CfA Press Release
Release No.: 02-07 |
For Release: March 7, 2002
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
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,
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://www.cfa.harvard.edu/news/reid_images.html.
For more information, contact:
David A. Aguilar, Public Affairs
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7462 Fax: 617-495-7468