Australian Sky & Telescope — November-December 2017

26 AUSTRALIAN SKY & TELESCOPENovember| December 2017

Sun Mercury

0.38 a.u.

Venus

0.72 a.u.

Earth

1 a.u. 1.52 a.u.

Now Gradual warming

Birth 1 2 3 4

Billions of years (approx.)

567

TIMELINE: WIKIMEDIA COMMONS / OLIVERBEATSON; CHARTS: C. REED /

S&T

,SOURCE:SACK-

MANN ET AL /

ASTROPHYSICAL JOURNAL

1993; EXPANDING SUN: G. DINDERMAN /

S&T

, SOURCE:

K.-P. SCHRÖDER (UNIV. OF GUANAJUATO) & R.C. SMITH (UNIV. OF SUSSEX) /

MNRAS

, 2008

Even as it bloats during these red giant phases, the Sun

paradoxically will lose much of its mass in a outflowing solar

wind. By the end of the AGB phase, it will have sloughed

offsomuchofitserstwhileselfthatitmightwellforman

expandingplanetarynebula.Eventuallyitwillendupasa

white dwarf, a stellar remnant that has used up all its possible

sourcesoffuelforthermonuclearfusion.Atthisstage,12.5

billion years after it first ignited, our once-massive star will

haveamassabouthalfoftoday’sandasizenotmuchlarger

than Earth’s. The Sun will remain bright — about 35 times

asluminousastoday—anditssurfacetemperaturewillbea

ferociouslyhot120,000°C.Itwillalsobeextremelycompact,

and the leftover internal heat will take billions of years to

leak to the surface.

Eventually,perhaps100billionyearsfromnow,itwill

havecooledtothepointwhereitbecomesablack dwarf,too

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

(UniversityofMichigan).“Soit’sgoingtotakeawhile.”

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

Today

Life Cycle of the Sun

1

10

100

0 5

Mars

First

red-giant

peak

Helium

shell

flashes

White

Now dwarf

Earth

Venus

Mercury

(^10) 12.2
Sun’s age (billions of years)
Diameter (times present value)
12.1 12.3 12.3650 12.3655
Orbit of:
Sun’s Diameter
Contraction
Leaves main
sequence
1
10
100
1,000
10,000
Contraction
Sun’s Luminosity
Leaves main
sequence
Now
Luminosity (times present value)
First
red-giant
peak Second
red-giant
peak
Helium
shell
flashes
(^0510) 12.2
Sun’s age (billions of years)
12.1 12.3 12.3650 12.3655
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...
SFUTURE 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.
FATE OF THE SOLAR SYSTEM