Colour changes visible in Mira

In these days, MIRA, the most famous variable star in the sky, is once again visible at the naked eye: it is literally capable of appearing and disappearing from the sky more than any other variable star.

Hevelius called it Mira – the “Marvelous” – probably in reference to the words that David Fabricius wrote to Kepler in 1609, after rediscovering it 15 years after its first sighting: “Res Mira!”, what a marvelous thing!

Given the immutability of the sky as imagined at the time, this was truly incredible; yet centuries later, being able to see an extra dot shining in the sky with the naked eye still has the same appeal today.

Even more so when we discover that this variation in brightness also corresponds to a variation in colour: Mira can literally change from copper red to golden yellow. And it does so in the space of a single month, as shown by these images – in the form of scribbles – taken with the same instrument and the same parameters, tapping the telescope to better capture the colour of the point-like star.

Fabricius, who described it as “reddish like Mars”, could not notice this colour change because he could only see the star with the naked eye. However, Hevelius did so in the 18th century: “the colour has changed in a sense, from white to dusky, or red”; and W. Sadler Franks continued to monitor this in the XX century.

Mira then, is doubly marvellous because it can change colour.

Mira is NOT the only star in which it is possible to admire a colour variation: as I have been able to observe in recent years, other variables also show something similar, such as Chi Cygni, R Leonis, S Cephei, W Orionis, V Cygni, V Hydrae and a few others.

In Mira and very few others, however, this colour variation can also be appreciated with small instruments.

Dobson Stargate telescope Sky-Watcher 18" 285x and Huawei p30 pro ISO1250 0.3s.

Mosaic of two snaps.