Even stars use sunscreen!
Posted: March 8, 2002

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.

At its maximum brightness, the variable star Mira floods a hypothetical planetary companion with light in this artist's conception. Credit: David Aguilar, Harvard-Smithsonian Center for Astrophysics
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."

At its minimum brightness, the variable star Mira expands and cools. In this artist's conception, the dimming of Mira makes its small, hot white dwarf companion visible in the sky of a hypothetical planetary companion. Credit: David Aguilar, Harvard-Smithsonian Center for Astrophysics
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.