samedi 6 septembre 2008

Cepheus 7

If not very bright, the constellation is still quite noticeable, just to the west of Cassiopeia's chair. The stars are mostly third and fourth magnitude.
The constellation has numerous binaries, several significant variables, and a few interesting deep sky objects.

Alpha Cephei is known as Alderamin ("The Right Shoulder"). In another 5500 years this will become the Pole Star.
Beta Cephei is called Alfirk ("The Herd"). This is a visual binary as well as a variable (see below).
Gamma Cephei is Er Rai (Shepherd). Long before Aldemarin becomes the Pole Star, this one will assume that title (around 4000 AD).
Delta Cephei is a prototype for one of the more significant types of variables (see below). The star is also a very fine binary with a colour contrast.
Mu Cephei is a brilliantly coloured star, a deep red which moved William Herschel to call it "The Garnet Star". The colour depends on the size of one's telescope; the larger scopes bring out an orange element. It is also a semiregular variable (see below).

Double stars in Cepheus:
The constellation has a number of very fine binaries, some quite easily resolved by small scopes, others rather more difficult.
Beta Cephei is a blue giant with a faint companion easily resolved: 3.2, 8; PA 250º, separation 13.6"
Delta Cephei is a fixed double, a yellow giant with blue companion: 3.8, 7.5; PA 191º, separation 41".
Xi Cephei is considered to be the most attractive binary in Cepheus, a blue-white primary and yellow (or reddish) companion that orbits every 3800 years: 4.4, 6.5; PA 275º, separation 8.2". 6.5;
Kruger 60 is a famous binary only 12.9 light years away, comprised of two red dwarfs. Observers had reported seeing flare-ups on the surface of the companion, which is orbiting the primary every 44.6 years.
AB: 9.8, 11.4; presently the companion is at PA 109º and separation 3.2".
The binary is less than one degree SSW of delta Cephei. Burnham has a finder's chart (p. 600).
Struve 2816 is a multiple binary, a very attractive triple:
AC: 6.3, 8.1; 121º, separation 11.7" D: 8.0; PA 339º, 19.9"
In the same field are Struve 2813 and Struve 2819, all centred in the middle of the large but faint diffuse nebula IC 1396, just south of mu Cephei.

Variable stars in Cepheus:
Beta Cephei is the prototype of a class of pulsating variables. These are hot, luminous, and very massive stars with a specral type of O9 to B3. The group is also called 'beta Canis Majoris' stars, for this star too is a member of the group. Indeed, it is the brightest member of the group.
The variations in visual magnitude are of very small amounts, barely as much as a quarter of a magnitude. Beta Cephei varies only 0.04 magnitude from its typical brightness of 3.2 and a period of 4 hours 34 minutes.
Delta Cephei is also the prototype of a class of variables.
Delta Cephei variables are immensely useful stars. Studies of the period of pulsation and the apparent magnitude led investigators to devise a method of gauging the distance to outlying galaxies (the so-called period-luminosity relation).
Delta Cephei varies from 3.48 to 4.37 every 5 days, 8 hours, 47 minutes, 31.9 seconds. This means its maximum brightness can be calculated with a reasonable degree of accuracy.
After the maximum is reached, a gradual diminuition of magnitude occurs over the next three days, only for the star to again increase over the following two days until it again reaches its maximum.
Comparing its magnitude with zeta, just to the west, will tell you if it has reached its brightest. Zeta has a visual magnitude of 3.4, while delta Cephei varies from 4.4 to 3.5. That is, for most of its cycle, it will be rather more dim than zeta, but as it reaches its maximum, it should appear to have a magnitude quite similar to its neighbour.
Mu Cephei is a semiregular supergiant that also varies roughly from 4.5 to 3.5, with a very long period: 730 days.
S Cephei is a long-period Mira-type variable, 7.4-12.9, with a period of 486 days. In the year 2000 the maximum should occur in late January or early February.
T Cephei is also a long-period Mira-type variable, ranging from 5.2 to 11.3 every 388 days. It should reach a maximum in the last week of December of 1999.

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