Stargazing: Cepheus, the father of Princess Andromeda

Northern Circumpolar
This chart shows the stars that can be found high in the northern sky, as seen from mid-northern latitudes, including Great Britain, and depicts the constellations surrounding Ursa Minor (the Little Bear) and its prominent leading star, Polaris, the Pole Star. As well as the familiar constellations Ursa Major (the Great Bear), Ursa Minor (the Little Bear) and Cassiopeia, less well known groups grace this area of sky, including the large but relatively inconspicuous Cepheus.

Located at or around the overhead point (as seen from mid-northern latitudes) during autumn evenings, Cepheus is not the most eye-catching constellation. However, it does have a fairly distinctive shape and is a familiar sight to backyard astronomers. As with many of the constellations, Cepheus doesn’t particularly resemble the character it depicts, this being King Cepheus, the husband of Queen Cassiopeia and father of the beautiful Princess Andromeda.

Cepheus is one of the oldest star patterns, and featured in the catalogue of constellations compiled in around 150AD by the Greek astronomer Ptolemy, the list he produced detailing the 48 constellations that were known to, and recognised by, the astronomers of the time. The group can be located by using the two end stars in the ‘bowl’ of the Plough as pointers. Depicting the hindquarters and tail of the Great Bear, the seven main stars forming the Plough are arranged rather like a gigantic heavenly spoon and form what is probably the best known pattern of stars in the heavens. During autumn evenings, the Plough can be seen fairly low down in the north-western sky. 

To find your way to Cepheus, first of all locate Merak and Dubhe, the two end stars in the ‘bowl’ of the Plough. Extending an imaginary line from Merak through Dubhe will take you through Polaris and on to Alrai. This is one of the stars forming the main shape of Cepheus and from here the rest of the group can be picked out.

Marking the right shoulder of Cepheus is Alderamin, the brightest star in the constellation and which shines at magnitude 2.45 from a distance of around 50 light years (read more about magnitudes here). To the north of Alderamin is Alfirk, which lies at a distance of nearly 700 light years and marks the waist of Cepheus. Alfirk is actually a double star and those of you with a small telescope may be able to resolve the two stars that make up the Alfirk system, although you will need to look carefully as the companion star is quite faint.

Some of the stars in Cepheus have colours that can be detected with binoculars, including the yellow-orange tint of the magnitude 3.21 orange giant, Alrai, the star which marks the left knee of Cepheus and whose light has taken around 45 years to reach us. Another orange giant star is Iota Cephei (for more about star names see here) although, being slightly fainter than Alrai, its colour is a little more difficult to make out. The main outline of Cepheus is completed by the orange super giant star Zeta Cephei, the light from this star having set off towards us over eight centuries ago.

One star of particular interest is the red super giant Mu Cephei, which lies near the southern border of Cepheus and, although its exact distance is not known, is thought to shine from between 2,500 and 3,000 light years away. The colour of Mu is very distinctive, so it’s much easier to make out than other stars in Cepheus, and even a pair of binoculars will bring it out very well. The astronomer William Herschel described this star as being ‘a very fine deep garnet colour’, leading to it being popularly known as Herschel’s Garnet Star. As well as being one of the most luminous stars known, Mu Cephei is also one of the largest of the naked-eye stars. Indeed, it is so huge that, if it were put in the place of our own Sun, its surface would extend out to well beyond the orbit of Jupiter. Cepheus

A lunar eclipse for early risers

Rather than producing its own light, the Moon shines because its surface is lit up by the Sun’s rays, lunar eclipses occurring at Full Moon when the Sun and Moon are opposite each other in the sky and the Moon’s orbit carries it into the Earth’s shadow. At first it may be thought that a lunar eclipse takes place every month, although this is not the case. The reason for this is that the plane of the Moon’s orbit around the Earth is inclined a little to the plane of the Earth’s orbit around the Sun. This means that the lining up of the Sun, Earth and Moon isn’t always exact, the upshot being that the Moon usually passes either slightly above or below the Earth’s shadow at the time of Full Moon.

During a total lunar eclipse, totality takes place when the Moon lies completely inside the umbra, or darkest part, of the Earth’s shadow, at which point the sunlight which normally illuminates the lunar surface is cut off and our satellite is plunged into darkness. However, it is only rarely that the Moon disappears completely from view. A small amount of sunlight is usually bent, or refracted, onto the lunar surface by the Earth’s atmosphere, resulting in the Moon taking on a deep coppery-red colour.

On the early morning of 28 September, the Moon passes through the Earth’s shadow and undergoes a total lunar eclipse that will be visible from the UK and Ireland. The main part of the eclipse begins when the Moon enters the umbra at 2.05am, from which time the Earth’s shadow will be seen slowly creeping across the lunar surface. Full totality occurs between 3.10am and 4.23am, the Moon finally leaving the umbral part of the Earth’s shadow at 4.23am. Let’s hope that the sky is clear and reasonably cloud free!

If you happen to be trekking through a Dark Sky Discovery Site, or even wandering around your own backyard, take a look at Cepheus, as well as setting your alarm clock for the early hours of 28 September, so you can watch the lunar eclipse. Happy stargazing!

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