A bright star is shining in the evening sky where none was visible just a few months ago. It is not a nova but an unusually brilliant apparition of Mira, a variable star with a great range in magnitude in the constellation of Cetus.
Most naked eye variables change in brightness by a fairly small amount. Mira is different because it can be seen easily with the unaided eye at its best but fades until it can only be seen in binoculars or a telescope when at minimum. At the moment it already seems to be brighter than magnitude 2.5 and still rising..
The name Mira means “wonderful” so it was clearly named for this impressive behaviour. Its catalogue name is omicron Ceti and it lies about 420 light years away from Earth.
Its variability is said to have been first spotted by the astronomer David Fabricius who mistook it for a nova in 1596. Today we know that Mira is the brightest in a class of star called the long-period variables – red giants of which around 6,000 are known.
Usually, Mira only reaches around magnitude 3.5 at brightest and may only get up to fifth magnitude in a cycle that lasts around 11 months. This year’s maximum is not due for another month and so will clearly be one of the brightest recorded.
The extreme observed range of brightness for Mira is 1.7 at its maximum and 10.1 at minimum with a period of about 332 days. This means that the maximum occurs about a month earlier every year. Unfortunately next months will happen when Cetus is close to the Sun in the sky.
At the moment, however, it is easy to spot. The way I always find it is to look for a pattern of three stars in the tail of Cetus which resemble a “false Aries” as they make a similar pattern to the zodiacal constellation that can also be seen nearby. Follow a line through the two stars making the longest side of this triangle for the same distance to the west and you will hit Mira.
I produced the accompanying chart with the free and excellent planetarium program Stellarium. You can find another map on the Society for Popular Astronomy website with links to observing charts carrying magnitudes of suitable comparison stars so that you can make your own estimates of Mira’s brightness. Or just take a look for yourself and marvel at one of the most wonderful stars in the naked-eye sky.