Mira (Omicron Ceti)
|Mira's 13 light-year tail. Image credit:
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 and its companion star as seen from a hypothetical
planet. Art by David Anguilar, Harvard-Smithsonian Center for Astrophysics
Mira is the only known star with a comet-like tail. It is also enveloped
by a puzzling spiral structure.
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.
|Image of Mira acquired by the Hubble Space Telescope
revealing the star's oval shape
Mira's great size and instability result in a stellar
wind that will eventually blow away the star's outer envelope, forming
a planetary nebula and leaving behind a white dwarf.
| Mira A
||2.0 to 10.1
||M6e to M9e III
||420 light-years (130 pc)
||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 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
|Mira A (yellow, right) is shedding material (green)
that flows into a disk (red) around Mira B (blue, left). Image credit:
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.