August 6, 1997
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.
The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract with NASA's Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).
Image files in GIF and JPEG format and captions may be accessed on the Internet via anonymous ftp from oposite.stsci.edu in /pubinfo. GIF JPEG PRC97-26 Mira gif/mira.gif jpeg/mira.jpg Higher resolution digital versions (300 dpi JPEG) of the release photograph are available in /pubinfo/hrtemp: 97-26.jpg (color) and 97-26bw.jpg (black & white).
GIF and JPEG images, captions and press release text are available via the World Wide Web.
PHOTO CAPTION NO.: STScI-PRC97-26
In the lower two panels the reconstructed images of Mira are magnified and the companion is out of the field of view to the left from Mira.
Hubble's visible-light image of the disk of Mira reveals it 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 measurements show the red giant star is 700 times larger than our Sun.
In UV light, Hubble resolves a small hook-like appendage extending from Mira, in the direction of the companion that could be material from Mira being gravitationally drawn toward the smaller star. Alternately, it could be that material in Mira's upper atmosphere is being heated due to the companion's presence.
In this magnified image of Mira, displayed on a log scale to show better the extension toward the companion, the companion is out of the field of view - to the left of Mira.
PHOTO CREDIT: Margarita Karovska (Harvard-Smithsonian Center for Astrophysics), and NASA