Astronomy 294
Life in the Universe
Prof. Scott Gaudi

Lecture 1: Death on Earth and Death in the Universe

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.

The Universe is Expanding

The Universe is Accelerating

Milky Way will collide with Andromeda

No other galaxies will be visible

Five Ages of the Universe:

--Primordial Era - big bang -> 10⁶ years

--Stelliferous Era - 10⁶ years -> 10¹⁴ years

--Degenerate Era - 10¹⁴ years -> 10⁴⁵ years

--Black Hole Era - 10⁴⁵ years -> 10¹⁰⁰ years

--Dark Era - >10¹⁰⁰ years

After the dark era, life likely becomes 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.

Death on Earth

After ~12.4 billion years

Sun ends up as a White Dwarf with R~R_earth

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

Earth has lost its

Sun as white dwarf begins to slowly cool and fade away into a long night...

Cosmic Collisions

In a few billion years, the Milky Way will collide with Andromeda.

Will form one big Galaxy.

The Empty Empty Sky

As the Universe expands:

--Expansion continues forever at a faster rate

--Space between galaxy clusters widens

--Universe cools down at a faster rate

The "Cosmological Horizon"

--Distance beyond which we can't observe due to finite speed of light

--Nearest galaxies not bound to local group will disappear after ~10¹¹ years

--The sky will be devoid of Galaxies

A Dying Universe

Five Ages of the Universe

Primordial Era - big bang -> 10⁶ years

Stelliferous Era - 10⁶ years -> 10¹⁴ years

Degenerate Era - 10¹⁴ years -> 10⁴⁵ years

Black Hole Era - 10⁴⁵ years -> 10¹⁰⁰ years

Dark Era - >10¹⁰⁰ years

Stelliferous Era: 10⁶ years -> 10¹⁴ years

Encompasses the present day

--Star formation continues

--Only stars with M>0.8M? have had time to use up their hydrogen

Future evolution

--Star formation will eventually cease

--Low-mass stars will dominate

End of Star Formation

After t=10¹⁴ years:

Successively more matter is locked up in stellar remnants, depleting the free gas reserves.

Cycle of star birth & death is broken:

Nuclear fuel is exhausted

Red dwarfs burn out as low-mass white dwarfs

Remaining matter is locked up in black dwarfs, cold neutron stars, and black holes

The last stars fade into a long night...

The Littlest Stars

Smallest mass star capable of fusing hydrogen is ~0.08 Solar Masses

These stars have a lifetimes of 10¹³ years

End of Life?

Without nuclear energy generation in stars, sustaining life is more difficult

Still some hydrogen, so fusion still possible

Could 'harvest' brown dwarfs

Degenerate Era: 10¹⁴ years -> 10⁴⁵ years

Solar System "Evaporation"

After t=10¹⁵ years:

Gravitational encounters between stars are rare, but disrupt orbiting systems:

--Planetary systems get disrupted by stellar encounters and their planets scattered.

--Wide binary systems are broken apart.

--Close binary stars coalesce into single remnants.

Dissolution of Galaxies

After t=10¹⁹ years:

Stellar remnants within galaxies interact over many many orbits.

Some stars gain energy from the interaction and ~90% get ejected from the galaxy.

After t=10²⁴ years

Others lose energy and sink towards the center.

After t=10³⁰ years

The last 10% of remnants coalesce into Supermassive Black Holes.

Dissolution of Matter?

After t=10^32-45 years: Some particle models predict that protons are unstable. Protons decay into electrons, positrons, to neutrinos. All matter not in Black Holes comes apart.

End of Life?

Without nuclear energy generation, life as we know it becomes effectively impossible

Only sources of energy are extremely weak background radiation and gravity

Would have to learn how to harvest gravitational energy

Black Hole Era: 10⁴⁵ years -> 10¹⁰⁰ years

Only black holes left

Two kinds

--Supermassive black hole in the center of the Galaxy

--Stellar-mass black holes

Evaporation of Black Holes

After t=10⁶⁷ years:

--Remaining stellar-mass black holes evaporate by emitting particles via Hawking Radiation.

After t=10¹⁰⁰ years:

--Supermassive Black Holes evaporate one-by-one.

End of the epoch of organized matter

Dark Era: >10¹⁰⁰ years

After black holes have all evaporated:

--Universe continues to cool off towards a Radiation Temperature of absolute zero.

--Only matter is a thin, formless gas of electrons, positrons, neutrinos.

--Only radiation is a few increasingly redshifted photons.

The end is cold, dark, and disordered...

End of Life?

Everything is at the same temperature and same entropy

According to second law of thermodynamics, there are then no sources of energy

Life becomes effectively impossible.

The End

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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