Extended shrouds of gas and dust surround the Pole Star and two other stars of the same type, new observations reveal. Astronomers say the shrouds may be related to the stars鈥� pulsations, which are used to measure astronomical distances.
The Pole Star (Polaris) is also known as the North Star and is the nearest example of a Cepheid variable star. Cepheids are massive stars whose atmospheres become unstable when they are middle-aged, which occurs when they are about 50 million years old. The instability causes the stars to oscillate in size 鈥� and therefore brightness 鈥� for several million years. Because this oscillation is related to their intrinsic brightness, Cepheids are useful yardsticks for judging astronomical distances.
Observations with an infrared satellite in 1983 showed little evidence that Cepheids are surrounded by any gas and dust, which glows at infrared wavelengths. But now, two teams of astronomers have used the higher resolving power of multiple, linked telescopes called interferometers and detected dust shrouds 鈥� or 鈥渆nvelopes鈥� 鈥� around three Cepheids, including the Pole Star.
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The Pole Star, and another star called Delta Cephei, were viewed with the Center for High Angular Resolution Astronomy (CHARA) interferometer in Mount Wilson, California, US. Another star, in the southern hemisphere, was studied with the Very Large Telescope Interferometer in Cerro Paranal, Chile.
Losing mass?
The researchers, led by Pierre Kervella and Antoine M茅rand of the Paris Observatory, France, found that the envelopes were two to three times as large as the stars themselves. It is still not clear how the envelopes formed, but M茅rand says: 鈥淚t is most probable that the environments were created by matter ejected by the star itself.鈥�
Team member Jason Aufdenberg at the National Optical Astronomy Observatory in Tucson, Arizona, US, agrees. 鈥淭hese are pulsating stars, so it鈥檚 likely that in the pulsation, they release gas and dust,鈥� he told New 杏吧原创.
Nancy Evans of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US, says the research will be most important for understanding the physics of Cepheids. 鈥淚f it turns out they are losing mass, we will have to go back and rework the models to be confident we understand what state they鈥檙e in,鈥� Evans told New 杏吧原创. But she says the discovery of the envelopes should not affect how the stars are used to measure distances.
Evans, who recently used the Hubble Space Telescope to image one of Polaris鈥檚 two companions for the first time, says the research was unexpected. 鈥淚 was surprised 鈥� I don鈥檛 think there was any previous observational evidence to suggest that this was the case,鈥� she says. 鈥淚t opens a door to something else going on with them.鈥�
