Cepheus

When Cassiopeia is high up in the northeast, if we look

Cepheus

Star

Name

Magnitude

Distance (Light-years)

a

Alderamin

2.44

46

P

Alphirk

3.23

750

8

Variable

1337

e

4.20

98

c

3.60

717

just west of it we can see a moderately bright star, Alpha Cephei (mag. 2.44) in the constellation of Cepheus, the Sea Monster. We can easily locate it if we extend the line joining the stars Alpha and Beta of Cassiopeia towards west. The constellation of Cepheus is not very prominent because apart from Alpha Cephei, it has no star brighter than the fourth magnitude. But on a clear moonless night we can easily make out the pentagonal shape of the constellation.

Although it has no bright star, Cepheus has one remarkable star Delta Cephei, which is the first of a kind of variable stars which astronomers use for measuring distances. The regular periodic variation of its brightness was discovered in 1784 by an English amateur astronomer named John Goodricks, who was deaf and dumb but had a keen power of observation. Known as Cepheid variables, the period of variation of brightness of such a star is known to be proportional to its absolute brightness. The brighter the Cepheid variable the longer will be its period of variation, that is, it will take longer to change from dim to bright to dim again.

Like all Cepheid variables, the brightness of Delta Cephei varies with remarkable regularity, changing from magnitude 3.51 to 4.3 every 5 days 9 hours. If we want to see it ourselves there is an easy way. Near Delta Cephei, just to the west of it, we can see two stars Zeta Cephei (mag. 3.6) and Epsilon Cephei (mag. 4.2). The two stars have magnitudes matching the brightness of Delta Cephei when it is brightest and dimmest respectively. So, when at its brightest Delta Cephei will be as bright as Zeta Cephei and when dimmest it will be dimmer than Zeta Cephei and almost as bright as Epsilon Cephei. It can be real fun watching if we are lucky to have a week or so of clear, dark nights. Cepheus culminates at around 9.00 p.m. during the third week of October.

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