34 ASTRONOMY • FEBRUARY 2018A: The density of the plasma in
the Sun’s atmosphere falls off
precipitously as we move out-
ward from the photosphere (its
visible surface) to the corona.
Because of this, the light we
receive from the Sun over-
whelmingly comes from its
photosphere; only a tiny frac-
tion comes from its corona.
This is also why the corona can
be seen only when the light
from the photosphere is blocked
(via a total eclipse or an occult-
ing disk in a coronagraph). In
fact, the photosphere is often
referred to as the “surface” of
the Sun, even though there is no
real solid surface.
The color of light a star emits
is related to its temperature. This
means that we can determine
the effective temperature of the
Sun by measuring the amount of
light it emits at each wavelength
and comparing the resulting
spectrum we see to models.
Another approach is to record
which absorption lines are pres-
ent in the solar spectrum and
determine their strengths; both
the elements present and their
strengths are sensitive to tem-
perature. These different meth-
ods all show that the effective
temperature of the Sun’s surface
is around 5,800 kelvins (9,980
degrees Fahrenheit [5,520
degrees Celsius]).
So although the Sun’s corona
at a temperature of over a mil-
lion kelvins (1,800,000 F
[1,000,000 C]) is significantly
hotter than the photosphere,
the vast majority of the light weAstronomy’s experts from around the globe answer your cosmic questions.OUR SOLAR
SYSTEM
gases like ammonia (NH 3 ) and
hydrogen sulfide (H 2 S) con-
dense to form cloud decks too,
in addition to water.
So the giant planets present
a unique situation — NH 3 con-
denses at the coldest atmo-
spheric temperatures to form
the ref lective cloud decks we
see through a telescope.
Beneath that, there’s a chemical
compound formed from a com-
bination of NH 3 and H 2 S. And
a few hundred miles or kilome-
ters below that, we expect water(H 2 O) clouds, just as on Earth.
This water cloud is so deep that
it is hidden from Earth-based
telescopes and even visiting
spacecraft. Water ice is only
ever seen where the stormy
atmosphere exhibits powerful
convective rising motions,
dredging up materials from
deeper within the planet. The
situation is even more complex
for the ice giants Uranus and
Neptune, where supercold tem-
peratures allow methane (CH 4 )
to condense above the NH 3 ,
H 2 S, and H 2 O, which is why
their clouds look so different
from the gas giants’.
These clouds are moved
around by powerful winds,
swirling eddies, and vortices to
create the beautiful patterns
that we see. However, pure
NH 3 ice should simply be
white, so the extra colors (the
reds, browns, greens, blue-
grays of Jupiter) must be due to
contaminants mixed with the
condensed aerosols. These are
other chemical species eitherASKASTR0
Q: HOW IS THE TEMPERATURE OF THE SUN’S
SURFACE MEASURED THROUGH ITS MUCH
HOTTER ATMOSPHERE, THE CORONA?
David Kennedy, Auburndale, Floridause to measure the effective
temperature of the Sun comes
from its photosphere. The con-
tribution from the corona is
minuscule in comparison.
Stuart Jefferies
Astronomer, University of Hawaii
Institute for Astronomy, Maui, HawaiiQ: HOW DO CLOUDS FORM
ON JUPITER OR OTHER
GAS GIANTS, AND HOW
DEEP DO THEY EXTEND?
Craig, Riley, and August Pritzlaff
Cedar Park, TexasA: Despite the seemingly alien
environments of our two gas
giants, the mechanism for
cloud formation is surprisingly
Earth-like: When the tempera-
ture drops low enough, gases
within the atmosphere start to
condense or freeze to form
droplets known as aerosols
— the icy crystals that make up
clouds. The key difference is
that jovian and saturnian tem-
peratures are low enough thatThe Sun’s corona is visible only during a total solar eclipse or when
the brightest portion of the star is blocked by an instrument called
a coronagraph, as shown here. A coronal mass ejection of hot plasma
appears at lower left. Coronal gases reach temperatures of 1,800,000
degrees Fahrenheit (1,000,000 C) or more. The photosphere, or visible
surface of the Sun, typically measures up to 10,000 F (5,540 C). ESA, NASA/SOHOJupiter’s intricately swirling atmosphere shows a stunning variety
of cloud colors in this photo snapped by JunoCam, the Juno spacecraft’s
optical camera. DAVID MARRIOTT