Cetus

Cetus-lowThe constellation Cetus (the Whale or Sea Monster) represents the sea monster sent by Poseidon, the god of the sea, to which the beautiful princess Andromeda, daughter of King Cepheus and Queen Cassiopeia of Ethiopia, was to be sacrificed (see Perseus). As we have seen, Perseus saved the day (and Andromeda into the bargain) by killing the monster, following which it was placed in the sky where it can be seen to this day alongside the other characters depicted in the legend.

The constellation can be found to the south and south east of Pisces, and the star Alrescha in Pisces is included on this chart (as well as on the chart showing Northern Autumn / Southern Spring stars) as an aid to identification. Cetus is best placed for observation during October and November and can be seen in its entirety from virtually anywhere south of central Canada, northern Europe and northern Russia with at least part of the group visible from any location on the planet.

The brightest star in Cetus is Deneb Kaitos, a magnitude 2.04 yellow giant whose light has taken around 95 years to reach us and which depicts the sea monster’s tail. Slightly fainter is the magnitude 2.54 red giant Menkar. Shining from a distance of around 250 light years, this star derives its name from the Arabic ‘al-minkhar’ meaning ‘the Nostrils’. Named from the Arabic ‘batn qaitus’ meaning ‘the Sea Monster’s Belly’, the orange giant star Baten Kaitos shines at magnitude 3.74 from a distance of 234 light years.

The light from magnitude 3.49 Tau Ceti has taken just 11.9 years to reach us, making it one of the closest stars to our solar system. Located a short way to the north west of Tau is Dheneb which, at magnitude 3.46, shines from a distance of around 120 light years. Southernmost of the circlet of stars forming the head of Cetus is magnitude 3.47 Kaffaljidhma, the light from which set off towards us around 80 years ago.

The most famous object in Cetus is the long period variable Mira, the star which gave its name to all the other variable stars of its kind. Mira also holds the distinction of being the first variable to be discovered. On 13th August 1596 the Dutch astronomer David Fabricus observed the star, mistaking it for a nova (a star that suddenly flares up to several times its original brightness before returning to its original state). It was next spotted in 1603 when the German astronomer Johann Bayer catalogued it as a 4th magnitude star, including it as Omicron Ceti in his star atlas Uranometria. Not long after this Omicron Ceti disappeared from view, only to reappear almost a year later. Subsequent observation finally revealed its true nature. Because Omicron Ceti was the first star known to vary in brightness, it was naturally regarded as being highly unusual, and the Polish astronomer Johannes Hevelius named it Mira, meaning ‘wonderful’, in his book Historiola Mirae Stellae published in 1662 and in which he describes the star.

Mira-type variables are long period variable stars whose brightness oscillates over periods of several months. They are all pulsating red giants, their amplitudes (ranges in brightness from maximum to minimum) averaging out at around 5 or 6 magnitudes, although some are known to vary by as much as 9 or 10 magnitudes. The variations are by no means regular and substantial differences between successive cycles of variability occur. Their periods can be anything from less than 100 days to 700 days or more, although successive periods can differ markedly. Mira itself varies between around magnitude 3 (slightly fainter than Deneb Kaitos) to magnitude 9 or 10 although on one occasion, during observations made by William Herschel in 1779, it almost reached 1st magnitude. At other times however it has barely attained 4th magnitude. In addition, its period of variability, although averaging out at 331 days, has been known to vary dramatically, durations as short as 304 days and as long as 355 days having been recorded.

Located at a distance of around 400 light years, Mira is the brightest of the long-period variables and only moderate optical aid is required to follow its complete cycle. It is easily located roughly two-fifths of the way from Delta Ceti to Baten Kaitos as shown on the chart and at times it can be seen with the naked eye. However, for most of its cycle of variability it cannot be seen without the help of either binoculars or a small telescope.

Get to know the stars of Cetus and, once you have picked Mira out, keep an eye on it over the following few months and watch it as it slowly undergoes changes in magnitude. Continued observation of Mira over a period of several weeks will reveal these changes although, as is the case with any variable star which remains at or near minimum brightness for extended periods, Mira may well be out of view when you look for it. If this is the case, keep checking and it will eventually reappear, following which you can keep an eye on its progress.

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