauri]. After more than a year, it gradually became invisible .."

Supernova recorded at Chaco Canyon, NM?

Cygnus Loop ~13,000 BC

Vela Supernova Remnant (~10,000 BC) Interstellar medium ‘seeded’ with heavy elements.

Cassiopeia A Supernova Remnant Neutron star? Black hole? X-ray

Iron Cassiopeia A Supernova Remnant Silicon