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Harvard-Smithsonian Center for Astrophysics

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David A. Aguilar, Public Affairs
Harvard-Smithsonian Center for Astrophysics
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daguilar@cfa.harvard.edu

For Release: March 7, 2002

Release No.: 02-07

Even Stars Use Sunscreen!

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
http://cfa-www.harvard.edu/press/reid_images.html