Mira (Omicron Ceti)
Figure 1. Mira's 13 light-year tail. Image credit: Caltech/GALEX/NASA.
Figure 2. Mira Ceti lies in a barren area of the sky and thus is easy to locate when at its brightest. But it is visible to the naked eye for only a few weeks each year, although regularly in each period.
Figure 3. Mira is a long period variable visible to the naked eye. The average period is 331 days; the magnitude ranges from 1.7 to 4 at maximumand down to 10 at minimum
Figure 4. Mira and its companion star as seen from a hypothetical planet. Art by David Anguilar, Harvard-Smithsonian Center for Astrophysics.
Figure 5. Mira A (yellow, right) is shedding material (green) that flows into a disk (red) around Mira B (blue, left). Image credit: M. Ireland/Caltech.
Mira (Omicron Ceti) is a binary star system in the constellation Cetus. It consists of a cool, pulsating giant, the famous star Mira A, and a low-mass companion, Mira B, which is accreting matter that has been shed by its larger partner.
Mira is the only known star with a comet-like tail. It is also enveloped by a puzzling spiral structure.
|Image of Mira acquired by the Hubble Space Telescope
revealing the star's oval shape
Mira A lies on the so-called asymptotic giant branch of the Hertzsprung-Russell diagram. It is pulsating variable whose brightness ranges from 2.0 to 10.1 and spectral type from M6e to M9e III over a period of 331.96 days as its surface rises and falls. Visible to the naked eye at its brightest, Mira can be seen only with optical aid for most of its cycle. It is the prototype and brightest long-period variable or Mira star, and was give its name (meaning "wonderful") by David Fabricius, who was the first to record its brightness fluctuations in 1596. This very luminous and hugely distended star has a diameter of about 650 million km (over 400 million miles) so that it could comfortably swallow the orbit of Mars. Hubble Space Telescope observations have shown that, like some other stars of its type (including R Leonis and W Hydrae), it is conspicuously egg-shaped.
|visual magnitude||2.0 to 10.1|
|spectral type||M6e to M9e III|
|surface temperature||3,000 K|
|distance||420 light-years (130 pc)|
|position||R.A. 02h 19m 20.7s, Dec. -02° 58' 39"|
Mira B and its captured disk of matter
Mira B, also known as VZ Ceti, is separated from Mira A by an average distance of about 100 astronomical units (AU) and the two stars complete an orbit around each other in roughly 500 years. Although astronomers used to think that Mira B was a white dwarf, recent evidence suggests that it is a main sequence star with about half the mass of the Sun. The erratic variability of Mira B are probably connected with its capture of matter from Mira's stellar wind, making Mira B what is known as a symbiotic star. Mira B accretes as much as one percent of the matter lost by its primary.
In January 2007, a group of astronomers using the the Keck Observatory in Hawaii and the Gemini South telescope in Chile reported the discovery of a protoplanetary disk around Mira B raising the unexpected possibility that planets may form from material shed by dying stars. Though planet formation is perhaps unlikely as long as the disk is in active accretion, it may proceed rapidly once Mira A passes through its giant phase and becomes a white dwarf. Furthermore, if planets do form they will have a plentiful source of the raw materials necessary for life, including carbon.
The disk around Mira B currently contains less than a Jupiter's worth of material. However, this is likely to increase to between three and five Jupiters' worth of matter before the accretion process ends – roughly the mass needed to form a planetary system like our own. In addition to the light from the ordinary star, any planets that form will be bathed in the pale glow of the white dwarf, which would appear about as bright as a crescent Moon.
Although this is the first disk known to have formed this way, it may not be unusual. Two thirds of bright star systems are binaries, and about a quarter of these should evolve into systems like Mira, with a red giant donating material to an ordinary star.