Caption: The plot
shows the evolution of the Sun's
luminosity,
radius, and effective temperature
with time.
Thus, the illustrates key aspects of the
Sun's life phases.
Features:
- The quantities are normalized to
present-day
values for the Sun.
- The time-zero
is the zero-age main sequence (ZAMS) phase
and past ∼ 10 Gyr
the Sun is evolving to
its red giant phase.
- The Sun's red giant phase
is estimated to begin in about 5.4 Gyr
(see Wikipedia: Sun:
After core hydrogen exhaustion).
The plot is NOT
quite consistent with the estimate, but exact consistency between different astro
sources is hard to find.
Calculations of the model quantities vary somewhat between sources.
This is a consequence of uncertainties in the modeling.
- As you can see in the plot,
soon after formation when
the Sun just arrived on the
main sequence,
it was probably ∼ 30 % less bright than now.
About 3.5 Gyr from now, the Sun will probably be
∼ 30 % brighter than now.
- The
secular increase
(i.e., long-term increase)
in the Sun's
luminosity
(i.e., the solar brightening)
is a purely theoretical prediction since it is unobservably slow over
human history.
But it is as certain as pure
theory can be since
we understand main-sequence stars
very well in their main behaviors.
The solar brightening
is a pretty modest change for the Sun for
next 5 Gyr or so.
- But alas, the solar brightening
spells the doom of
complex life
on Earth and then later
all life
on Earth
(see IAL 11: The Earth;
Wikipedia: Future of Earth: Climate Impact;
Rampino, M. R. & Caldiera, K. 1994,
ARAA, 32, 83-114, The Goldilocks Problem: Climatic Evolution and Long-Term Habitability of Terrestrial Planets):
- First, within
a gigayear or so
carbon dioxide (CO_2) will probably
be removed from the Earth's atmosphere
by weathering which locks it up in
calcium carbonate (CaCO_3).
The volcanic outgassing
of CO_2 probably can't keep up with
the weathering which speeds up as the
Earth gets hotter with the
solar brightening.
With no CO_2, most
plant life must cease
(because NO CO_2,
NO photosynthesis) and
necessarily most animal life
pari passu.
- Second, the extra heating of the Earth will
cause the troposphere to extend much
higher than it now does.
Then convection will
carry water vapor much higher
where increased photodissociation
by stronger ultraviolet (UV) light
(see
Wikipedia:
Electromagnetic absorption by water: Electronic spectrum)
will break up water molecules (H2O)
and the freed hydrogen will
undergo atmospheric escape.
On the time scale of 2 Gyr, the
Earth's
liquid water will be gone.
This will turn off weathering
to calcium carbonate (CaCO_3).
This will then lead
volcanic outgassing
to replenish the atmosphere CO_2
and then give the Earth
a thick CO_2 dominated
planetary atmosphere like
the Venusian atmosphere
which lead to a runaway greenhouse effect
which will make Earth hellish like
Venus.
We don't have to worry about this melancholy story since it is on a time scale far longer
than human history.
We only contemplate it philosophically.
Credit/Permission: ©
User:RJHall,
2011 /
Creative Commons
CC BY-SA 3.0.
Image link: Wikipedia:
File:Solar evolution (English).svg.
Local file: local link: sun_evolution.html.
File: Sun file:
sun_evolution.html.