An extract on #travelawsome
Mira A is a well-known example of a category of variable stars known as Mira variables, which are named after it. The 6,000 to 7,000 known stars of this class are all red giants whose surfaces pulsate in such a way as to increase and decrease in brightness over periods ranging from about 80 to more than 1000 days.
In the particular case of Mira, its increases in brightness take it up to about magnitude 3.5 on average, placing it among the 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 1700 times. Since Mira emits the vast majority of its 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 the return to minimum taking twice as long.
Contemporary approximate maxima for Mira:
From northern temperate latitudes, Mira is generally not visible between late March and June due to its proximity to the Sun. This means that at times several years can pass without it appearing as a naked-eye object.
The pulsations of Mira variables cause the star to expand and contract, but also to change its temperature. The temperature is highest slightly after the visual maximum, and lowest slightly before minimum. The photosphere, measured at the Rosseland radius, is smallest just before visual maximum and close to the time of maximum temperature. The largest size is reached slightly before the time of lowest temperature. The bolometric luminosity is proportional to the fourth power of the temperature and the square of the radius, but the radius varies by over 20 % and the temperature by less than 10 %.
In Mira, the highest luminosity occurs close to the time when the star is hottest and smallest. The visual magnitude is determined both by the luminosity and by the proportion of the radiation that occurs at visual wavelengths. Only a small proportion of the radiation is emitted at visual wavelengths and this proportion is very strongly influenced by the temperature (Planck's law). Combined with the overall luminosity changes, this creates the very big visual magnitude variation with the maximum occurring when the temperature is high.
Infrared VLTI measurements of Mira at phases 0.13, 0.18, 0.26, 0.40, and 0.47, show that the radius varies from 332 R at phase 0.13 just after maximum to 402 R at phase 0.40 approaching minimum. The temperature at phase 0.13 is 3,192 K and 2,918 K at phase 0.26 about halfway from maximum to minimum. The luminosity is calculated to be 9,360 L at phase 0.13 and 8,400 L at phase 0.26.
The pulsations of Mira have the effect of expanding its photosphere by around 50 % compared to a non-pulsating star. In the case of Mira, if it was not pulsating it is modelled to have a radius of only R.