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Astronomy 141
Life in the Universe
Prof. Scott Gaudi

Lecture 1: Death on Earth


Key Ideas

The Sun is getting brighter,
--Thus the Earth is getting warmer
In 1 Gyr, moist greenhouse may be triggered
In 4.5 Gyr, the runaway greenhouse will be triggered
In 6.5 Gyr, the Sun will run out of hydrogen
Thus begins the inexorable death of the Sun
--Red Giant
--Horizontal Branch
--Asymptotic Giant w/unstable puslations
--White Dwarf
Without additional energy sources, life in the solar system will be difficult or impossible.


Death in the Universe

What are the long-term prospects for life:

--on Earth?

--in the Solar System?
--in the Universe?


Death on Earth

Near-term: human foibles
Mid-term: Impact threats
Long-term: Our Sun

Impact Threats

Extinction-level collisions every ~100 million years.
--Likely enough time for technology to develop enough to allow a way to avoid catastrophe


The Sun Today

Age: 4.55 Billion Years
Mass: 1 Msun (1.99 ?1030 kg)
Radius: 1 Rsun (700,000 km)
Luminosity: 1 Lsun (3.83?1026 Watts)
Temperature: 5779 K
Fuel Supply: 50% of the core Hydrogen has been consumed.

"Quiet Adulthood"

The Sun took ~ 50 Myr to form.
Reached the Main Sequence ~4.50 Gyr ago
--Little Fainter: 0.70 Lsun
--Little Smaller: 0.897 Rsun
--Little Cooler: 5586 K
As it burns H to He in its core, it slowly gets slightly hotter & slightly brighter.

Current Habitable Zone

a=(0.84-1.7)AU -- "optimistic"
--Inner edge set by runaway greenhouse
--Outer edge set by the place where the greenhouse can no longer prevent liquid water from freezing

a=(0.95-1.4)AU -- "conservative"
--Inner edge set by "moist greenhouse"
--Outer edge set by "C02 freeze-out" in atmosphere


Mid-Life Crisis for the Earth (First Crisis)

T=5.5 Gyr (1 Gyr from today):
Sun ~10% brighter than today (~1.1 Lsun)
Extra solar energy triggers a Moist Greenhouse Effect
Atmosphere dries out as water vapor is lost to space.


Venus on Earth (Second Crisis)

T=9 Gyr (4.5 Gyr from today):
--Sun 40% brighter (~1.4 Lsun)
--Extra solar energy triggers a Runaway Greenhouse Effect
--The oceans will evaporate into space.
--CO2 locked in marine sediments is released into the atmosphere
Earth will be like Venus is today
--Hot, heavy, bone-dry CO2 atmosphere

Can we survive the first two crises?

Move to Mars?
--Insuffient atmosphere on Mars
Move the Earth?
--Exchange energy with giant planets using comets or asteroids!
--Move Earth further away from the Sun
--Better be careful you get it right!
Sunshields?
--block the rays of the Sun

However, whatever we do we are ultimately doomed, as the Sun will stop shining.


Hydrogen Core Exhaustion (Third and Final Crisis)

T=11 Gyr (6.5 Gyr from today):
Sun's core runs out of Hydrogen
--Inert He core starts to contract & heat up
--H burning moves out into a shell
--T = 5517 K
--R = 1.575 Rsun
--L = 2.21 Lsun
Sun leaves the Main-Sequence and becomes a Sub-Giant star.


The Death of the Sun (and so Life on Earth and Life in the Solar System)

"Lively Old Age"

Over the next 0.7 Gyr:
Sun expands at a near-constant L?2.2 Lsun
Sun swells from 1.58 Rsun to 2.3 Rsun
Surface cools from 5517 K to 4902 K
Start of slow mass loss in a stellar wind:

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 outwards:
--Venus ~1 AU, Earth ~1.4 AU
At the Tip of the Giant Branch:
--T = 3107 K (M0 Star)
--L = 2350 Lsun
--R = 166 Rsun (0.775 AU)
Sun's envelope swallows Mercury!

The Helium Flash

When the Sun reaches the tip of Red Giant Branch
--Core Helium Burning to C & O ignites rapidly in a Helium Flash
--Extra energy stabilizes the core against collapse
--Hydrogen Shell Burning layer forms outside the He burning core.
Descends quickly onto the Horizontal Branch in ~1Myr

The Horizontal Branch

With He fusion in the core, the Sun briefly regains Hydrostatic & Thermal Equilibrium:
--Radius: R=9.5 Rsun
--Temperature: T = 4724 K
--Luminosity: L = 41 Lsun
But, He fusion is ~100? less efficient, so this phase will be very short-lived: ~100Myr

An All-Too-Brief Retirement

After ~100Myr, Sun runs out of core Helium:
--C-O "ash" Core contracts & heats up
--Remaining He displaced into an He-burning shell, surrounded by a thin H-burning shell
Sun starts to swell up & gets brighter:
--R = 18 Rsun
--T = 4450 K
--L = 110 Lsun

Helium Core Exhaustion

Sun now climbs the Asymptotic Giant Branch in ~20 Myr:
--C-O core steadily contracts & heats up
--He-burning shell surrounded by a thin H-burning shell
Sun swells rapidly, getting cooler & brighter:
--R = 180 Rsun (0.84 AU)
--T = 3160 K
--L = 3000 Lsun

AGB Phase Mass Loss

AGB ascent is accompanied by mass loss in the form of a stellar wind:
--The Sun's mass is reduced to ~0.6 Msun
Surviving planets move further outward:
--Venus at 1.22 AU
--Earth at 1.69 AU
Near the tip of the AGB, thermal pulsations in the envelope begin?

The Tremors of Old Age

At tip of the AGB, unstable thermal pulsations start in the He-burning shell:
--Models predict 4 pulses spaced by 100,000yrs
--Puffs Sun up to 213 Rsun (~1 AU)
--Largest pulse is #4, with L ~ 5200 Lsun!
Pulses progressively eject most of the remnants of the envelope.
Each pulse of mass loss moves the planets further outwards.

A Final Flowering

Last thermal pulse blows off the rest of the envelope in a few thousand years.
Hot C-O Core is unveiled:
--T goes from 4000K to 120,000K
--L stays constant at ~3500 Lsun
--Core traverses the H-R diagram
UV photons from the core ionize the ejected envelope gas, forming a Planetary Nebula


The Final Fate of the Solar System

All that remains of the Sun is a Carbon-Oxygen Core, now ~0.54 Msun, which evolves into a cooling White Dwarf with R~Rearth
Mass loss over, the remaining planets settle into their final orbits:
--Venus at 1.34 AU
--Earth at 1.85 AU
--Mars at 2.8 AU
Sun as white dwarf begins to slowly cool and fade away into a long night...

With no additional energy source, the Sun gradually fades to oblivion.

Ultimately, the Earth cools to a few degrees above absoluate zero.


All stars will eventually share a fate similar to that of the Sun.


See A Note about Graphics to learn why the graphics shown in the lectures are generally not reproduced with these notes.

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