Lecture 22

The Once and Future Sun

The Fate of the Sun

Question: How will the Sun evolve?

Sackmann, Boothroyd, & Kraemer (1993), paper in the Astrophysical Journal (1993, Vol. 418, 457):

• State-of-the-Art Stellar Evolution program
• Detailed Solar Structure Model
• Inclusion of realistic Mass-loss

Self-consistent model tracing the post-MS evolution of the Sun.

The Present Day Sun

Properties:

• Age = 4.55 Gyr
• Mass = 1 M_sun = 1.99x10³³ g
• Radius = 1 R_sun = 700,000 km
• Luminosity = 1 L_sun = 3.83x10²⁶ Watts
• Temperature = 5779 K
• ~50% of the core H has been burned

Note: This and subsequent pictures show the inner solar system with the Sun drawn to scale with respect to the orbits of the planets. The scale is also the same between each solar system graph, so you can see how much the planets move outward as mass loss proceeds.

"Quiet Adulthood"

The Sun took ~ 50 Myr to form.

Reached the Main Sequence ~4.50 Gyr ago

• Little Fainter: 0.70 L_sun
• Little Smaller Radius: 0.897 R_sun
• Little Cooler: 5586 K

Continues to burn H to He in its core, slowly getting slightly hotter & brighter.

Mid-Life Crisis for the Earth

5.6 Gyr (1.1 Gyr from today):

• Sun 10% brighter: ~1.1 L_sun
• Cause a moist greenhouse effect on Earth, driving away most of the water in the air.

9 Gyr (3.5 Gyr from today):

• Sun 40% brighter: ~1.4 L_sun
• Oceans totally evaporate in a runaway greenhouse effect: Earth like Venus today.

End of the Main-Sequence Life

11 Gyr: H exhausted in the core

• Inert He core starts to collapse
• H burning moves into a shell
• T = 5517 K
• R = 1.575 R_sun
• L = 2.21 L_sun

Sun leaves the Main-Sequence.

"Lively Old Age"

Next 0.7 Gyr:

• Sun expands an near-constant L ~2.2 L_sun towards the base of the Red Giant Branch.
• Sun swells in size from 1.58 R_sun to 2.3 R_sun
• Cools from 5517 K to 4902 K

Start of slow mass loss in a stellar wind:

• Wind picks up as the Sun approaches the base of the Red Giant Branch

Climbing the Red Giant Branch

Takes ~0.6 Gyr to make the climb.

Loses 28% of its mass in a strong wind

• Causes the planets to move out some:
• Venus ~1 AU, Earth ~1.4 AU

At the Top, just before the Helium Flash:

• T = 3107 K (M0 III Star)
• L = 2350 L_sun
• R = 166 R_sun (0.775 AU) engulfing Mercury.

Helium Flash

Helium Flash occurs at Top of the Red Giant Branch:

• Core Helium Burning to C & O (Triple - alpha)
• Shell Hydrogen Burning

Descends to the Horizontal Branch in 1 Myr:

• Rapid envelope contraction to R=9.5 R_sun
• Gets a little hotter: T = 4724 K
• Gets a lot fainter: L = 41 L_sun

Horizontal Branch

Burns He to C&O in core for ~0.1 Gyr:

• He core starts to run out
• C-O Core contracts & heats up

Star swells up a little & gets brighter:

• R = 18 R_sun
• T = 4450 K
• L = 110 L_sun

Ascent of the Asymptotic Giant Branch

Climbs the Asymptotic GB in ~20 Myr:

• Collapsing C-O core.
• He burning shell + H burning shell

Star swells, gets cooler & brighter:

• R = 180 R_sun (0.84 AU)
• T = 3160 K
• L = 3000 L_sun

AGB Phase Mass Loss

AGB ascent is accompanied by mass loss in the form of a stellar wind:

• The Sun's mass gets reduced to ~0.6 M_sun

Remaining planets move further outward:

• Venus at 1.22 AU
• Earth at 1.69 AU

Near the top: thermal pulsations start.

Envelope Ejection

At top of the AGB, unstable thermal pulses start in the He-burning shell:

• Models predict 4 pulses, every 100,000 years
• Puffs star up to 213 R_sun (~1 AU)
• Peak pulse is #4, with L ~ 5200 L_sun

Pulses eject most of the envelope.

Mass loss in pulses moves planets out more.

Planetary Nebula Phase

Last pulse blows off what is left of the envelope in ~100,000 years:

• T goes from 4000K to 120,000K
• L stays constant at ~3500 L_sun
• Core traverses the H-R diagram

UV photons from the core ionize the ejected envelope gas, forming a Planetary Nebula

The Final Configuration

Core, now ~0.54 M_sun, evolves into a slowly cooling White Dwarf with R~R_earth.

Mass loss is now ended, and the remaining planets settle into their orbits:

• Venus at 1.34 AU
• Earth at 1.85 AU
• Mars at 2.8 AU

The Seven Ages of the Sun

1. 1 M_sun Main-Sequence Star: 11 Gyr
2. Red Giant Branch Ascent: 1.3 Gyr
3. Horizontal Branch: 100 Myr
4. Asymptotic Giant Branch Ascent: 20 Myr
5. Thermal Pulse Phase: 400,000 yr
6. Envelope Ejection: < 100,000 yr
7. 0.54 M_sun White Dwarf: …

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