STAR LIFE DEATH Life on the Main Sequence

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 Low-mass stars: luminosity increases with age zero-age main sequence 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 < 8 M )

‘Evolutionary tracks’ Surface cools, core contracts & heats, radius expands. 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. . . 12 C + 4 He 16 O + gamma ray

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

Helium-burning, Horizontal Branch star p. 279

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

* Supergiants lose mass: > Stellar winds > ‘Flashes’ in helium-burning shell Old stellar core Planetary Nebula Ejected stellar envelope Ring Nebula 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 > 8 M )

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. core re-ignition core exhaustion p. 283

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. . . p. 284

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.

Supernova 1987 A Before After

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

SN blast wave reaches inner ring

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

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

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

". . In the 1 st year of the period Chih-ho, the 5 th 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