Although the giant star Mira has been known for about 400 years, astronomers have had to wait for NASA's Hubble Space Telescope to provide the first ultraviolet images of the extended atmosphere of the cool red giant star and its nearby hot companion.
By giving astronomers a clear view of the individual members of this system, Hubble has provided valuable insights into other types of double star systems where the stars are so close they interact with one another.
The separation between Mira and its companion is about 70 times more than that between Earth and the Sun, (equal to an angular size of only 0.6 arcseconds the apparent diameter of a dime at four miles away) even smaller than the typically fuzzy ground-based telescopic image of a single star as smeared out by Earth's turbulent atmosphere.
Using the European Space Agency's Faint Object Camera aboard Hubble, Margarita Karovska and John Raymond of the Harvard-Smithsonian Center for Astrophysics, Cambridge, MA; Warren Hack of the Space Telescope Science Institute, Baltimore, MD; and Edward Guinan of Villanova University, Villanova, PA, obtained both ultraviolet and visible light images and spectra of the two separate stars in the Mira system. The results appear in the June 20 Astrophysical Journal Letters.
In ultraviolet light, Hubble has resolved a small hook-like appendage extending from Mira in the direction of the companion, which might be material from Mira being gravitationally drawn toward the smaller star. Alternately, it could be material in Mira's upper atmosphere being heated due to the companion's presence.
Hubble's visible-light images show that Mira has an odd, asymmetrical shape resembling a football. This may be tied to dramatic changes occurring during its expansion-contraction cycles, or to the presence of unresolved spots on its surface. Hubble allows astronomers to measure the star's size at about 60 milliarcseconds, corresponding to a diameter some 700 times larger than our Sun. If Mira were at the center of our solar system, it would extend out more than 300 million miles, well beyond Mars' orbit and nearly two-thirds of the way to Jupiter.
Mira (officially called Omicron Ceti in the constellation Cetus) is the prototype for an entire class of stars known as "Mira-type variables." Although once like our Sun, Mira is now at the end of its life, and has evolved into a cool red giant star that is highly variable in brightness. Contracting and expanding every 332 days, Mira sheds vast amounts of material through its powerful "wind" of gas and dust.
Mira's companion is a burned-out star called a white dwarf that is surrounded by material captured from Mira's wind. At a distance of about 400 light-years, Mira is the closest wind-accreting binary system to Earth.
Separating the spectra of Mira and its companion something astronomers previously have tried to do through indirect means is a crucial step for studies of physical processes associated with wind accretion in binaries.
Mira was discovered on August 13, 1596, by Dutch astronomer David Fabricus, who mistook it for a nova because it later faded from view. He called it Mira, meaning "The Wonderful." Astronomers later realized it was really the first case of a variable star.
NASA Headquarters, Washington, DC
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Space Telescope Science Institute, Baltimore, MD
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA