Mira

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Mira
Observational Data
Designation Omicron Ceti
Mira Ceti
Right ascension 02h 19m 20.79210s[1]
Declination -02° 58′ 39.4956″[1]
Constellation Cetus
Type of object Variable star
Magnitude Apparent Mag: +6.53[1]
Absolute Mag: +1.38[2]
Astrometry
Distance from Earth 350 ly
Radial velocity 63.50±0.6 km/s
Proper motion RA: 9.33 mas/yr[1]
Dec.: -237.5 mas/yr[1]
Parallax 10.91±1.22 mas[1]

Mira (Omicron Ceti, Mira Ceti) is a well known variable star in the Constellation of Cetus and was the first variable star discovered that wasn't a supernova.[3] It was discovered in 1596 by David Fabricius in 1596, a German astronomer, though there is some evidence its variable nature may have been observed earlier by the Babylonians.[4] Another variable star, Algol, was discovered earlier but its variable nature wasn't confirmed until 1667. Astronomers were amazed by the way the star's brightness changed and named it Mira, from the Latin for "miraculous" or "wonderful".

Being a variable star, the star's magnitude (or brightness) varies over time with a period of roughly 330 days.[3] The star's apparent magnitude peaks at approximately +3 and is at +9 at its lowest, 1,500 times fainter.[3][5] This means that depending where it is on its cycle, Mira may or may not be visible to the unaided eye.

History

The first definitive recorded observation of the star was by David Fabricus (1564-1617) of the Netherlands.[6] He first saw the star in 1596 and originally believed it to be a nova. However later in 1609 he observed the star again and realised it therefore couldn't be a nova.[4] Meanwhile, in 1603 Johann Bayer catalogued the star as Omicron Ceti.[6] The star was then more or less forgotten until Johannes Holwarda, a Dutch astronomer rediscovered the star and was the first to determine its period of roughly eleven months. The star was named Mira by Johannes Hevelius in 1662.[5]

Properties and Structure

Mira is in fact a binary system, where one smaller star orbits another.[4] These two stars are designated Mira A and Mira B. The larger star (Mira A) is a red giant with spectral type M7 IIIe. Mira A is an oscillating variable star, where the star periodically expands and contracts. As the surface area of the star changes, so does its luminosity and so apparent magnitude. Mira forms the prototype for this group of stars, of which there are around 6,000-7,000 known.[4] This class of variable stars are known as pulsating variables.
The light curve of the variable star Mira
The light curve of Mira showing how its brightness oscillates.

The companion star Mira B is a White dwarf and is thought to be accreting material from its neighbour, Mira A. Mira B completes one orbit around Mira A every 500 years.[7]

The system is though to be situated 350 light years from Earth, though some sources suggest it is higher at 420 ly.[3][4] The system also appears to be racing round the Milky Way around 130 km/s faster than other stars.[7] Ultraviolet observations in 2006 by NASA's Galaxy Evolution Explorer have shown Mira A is shedding vast quantities of material, which has formed a tail resembling that of a comet.[7]

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 Mira from the SIMBAD Astronomical Database
  2. By direct calculation, using distance of 350 ly and an apparent magnitude of +6.53.
  3. 3.0 3.1 3.2 3.3 Mira Ceti. Retrieved on 2018-12-27.
  4. 4.0 4.1 4.2 4.3 4.4 Cetus. Retrieved on 2018-12-27.
  5. 5.0 5.1 Time to see Mira the Wonderful. Retrieved on 2018-12-27.
  6. 6.0 6.1 Hoffleit, D. (1997). History of the Discovery of Mira Stars. The Journal of the American Association of Variable Star Observers, 25(2), pp.115-136. Bibcode:1997JAVSO..25..115H
  7. 7.0 7.1 7.2 Mira Soars Through the Sky. Retrieved on 2018-12-27.