Sky and Telescope - October 2017 - 24

Fate of the Solar System

Life Cycle of the Sun

Birth

1

Now

2

3

4

Gradual warming

5

6

7

10,000
1,000

Sun's Luminosity
Helium
shell
flashes

100
Contraction
Leaves main
sequence

10
1

First
red-giant
peak

Now
0

5

10 12.1 12.2 12.3

Second
red-giant
peak

12.3650

cold to emit any optical radiation at all. The light will have
gone out in our solar system.
Uncertainties about the Sun's future remain. One concerns mass loss. It's hard for astrophysicists to know just
how much of its bulk the Sun will throw off after it exits the
main sequence. To improve their understanding, they need to
collect loads more data on other solar-size stars undergoing
mass loss. Rates of such dissipation depend on a star's mass,
age, metallicity (the fraction of its mass that's not hydrogen
or helium), and perhaps even angular momentum. "That's 3
or 4 variables, and to survey some function that has 3 or 4
variables is an enormous amount of data," says Fred Adams
(University of Michigan). "So it's going to take awhile."
Another ambiguity is convection, or precisely how heat circulates in the outer layers of the Sun. "It's sort of the elephant
in the room," says Robert Smith (University of Sussex, UK).
Diameter (times present value)

Luminosity (times present value)

Even as it bloats during these red giant phases, the Sun
paradoxically will lose much of its mass in a ballooning solar
wind. By the end of the AGB phase, it will have sloughed
off so much of its erstwhile self that it might well form an
expanding planetary nebula. Eventually it will end up as a
white dwarf, a stellar remnant that has used up all its possible
sources of fuel for thermonuclear fusion. At this stage, 12.5
billion years after it ﬁrst ignited, our once-massive star will
have a mass about half of today's and a size not much larger
than Earth's. The Sun will remain bright - about 35 times
as luminous as today - and its surface temperature will be a
ferociously hot 120,000°C. It will also be extremely compact,
and the leftover heat from fusion will take billions of years to
leak to the surface.
Eventually, perhaps 100 billion years from now, it will
have cooled to the point where it becomes a black dwarf, too

Orbit of:
Mars
Earth
Venus
Mercury

100

Sun's Diameter
10

Contraction
Leaves main
sequence

1

Sun's age (billions of years)

First
red-giant
peak

Now
0

12.3655

Helium
shell
flashes

5

10 12.1 12.2 12.3

White
dwarf
12.3650

12.3655

Sun's age (billions of years)

p FUTURE SHOCK The Sun is already growing in both brightness and size - slowly for the next 7 billion years, then enormously as it enters
its red giant phase. Note the three different time scales; they're expanded greatly near the end of the Sun's life to show comparatively rapid
changes. The orbits of the planets enlarge when the Sun loses mass. In this particular model, by I.-Juliana Sackmann, Arnold Boothroyd, and
Kathleen Kraemer, the Sun swells large enough to engulf Mercury but not quite Venus or Earth.

Today

Sun

Mercury
0.38 a.u.

Venus
0.72 a.u.

Earth
1 a.u.

Mars
1.52 a.u.

According to solar-evolution models by K.-P. Schröder (U. of Guanajuato, Mexico) and Robert C. Smith (U. of Sussex, U.K.), our Sun, as it expands . . .

24

O C T O B E R 2 0 17 * S K Y & T E L E S C O P E

TIMELINE: WIKIMEDIA COMMONS / OLIVERBE ATSON; CHARTS: C. REED / S&T, SOURCE: SACKM A NN E T A L / ASTR OPHYSICAL JOUR NAL 19 93; E X PA NDING SUN: G. DINDER M A N / S&T, SOURCE:
K .-P. SCHRÖDER (UNIV. OF GUA N A JUATO) & R.C. SMITH (UNIV. OF SUSSE X ) / MNR AS, 20 0 8

Billions of years (approx.)
Sky and Telescope - October 2017

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