Mira

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Biography

Mira ( or ) also known as Omicron Ceti (? Ceti, ? Cet), is a red giant star estimated 200-400 light years away in the constellation Cetus . Mira is a binary star , consisting of the red giant Mira A along with Mira B . Mira A is also an oscillating variable star and was the first non- supernova variable star discovered, with the possible exception of Algol . Apart from the unusual Eta Carinae , Mira is the brightest periodic variable in the sky that is not visible to the naked eye for part of its cycle. Its distance is uncertain; pre- Hipparcos estimates centered around 220 light-years , while Hipparcos data suggest a distance of 418 light-years, albeit with a margin of error of ~14%.

Observation history

Evidence that the variability of Mira was known in ancient History of China|China , Babylon or Ancient Greece|Greece is at best only circumstantial. What is certain is that the variability of Mira was recorded by the astronomer David Fabricius beginning on August 3, 1596. Observing the planet Mercury (planet)|Mercury , he needed a reference star for comparing positions and picked a previously unremarked third-magnitude star nearby. By August 21, however, it had increased in brightness by one Apparent magnitude|magnitude , then by October had faded from view. Fabricius assumed it was a nova, but then saw it again on February 16, 1609.

In 1638 Johann Holwarda|Johannes Holwarda determined a period of the star's reappearances, eleven months; he is often credited with the discovery of Mira's variability. Johannes Hevelius was observing it at the same time and named it "Mira" (meaning "wonderful" or "astonishing," in Latin) in 1662's Historiola Mirae Stellae , for it acted like no other known star. Ismail Bouillaud then estimated its period at 333 days, less than one day off the modern value of 332 days, and perfectly forgivable, as Mira is known to vary slightly in period, and may even be slowly changing over time. The star is estimated to be a 6-billion-year-old red giant .

There is considerable speculation as to whether Mira had been observed prior to Fabricius. Certainly Algol 's history (known for certain as a variable only in 1667, but with legends and such dating back to antiquity showing that it had been observed with suspicion for millennia) suggests that Mira might have been known too. Karl Manitius, a translator of Hipparchus ' Commentary on Aratus , has suggested that certain lines from that second century text may be about Mira. The other pre-telescopic Western catalogs of Ptolemy , al-Sufi , Ulugh Beg , and Tycho Brahe turn up no mentions, even as a regular star. There are three observations from Chinese and Korean archives, in 1596, 1070, and the same year when Hipparchus would have made his observation (134 BC) that are suggestive, but the Chinese practice of pinning down observations no more precisely than within a given Chinese constellation makes it difficult to be sure.

System

Mira is a binary star system that consists of a red giant (Mira A) undergoing mass loss and a high temperature white dwarf companion (Mira B) that is accreting mass from the primary. Such an arrangement of stars is known as a symbiotic system and this is the closest such symbiotic pair to the Sun . Examination of this system by the Chandra X-ray Observatory shows a direct mass exchange along a bridge of matter from the primary to the white dwarf. The two stars are currently separated by about 70& nbsp; astronomical unit s.

Component A

Mira A is currently an Asymptotic Giant Branch (AGB) star, in the thermally pulsing AGB phase. Each pulse lasts a decade or more, and an amount of time on the order of 10,000& nbsp;years passes between each pulse. With every pulse cycle Mira increases in luminosity and the pulses grow stronger. This is also causing dynamic instability in Mira, resulting in dramatic changes in luminosity and size over shorter, irregular time periods.

The overall shape of Mira A has been observed to change, exhibiting pronounced departures from symmetry. These appear to be caused by bright spots on the surface that evolve their shape on time scales of 3–14 months. Observations of Mira A in the ultraviolet band by the Hubble Space Telescope have shown a plume-like feature pointing toward the companion star.

Variability

Mira A is a well-known example of a category of variable star s known as Mira variable s, which are named after this star. It (and the other ca 6,000 to 7,000 known stars of this class from the General Catalogue of Variable Stars|GCVS catalogue (statistics at the end of the file indicate 6,006 mirae and 1,237 probable mirae)) are all red giant s whose surfaces oscillate in such a way as to increase and decrease in brightness over periods ranging from about 80 to more than 1,000 days.

In the particular case of Mira, its increases in brightness take it up to about Apparent magnitude|magnitude 3.5 on average, placing it among the List of stars in Cetus|brighter stars in the Cetus constellation . Individual cycles vary too; well-attested maxima go as high as magnitude 2.0 in brightness and as low as 4.9, a range almost 15 times in brightness, and there are historical suggestions that the real spread may be three times this or more. Minima range much less, and have historically been between 8.6 and 10.1, a factor of four times in luminosity. The total swing in brightness from absolute maximum to absolute minimum (two events which did not occur on the same cycle) is 1,700 times. Interestingly, since Mira emits the vast majority of its solar radiation|radiation in the infrared , its variability in that band is only about two magnitudes. The shape of its light curve is of an increase over about 100 days, and a return twice as long.

Contemporary approximate maxima for Mira: http://spider.seds.org/spider/Vars/mira.html

Oct 21-31, 1999

Sep 21-30, 2000

Aug 21-31, 2001

Jul 21-31, 2002

Jun 21-30, 2003

May 21-31, 2004

Apr 11-20, 2005

Mar 11-20, 2006

Feb 1-10, 2007

Jan 21-31, 2008

Dec 21-31, 2008

Nov 21-30, 2009

Oct 21-31, 2010

Sep 21-30, 2011

Aug 21-31, 2012

Jul 21-31, 2013

Jun 21-31, 2014

May 21-31, 2015

Mass loss

Ultra-violet studies of Mira by NASA 's Galaxy Evolution Explorer (Galex) space telescope have revealed that it sheds a trail of material from the outer envelope, leaving a tail 13 light-years in length, formed over tens of thousands of years.Minkel, JR., "The Scientific American", August 15, 2007 Accessed August 21, 2007. It is thought that a hot bow shock|bow-wave of compressed plasma/gas is the cause of the tail; the bow-wave is a result of the interaction of the stellar wind from Mira A with gas in interstellar space, through which Mira is moving at an extremely high speed of 130 kilometres/second. The tail consists of material stripped from the head of the bow-wave, which is also visible in ultra-violet observations. Mira's bow-shock will eventually evolve into a planetary nebula , the form of which will be considerably affected by the motion through the interstellar medium (ISM).

Component B

The companion star was resolved by the Hubble Space Telescope in 1995, when it was 70 astronomical unit s from the primary; results were announced in 1997. The HST ultraviolet images and later X-ray images by the Chandra X-ray Observatory|Chandra space telescope show a spiral of gas rising off Mira in the direction of Mira B. The companion's orbital period around Mira is approximately 400 years.

In 2007, observations showed a protoplanetary disc around the companion, Mira B. This disc is being accreted from material in the stellar wind|solar wind from Mira and could eventually form new planets. These observations also hinted that the companion was a main sequence star of around 0.7 solar mass es and spectral type K, instead of a white dwarf as originally thought. However in 2010 further research indicated that Mira B is in fact a white dwarf.

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