Sky & Telescope - January 2023 - 16

Stars and Planets, Part II
Half
Fraction of its mass the Sun will
lose as it ages
of a Sun-like star provides an informative evolutionary trajectory
for the current census of known exoplanets.
During the main sequence, a star remains in a state of
hydrostatic equilibrium, whereby the inward pull of gravity is
counterbalanced by the outward pressures of heat and light
produced by nuclear fusion. However, the star's core hydrogen
reserves are not limitless. Once a star has exhausted its
supply, fusion ceases. Without a supply of central energy, the
star's inert helium core begins to contract under its own gravity.
The outer regions of the star, still rich with unprocessed
hydrogen, also contract, causing the star's interior to heat up.
Eventually, a thin layer of hydrogen around the core reaches a
critical temperature, and nuclear fusion recommences.
Once hydrogen-shell burning begins, the star enters a
new evolutionary stage known as the subgiant branch. Fusion
slowly moves outward into higher sections as it depletes the
shell's hydrogen, adding new layers of helium ash to the stellar
core. A subgiant star produces more energy than its mainsequence
counterparts. The increased radiation pressure
pushes on the star's outer layers, causing the star to expand.
By the end of the subgiant phase, a Sun-like star will have
doubled in size.
The star grows even more during the next evolutionary
stage, known as the red giant branch. Stars like the Sun will
swell to become 100 times their original size. The internal and
external changes will cause the star to brighten by a factor of
1,000, and the distance at which a planet can sustain liquid
surface water will change significantly. In the case of our own
solar system, the habitable zone will sweep outward, briefly
baking the gas giants and their icy moons with an unfamiliar
warmth. (Earth's atmosphere and oceans will have long since
boiled away, perhaps during the subgiant phase.)
A far bleaker fate awaits the companions that closely
orbit their evolving stellar hosts. Such planets are doomed
to be engulfed by the expanding star at this stage. Mercury
and Venus will suffer this end. Earth might as well, but our
planet's future depends upon things like how the Sun's mass
loss will affect the planets' motions, which we don't know
precisely (S&T: Oct. 2017, p. 22). Assuming Earth does survive,
by the end of the Sun's red giant phase, average temperatures
on our planet will top 1000°C (2000°F).
Planetary-engulfment events at these late evolutionary
stages are common. Observations indicate that exoplanetary
engulfment will accompany the evolution of 30% of
all Sun-like stars. It's also the fate awaiting some 90% of
the confirmed exoplanet population. This is because we've
detected the vast majority of these alien worlds (95%) using
techniques that more easily find companions in close orbits.
Astronomers have seen signs of orbital decay in some planetary
systems, with the exoplanets' orbital periods decreasing
over time. These planets appear to be spiraling inward toward
their hosts on million-year time scales.
Stellar Indigestion
While exoplanet ingestion is not a rare phenomenon, there is
LIFE OF THE SUN The Sun is slowly brightening as it ages, pushing its habitable zone out. Once core hydrogen fusion ceases at the end of the
main sequence, the star will undergo several evolutionary stages, three of which appear here. Each brings extreme changes to the Sun's size and
luminosity and shifts the habitable zone's location. Orange numbers are the star's age during each stage (the AGB stage lasts about 20 million years).
1. Main Sequence: 0-11 billion years1. Main Sequence: 0-11 billion years
2. Red Giant Branch: 12-12.5 billion years2. Red Giant Branch: 12-12.5 billion years
Sun brightens and grows
slightly with age.
Sun brightens and grows
slightly with age.
Initial
Initial
habitable
zone
habitable
zone
Final
Final
habitable
zone
habitable
zone
Sun balloons to more than 100 times its current size
and brightens by at least a factor of 1,000.
Sun balloons to more than 100 times its current size
and brightens by at least a factor of 1,000.
3.
3
Su
S
Habitable zone
moves outward.
At the end of the main sequence,
the habitable zone will reach into
the asteroid belt.
At the end of the main sequence,
the habitable zone will reach into
the asteroid belt.
* Not to scale
Habitable zone
moves outward.
Habitable zone swoops through the outer solar system, moving
from Mars's current orbit out to the Kuiper Belt.
* Not to scale
Habitable zone swoops through the outer solar system, moving
from Mars's current orbit out to the Kuiper Belt.
Th
Sa
T
S
16 JANUARY 2023 * SKY & TELESCOPE
LIFE OF THE SUN: GREGG DINDERMAN / S&T
Sky & Telescope - January 2023

Table of Contents for the Digital Edition of Sky & Telescope - January 2023
