Look (star)

Mira (Ómicron Ceti / ο Cet / 68 Ceti) is a variable star in the constellation of Cetus, "the whale". One of the most remarkable stars in the night sky, its apparent magnitude varies between +2.0 —being at that time the brightest star in the constellation— and +10.1 —when it is not visible to the naked eye— with a period of 332 days. This has given rise to its name, Mira, from the Latin mira, "wonderful, amazing". How far away it is is uncertain; while measurements made before the Hipparcos satellite placed it 220 light-years from the Solar System, the Hipparcos data indicate a distance of 418 light-years, with a margin of error of 14%.

History of your observation

It may be that Mira variability was already known in ancient China, Babylonia, and Greece. What is certain is that Mira's variability was recorded by astronomer David Fabricius as early as August 3, 1596. Observing the planet Mercury, Fabricius needed a reference star to compare positions, choosing a previously unnoticed nearby third-magnitude star. -Look-. However, by August 21, the star's brightness had increased by one magnitude, while by October of that same year it was no longer visible. Fabricius assumed that she was a nova, until he saw her again on February 16, 1609.

In 1638, Johann Holwarda determined the period of the star's reappearances at eleven months; This Frisian astronomer is often credited with discovering the Mira variability. At the same time, Johannes Hevelius observed the peculiar star, calling it "Mira" —in the sense of "wonderful" or "astonishing"— in the Historiola Mirae Stellae of 1662, because its behavior deviated from the of any other known star. Ismail Bouillaud estimated his period at 333 days, which is less than a day different from the currently accepted period of 332 days.

There is considerable speculation as to whether Mira had already been observed before Fabricius. The story of Algol (β Persei)—certainly known as a variable in 1667, though different legends show that it had been watched with suspicion for millennia—suggests that Mira may have been known in antiquity. Karl Manitius, translator of Hipparchus of Nicaea's Commentary on Aratus, suggests that certain lines of that text from the 2nd century B.C. C. may be about Mira. Other catalogues, such as those of Ptolemy, Al-Sufi, Ulugh Beg and Tycho Brahe do not mention it, not even as a "normal" star. There are three observations from Chinese and Korean archives, from 1596, 1070, and 134 BCE. C. —the same year that Hipparco de Nicaea would have made the observations of it — which suggest that the star could already be known at that time.

Currently, Mira is the prototype for a class of variables named after it, Mira variables.

Physical characteristics

Image of Mira in ultraviolet light, where you can see the trace left by the star.

Mira is a red giant of medium spectral type M7IIIe; this varies between M5 and M9 —moment in which its temperature and brightness are lower—. As a consequence of its variability, it is problematic to define its temperature and size, since these parameters depend on the moment of the cycle in which the measurement is made and on the wavelength used; consequently, its luminosity is not unambiguous either. The relative proximity of Mira allows, however, to measure its angular diameter. This allows us to calculate its radius, which varies from 2 AU in visible light, to approximately double that in infrared light. Considering a surface temperature of 3,000 K, its luminosity can be estimated to be approximately 8,500 times solar luminosity—including a large amount of energy emitted as infrared radiation.

Mira is in the last phases of her stellar evolution. Billions of years ago it was a star similar to the Sun, but, having exhausted its hydrogen and helium fuel, it has become a highly distended and luminous star. Its variability comes from pulsations on its surface, changes in the size of the star —which can amount to 15% in each pulsation— that also affect its temperature and luminosity.

Observations carried out with the GALEX space telescope in the ultraviolet region have revealed that Mira leaves a trail of matter coming from its outer layers, creating a trail 13 light-years long —about three times the distance between the Sun from the nearest star, Proxima Centauri—formed over 30,000 years or more. A shock wave of plasma or compressed gas is thought to generate the wake; This shock wave results from the interaction between Mira's stellar wind and the gas in interstellar space, through which the star moves at a high speed —130 km/s. The mass of Mira's "trail" is estimated to be about 3,000 times that of Earth.

Ultimately, the lost material will form a planetary nebula, while the stellar remnant will condense into a white dwarf about the size of our planet.

Star System

Mira forms a binary system with a companion, Mira B, resolved in 1995 by the Hubble Space Telescope. Distant 70 AU from the primary, ultraviolet and X-ray images show a spiral of gas coming from Mira in the direction of Mira B. The orbital period of this companion is ~400 years.

Mira B is surrounded by a protoplanetary disk, formed from material from Mira's solar wind. Mira B is probably thought to be a K-type orange dwarf with a mass of about 0.7 solar masses, and not a white dwarf as initially believed. However, in 2010, further investigation indicated that Mira B is in actually a white dwarf.