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Astronomy 161
An Introduction to Solar System Astronomy
Prof. Scott Gaudi

Lecture 47: Death in the Universe


Key Ideas

The Sun is getting brighter
Without additional energy sources, life in the solar system will be difficult or impossible.

The Universe is Expanding and Accelerating

Five Ages of the Universe:
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


As it burns H to He in its core, it slowly gets slightly hotter & slightly brighter.

Venus on Earth

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?
--Insufficient 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

T=11 Gyr (6.5 Gyr from today):
Sun's core runs out of Hydrogen
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)

Over the next 1.4 Gyr, the Sun goes through several phases as it dies

Swells to enormous size
Losses mass
Starts to burn Helium, postponing its death
Runs out of Helium, swells again
Finally throws off its outer shell, leaving a hot core.
Core slowly cools over time to oblivion


The Final Fate of the Solar System

After ~12.4 billion years, 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 absolute zero.


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


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

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 toward the center.
After t=1030 years
The last 10% of remnants coalesce into Supermassive Black Holes.

Dissolution of Matter?

After t=1032-45 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 toward 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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