Sky & Telescope - December 2020 - 36

The Chemistry of Life

Volatile Chemistry

in our galaxy where stars and planets are
VAPORIZATION
being born today (S&T: May 2020, p. 34).
TEMPERATURES
Carbon atoms - as well as many other elements
We can also observe objects at different
- are created in the bellies of stars as products of
stages in the formation process. By doing
nuclear fusion and thrown into space during the
Silicate
1200K (930°C)
so, we can pull together the distinct stages
end stages of stellar evolution and death (S&T:
minerals
of planet growth and complete our story
June 2020, p. 58). Aging and dying stars also
of what happens to carbon throughout a
create stardust, the tiny particulate matter that
planet's creation.
provides the seeds of terrestrial worlds. We've
Such studies essentially "tune in"
observed carbon carried by grains, ice, and gas
to the frequencies of speciﬁc molecules
in space, and the form it takes affects how it's
around forming stars and planets, the
incorporated into forming planets.
same way you might tune the radio in
This brings up a central aspect of the chemisyour car to a local radio station. When
try of space: volatility. Volatility is a general term
a star is ﬁrst forming, it heats gases and
describing how likely it is that a given molecular
solids in close proximity to a couple
compound will transition to the vapor phase.
hundred kelvin, depending on how dense
Pressure and temperature determine when this
the material is. Within this hot gas, moltransition happens. But in space, the pressures
ecules are moving around randomly, but
are less than a quadrillionth (that is 15 orders
Carbon
400K (130°C)
grains
they are also spinning or rotating. Molof magnitude) less than the air we breathe! And
ecules that are spinning faster "want" to
the temperature in a star-forming cloud is 20K
spin slower, and they can spontaneously
to 30K above absolute zero, or about −250°C
slow down their rotation by emitting a
(−410°F) to me and you. Because of these
photon. This photon appears at microextreme conditions, there is no liquid phase in
200K (−70°C)
H2O
wave wavelengths and can be detected by
space chemistry: Compounds transfer directly
telescopes such as ALMA.
from solid rock or ice to the vapor phase.
From a spectrum of the gas around a
Stardust, whether silicate or carbonaceous, is
60K (−220°C)
CO2
young star, we can add up the carbon and
refractory, meaning it's hard to vaporize. Other
30K (−240°C)
CO
determine what molecular form it's in.
common molecules, such as water and carbon
We ﬁnd that just before the main phase
monoxide (CO), are referred to as volatiles, as
0K
of planet formation, carbon dominates
these species prefer to be in the gas phase and are
the chemistry of space: The majority
harder to incorporate into rocks and ices.
of detected molecules have carbon as a
Thanks to instruments such as the Atacama
central element. This is likely not an accident, as the carbon
Large Millimeter/submillimeter Array (ALMA) in Chile,
astronomers can detect a wide range of compounds in regions atom has the highest number of available bonds of the cosmi-

HOW THE SOLAR SYSTEM FORMED

Stars are born in clouds
of dust and molecular
gas.

Dense core

Dense regions of the
cloud collapse under
the weight of their own
gravity, rotating as they
do so.

DECEMBER 2 02 0 * SK Y & TELESCOPE

Inside the core, a star
forms surrounded by
a disk, fed by material
from the core as collapse
continues. To conserve
angular momentum, the
protostar ejects material
as outflows from its
poles.

Protoplanetary disk:
1-10 million years

Radiation and outflowing material remove the
planetary system's natal
envelope and cut the link
with the cloud. This is
when planetary building
blocks form - bodies
the size of asteroids or
even Mars.

Final assembly of
terrestrial worlds:
10-50 million years

The disk dissipates
and collisions continue, completing
planet formation.

THER M O ME TER: G REGG DINDER M A N / S&T; HOW THE SOL A R
SYSTEM FOR MED: G REGG DINDER M A N / S&T, SOURCE: K . PEEK

Molecular cloud

Protostar:
0-1 million years

Sky & Telescope - December 2020

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