Mira (star)

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Star
Mira (ο Ceti)
Mira observes with the Atacama Large Millimeter / submillimeter Array at a wavelength of 900 μm.
Mira observes with the Atacama Large Millimeter / submillimeter Array at a wavelength of 900 μm.
AladinLite
Observation
dates equinox :  J2000.0 , epoch : J2000.0
Constellation whale
Right ascension 02 h 19 m 20.79 s
declination -02 ° 58 ′ 39.5 ″
Angular expansion 47  mas
Apparent brightness 6.47 (2 to 10.1) mag
Typing
B − V color index +1.42 
U − B color index +1.09 
R − I index +1.90 
Spectral class M5e-M9e
Variable star type M (prototype)  
Astrometry
Radial velocity (+63.8 ± 0.9) km / s
parallax (10.91 ± 1.22)  mas
distance (300 ± 33)  ly
(92 ± 10)  pc  
Visual absolute brightness M vis +1.66 likes
Proper movement 
Rec. Share: (+9.3 ± 2.0)  mas / a
Dec. portion: (−237.4 ± 1.6)  mas / a
Physical Properties
Dimensions 1.2  M ☉
radius 400  R ☉
Luminosity

8400 to 9360  L ☉

Effective temperature 2918 to 3192  K
Other names
and catalog entries
Bayer name ο Ceti
Flamsteed name 68 Ceti
Bonn survey BD -3 ° 353
Bright Star Catalog HR 681 [1]
Henry Draper Catalog HD 14386 [2]
Hipparcos catalog HIP 10826 [3]
SAO catalog SAO 129825 [4]
Tycho catalog TYC 4693-1144-1 [5]
2MASS catalog 2MASS J02192081-0258393 [6]
Other names Mira, LTT 1179

Mira , ο Ceti (Omicron Ceti), is a double star in the constellation Whale , consisting of the red giant Mira A, or simply Mira, and the white dwarf Mira B or VZ Ceti. Mira A is a changeable star and namesake for the Mira stars . Mira is about 300 light years away from Earth.

Classification

The mutable star Mira photographed at two different times.

Mira A is a red giant belonging to the spectral class M (Hipparcos database). During a period of around 332 days, it changes its luminosity by up to 8 size classes. Neither the period nor the brightness minima and maxima are constant. At maximum it can reach the 2nd  magnitude class and is then a noticeably bright star in the night sky. But Mira also appears much weaker when passing through some maxima; and so the apparent brightness is at most 1.7 mag during a maximum, but only 4.9 mag in other cases. During the minimum, the apparent brightness can drop to the 10th magnitude, so that a telescope is required to observe it. The individual minima also show considerable differences with values ​​between 8.6 and 10.1 may. Mira is 1700 times brighter in its absolute maximum than in its absolute minimum - in the infrared range, in which Mira emits the majority of the radiation, however, it is only 6 times brighter than the minimum. The period from maximum to minimum is 206 days, and that from minimum to maximum 126 days.

Not only the brightness, but also the spectrum of Mira is variable. It is a red star with a relatively low surface temperature and changes its spectral type from M5e when passing through the maximum to the spectral type M9e during the minimum. When the star gradually becomes brighter again after the minimum, emission lines (above all of hydrogen ) appear in the spectrum from the 7th magnitude onwards , which superimpose the absorption spectrum. Their intensities rise steadily, briefly even after exceeding the maximum, before they then decrease again. The spectrum also shows noticeable titanium oxide bands.

Mira and her companion captured by the Hubble space telescope

The brightness fluctuations of Mira are caused by radial pulsations , in which the star expands and contracts again. Around the time it passes through the maximum, the star is smallest (approx. 330 solar diameter , ie approx. 460 million km) and has a surface temperature of approx. 3200 Kelvin; In contrast, the minimum diameter is around 400 solar diameters (approx. 550 million km) and the surface temperature around 2900 Kelvin. The large fluctuation range of the visual brightness is mainly attributed to the fact that at the maximum, due to the higher star temperature, a significantly larger proportion of the total radiation is emitted in the area visible to the eye than at the minimum.

At a distance of around 300 light years from Earth, the Hubble Space Telescope can resolve the star in terms of area , so that it is not only recognizable as a point.

discovery

Mira was discovered by the East Frisian pastor and amateur astronomer David Fabricius on August 13, 1596. At that time, the star passed its maximum brightness and was in the 2nd magnitude. Several months later, Fabricius searched for the star again, but could not find it. On the other hand, he succeeded in doing another observation on February 15, 1609, when Mira was shown as a third magnitude star. In spite of these observations, Fabricius is unlikely to have concluded that Miras was variable. As early as 1603 Johann Bayer had entered the star as Omikron Ceti in his sky atlas. In 1638 Johann Ph. Holwarda discovered that Mira changes its brightness more or less regularly. It is because of this strange property that Johannes Hevelius gave the star its name - Mira, the "miraculous" one. As early as 1660, the average period length was determined to be about 11 months; the French astronomer Ismael Boulliau , for example, estimated it to be 333 days. Nevertheless, the realization of Mira's variability only slowly became generally accepted in the further course of the 17th century. Mira was the first star ever to have fluctuations in brightness.

In 1923 a weak companion (VZ Ceti) was found by RG Aitken , who orbits Mira with a period of about 500 years. This is also variable and fluctuates with an irregular period between the 9th and 12th magnitude. Perhaps he is a white dwarf and is surrounded by an accretion disk created by Mira's stellar wind . In 2007 it was discovered on images taken by NASA's GALEX space telescope that Mira is the only known star to have a giant tail that is similar to that of a comet and extends over 13 light-years in length. It consists of matter from the outer shell that has been repelled by the star. The unusual structure is the result of the interaction of Mira's stellar wind with the gas of the interstellar medium , through which Mira moves at a high speed of 110 km / s. The star's annual mass loss is about one ten millionth of the solar mass ; and from the shed material a planetary nebula will form.

Ultraviolet image of the comet-like tail

Web links

Commons : Mira  - collection of images, videos and audio files

Individual evidence

  1. ↑ a b c Hipparcos catalog (ESA 1997)
  2. ↑ a b c omi Cet. In: VSX. AAVSO, accessed October 29, 2018 .
  3. ↑ a b c Bright Star Catalog
  4. ↑ a b c Hipparcos, the New Reduction (van Leeuwen, 2007)
  5. ↑ estimated based on the apparent brightness and distance
  6. ↑ a b c d Mira , in: Lexikon der Astronomie , Herder, Freiburg im Breisgau 1989, Vol. 1, ISBN 3-451-21491-1 , p. 430.
  7. ↑ Mira Predictions - 2019
  8. ↑ HC Woodruff, M. Eberhardt, T. Driebe, K.-H. Hofmann: Interferometric observations of the Mira star o Ceti with the VLTI / VINCI instrument in the near-infrared . In: Astronomy & Astrophysics . 421, No. 2, 2004, pp. 703-714. arxiv : astro-ph / 0404248 . bibcode : 2004A & A ... 421..703W . doi : 10.1051 / 0004-6361: 20035826 .
  9. ↑ A Star with a Comet's Tail ( Memento of the original from June 15, 2009 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , NASA Headline News, Aug. 15, 2007 @1@ 2Template: Webachiv / IABot / science.nasa.gov
  10. ^ A. Mayer: Herschel's view into Mira's head . In: Astronomy & Astrophysics . tape 531 , 2011, p. L4 , doi : 10.1051 / 0004-6361 / 201117203 .
  11. ↑ Mira , Jim Kaler, Stars .