Star Points for September, 2007; by Curtis Roelle
When the Sun Dies So Will the Earth

This month we will look forward far into the future at events 
that are predicted to affect the sun and, in turn, the earth. Our 
discussion is based on an article from the July issue of Sky & 
Telescope magazine by Gregory P. Laughlin entitled "From Here to 
Eternity: The fate of the Sun and the Earth."  To make it 
interesting we'll construct a scaled timeline based on a similar 
timeline we discussed in July in this column.

That timeline represented the history of the universe based on 
currently prevailing astronomical theories in which all events 
from the so-called "big bang" to the current time was compressed 
into a 24 hour day.  In that model you might recall jellyfish 
appearing around 11:00 p.m., 650 million years ago, and man 
taking his first steps on the moon at 11:59:59.9999 p.m. 

As in July we will use this scale:  One hour on the scale clock 
represents 625 million years.  Time begins at the present time 
and the scale clock starts at midnight.

Our sun, believed to be 4.5 billion years old, is slowly becoming 
hotter. As hydrogen fuses helium is created.  The helium sinks, 
increasing the mass of the core.  The increased core mass causes 
its temperature to increase. In the end it is the sun, the 
sustainer of life and energy on our planet, which will cause the 
earth's ultimate demise.  

At about 2:00 a.m. on our scale clock, or 1.2 billion years from 
now the sun's luminosity will have increased by 10% as it swells 
slightly due to the increased temperature at the surface. Around 
this time earth may experience a runaway greenhouse effect in 
which its temperature rapidly climbs.

On the earth the oceans have boiled dry.

The idea of a "runaway greenhouse" isn't new to our planet.  
Temperatures on earth have swung from one extreme to the other in 
the past, including two ancient so-called "snowball earths" in 
which the earth froze from pole to pole before life had emerged on 
the land.  The rate of change has always been slow and life has so 
far been able to adapt to variations in the environment.

One way to postpone the greenhouse effect is to move the earth 
farther away from the sun.  This could be achieved by transferring 
orbital energy from asteroids deflected toward earth where they 
make close fly bys.  Over time the excess orbital energy would 
cause the earth to maintain a constant temperature by slowly 
moving farther away from the sun.

But eventually the sun will grow even hotter. Just past 10 a.m., 
or 6.4 billion years from now, the sun will shine 2.2 times as 
bright as it is today.  Mars will then receive as much energy from 
the sun as the earth does today.  But its gravity is too weak to 
maintain an atmosphere.

Eventually the sun will enter its red giant stage.  At that time 
Jupiter's ice moons will develop atmospheres and start going 
through their own runaway greenhouse periods. That should occur 
7.7 billion years from now, or nearly half past Noon on our scale 
model clock.  On this scale clock it's been 1½ days since the 
theoretical Big Bang, 22.7 billion years earlier. 

On earth the atmosphere is gone and the crust has melted.

During its red giant stage up to 25% of its mass will be carried 
away by the solar wind.  It swells up 200 times its current size, 
almost reaching the earth.  

However, as it loses mass its gravitational pull will weaken.  
This will cause the planets to recede to orbits farther out and 
may postpone their ultimate demise.

The red giant phase ends when the core temperature gets hot enough 
to ignite the helium that has been accumulated.  The sun will then 
shrink again.

When the helium is spent a carbon core -- or "white dwarf" – will 
remain.  The outer layers of the sun will expand again to begin a 
second red giant phase.  Eventually the outer layers will be cast 
off to form a huge "planetary nebula" around the sun. 

The white dwarf will continue to cool, eventually becoming a 
cinder or "black dwarf" (not to be confused with a "black hole"). 
This cooling is estimated to take much longer than the current age 
of the universe. 

By this time the earth will be a cold lump.  An alternate theory 
predicts that tidal forces from the red giant sun will cause the 
planets to lose orbital energy and plunge into the sun.