Saturday, January 14, 2023

Mira variables

R Aquilae is a variable star visible in the constellation of Aquila 780 light-years away from Earth. At its maximum brightness, the star has an apparent magnitude of +5.8 and it is at the limit of visibility with the naked eye. At minimum, the star has an apparent magnitude of +12.0.

R Aquilae is classified as a Mira variable. This class of stars are red giants in the last stages of their life that within a few million years will eject their surface layers into space, forming planetary nebulae and becoming white dwarfs.

The Mira variables are pulsating stars: the entire star contracts and expands. This involves a variation in the radius and surface temperature and consequently in the brightness of the star.

R Aquilae on average has a diameter 25% greater than the solar one and is more than 3000 times brighter than our star.

The pulsations of the Mira variables occur on a time scale greater than 100 days. Usually the pulsation period is constant, but in some stars the period is variable.

R Aquilae is an example of the latter subclass. In fact, when it was discovered in 1915, its magnitude varied cyclically over a period of 320 days. Its period has now been reduced to 264 days. It is not yet clear what this additional effect is due to, but it is believed that it may be related in some way to inhomogeneities in the distribution of hydrogen fusion reactions inside the star's core.

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