Artist’s conception of a Mira variable star at maximum brightness Photo: David Aguilar, Harvard-Smithsonian Center for Astrophysics
The centuries-old mystery of why dying red giant stars dim and gradually disappear, only to reappear months later, has been solved, say US scientists. Computer modeling shows titanium oxide, the white pigment found in sunscreens, periodically forms in the atmosphere of the stars.
So-called Mira variable stars can dim to one thousandth of their maximum brightness. “That dimming is like exchanging a bank of stadium lights for a single night-light,” says Mark Reid at the Harvard-Smithsonian Center for Astrophysics in Cambridge, US, who was involved in the new research.
The temperature in the expanded outer atmosphere of the star varies between about 2000°C and 2800°C, says Reid. When the temperature is close to its peak, titanium and oxygen atoms become fully dissociated. But at the lower end of temperature range, they can form titanium oxide molecules, which absorb light from the inner, hotter regions of the star and cause it to dim.
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The same star at its minimum brightness Photo: David Aguilar, CFA
Titanium oxides shield visible light so well because electrons in the outer shells of the titanium and oxygen atoms absorb visible photons, which they later emit at a much lower temperature, almost entirely in the infrared part of the light spectrum.
Janet Mattei of the American Association of Variable Star Observers in Cambridge, Massachusetts, says it is exciting to learn more about Mira stars because most stars go through the Mira phase towards the end of their life. “We can learn about the future of our star and more about how these stars enrich the interstellar medium from which new stars form,” she says.
