Press Release

Even Stars Use Sunscreen

By SpaceRef Editor
March 7, 2002
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Cambridge, MA — Five billion years from now, a visitor to our solar
system may see a spectacular sight — our sun swollen into a giant
red orb that has swallowed the inner planets, including Earth. At
that size, the sun will shine with terrible ferocity. But over the
course of a year, it will expand even further, dimming to 1/1000th
its previous strength and fading to near-invisibility before
shrinking and brightening again. The sun will have joined the ranks
of the Mira variable stars, named after the red giant star Mira
(omicron Ceti) in the constellation Cetus the Whale.

The German astronomer David Fabricius discovered the ever-changing
nature of omicron Ceti in 1596 while searching for Mercury. In 1642,
the star was dubbed Mira, which means “The Wonderful.”

While astronomers have known of the existence of these dramatically
changing stars for hundreds of years, the cause of their variability
has been hard to identify. Now, two researchers at the
Harvard-Smithsonian Center for Astrophysics have solved this
long-standing mystery. The key, say Mark Reid and Joshua Goldston, is
the formation of light-absorbing chemicals in the star’s gaseous
atmosphere — the same chemicals found in sunscreen.

“Long before there were professional astronomers, people looked at
the heavens and noted that some stars seemed to vanish and then
reappear,” says Reid. “Only now are we beginning to understand more
fully why that happens.”

Their results will appear in the April 1, 2002 issue of the
Astrophysical Journal.

“Many variable stars change their brightness by small amounts because
they pulsate like a beating heart, alternately growing smaller and
hotter, then larger and cooler,” remarks Goldston. “But such
pulsations can only explain brightness changes up to a factor of 50,
which is like going from a 150-watt light bulb to a 3-watt
night-light.” Pulsations alone cannot cause the dramatic change seen
in Mira variables.

In 1933, Edison Pettit & Seth Nicholson suggested that molecules of
metallic oxides might form in a variable star’s atmosphere as it
expanded and cooled. Those molecules would then absorb light from the
star, causing it to dim. However, in 1933 they lacked the computing
power to model the star to see if this really worked.

Reid and Goldston realized that the formation of molecules like
titanium oxide (the white coloring agent used in many sunscreens and
paints) would increase the opacity of the star’s atmosphere. As a
result, light from inner, hotter regions is absorbed. Only light
coming from the outer, cooler layers of the star can reach us.

“These outer layers are so cool that most of the light is emitted as
heat at infrared wavelengths. There is so little optical light
emitted that a Mira variable seems to almost ‘disappear’ to the human
eye,” says Reid. “It’s amazing to realize what happens when a star
naturally forms sunscreen in its atmosphere. That is what Mira
variables do, and it has a powerful effect on how much light we see
with our eyes.”

The implications for a star that naturally forms sunscreen in its
atmosphere are dramatic. The apparent optical surface of the already
giant star expands even further, to nearly four times the Earth-Sun
distance. The temperature of the light-emitting material at that
distance is quite low, resulting in a thousand-fold dimming of the
star at visible wavelengths. “That dimming is like exchanging a bank
of stadium lights for a single night-light,” says Reid.

At slightly greater distances, the temperature drops even lower,
which allows the formation of silicate or graphite dust. Indeed,
shells of dust have been directly observed around some Mira
variables. That dust can block additional light and dim the star even
more. The dust is eventually dispersed into interstellar space, where
it contributes vital heavy elements to future generations of stars
(and potentially to future planetary systems where life may form).

Reid says, “The study of Mira variables is an exciting example of how
studying distant stars can tell us about our own sun’s future.
Dermatologists warn us that we need to use sunscreen to protect our
skin from our nearest star, the Sun. Little did we ever expect other
stars to be using it, too!”

Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian
Center for Astrophysics (CfA) is a joint collaboration between the
Smithsonian Astrophysical Observatory and the Harvard College
Observatory. CfA scientists organized into seven research divisions
study the origin, evolution, and ultimate fate of the universe.

Note to Editors: High-resolution images are available online at

SpaceRef staff editor.