O
n human timescales, it’s easy to think of
the Solar System as stable and unchanging.
But over the next 6.5 billion years, as the
Sun swells into a red giant and goes on to become a
white dwarf, its family of planets will undergo radical
alterations. Mercury will evaporate into the Sun,
and Pluto will develop lakes on its surface. There’s
a chance life could arise in liquid seas on distant
moons, that two or more of the inner planets could
collide, or that some planets could be flung right out
of the Solar System. For Earth in particular, all hell
will break loose, with our beloved planet becoming
a cinder devoid of all life, and possibly destroyed
altogether.
Only time separates us from this destiny.
Fortunately, there’s lots of it left.Our star
The Solar System’s future depends in large measure
on what the Sun does as it ages. In its broadest
brushstrokes, the fate of our star is well understood.
That’s because astrophysicists have the luxury of
studying other Sun-size stars in various stages of
their lifetimes, from brand-new stars to red giants
and beyond. (A star’s mass prefigures its endgame,
whether that’s becoming a white dwarf as our Sun
will or going supernova as stars much larger than
the Sun do.) In a widely cited 1993 scientific paper,
Juliana Sackmann (Caltech) and colleagues estimate
our star’s lifespan — from the time it began fusing
hydrogen in its core about 4.5 billion years ago, to
when it will have exhausted its fuel — will be about
12.5 billion years. Thus, it’s currently roughly a third
of the way through its entire lifetime.
For the next 6.5 billion years, the Sun will
gradually brighten, eventually becoming twice as
luminous as it is today. But that will be just the start
of its transformation, and the most straightforward.
Once it leaves the so-called main sequence and enters
the red giant branch phase, it will begin a frightful
growth in both luminosity and size, becoming, over
the course of just 600 million years, 2,300 times as
bright as today and 170 times its current diameter.
It will get even more climactic. There’s a second red
giant phase, called the asymptotic giant branch (AGB),
and during that stretch of about 20 million years, the
Sun will attain its greatest luminosity (5,200 times
today’s) and its largest extent of more than 200 times
its present width.WA SEARING FUTURE Late in its life, the Sun will swell
to such a size that it might swallow the Earth. Long before
that, it will have incinerated the surface of our planet.http://www.skyandtelescope.com.au 25