mercredi 16 juillet 2008

Taurus 11

The constellation shows mainly the horns, and exceedingly long horns they are. The left (southern) horn starts from the group of stars known as The Hyades, of which Aldebaran seems (erroneously) to be a member. It extends from Aldebaran to zeta Tauri, near the eastern edge of the constellation.
The right horn lifts up just west of the Hyades, from delta Tauri through tau Tauri and finally to its tip at beta Tauri (El Nath: remember this star as part of Auriga?)
The rest of the bull is rather disappointing; a slight body and two spindly legs. It may be that the bull is half-emerged in water, as it carries Europa across to Crete.
The stars of Taurus:

Taurus' eye is bright and piercing. This is Aldebaran (alpha Tauri), an orange giant about 40 times the size of the Sun. Aldebaran is an old star. For billions of years it has burned its supply of hydrogen until there is little left. Its future won't be as a spectacular explosion of a supernova but rather a gradual dimming into a white dwarf.
Following the lower horn out to its tip we find zeta Tauri. This is a shell star. Shell stars are main-sequence stars which rotate rapidly, causing a loss of matter to an ever-expanding shell.
Most of the interesting features of Taurus are found in the centre of the constellation and toward the west. Around Aldebaran are a number of stars which go by the collective name of The Hyades (see below).
Aldebaran is not a member of this group. Not only is it closer to us, but its proper motion is at a different angle. Aldebaran is moving at an angle of 161 degrees, the stars of the Hyades at around 102-109 degrees.
Double stars in Taurus

Taurus has an abundant selection of binary stars, including many Struve binaries that we haven't mentioned. Below is a very small selection of some of the easier doubles to resolve.
Theta2 and theta1 form a fixed binary of wide separation, theta2 just below and to the east. Note that theta2 is the primary: 3.4, 3.8; PA 346º and separation 337".
Kappa1 and kappa1 form an easily resolved binary: 4.2, 5.3; PA 328º, separation 5.3".
Sigma2 and sigma1 is another wide fixed binary. And again, sigma2 is the primary: 4.8, 5.2; PA 193º and separation 431".
80 Tauri is a difficult visual binary with an orbit of 189.5 years: 5.5, 8.0; current PA 17º and separation of 1.8" (very nearly its maximum separation).
Struve 422 is a wide visual binary with an orbit of over 2000 years: 5.9, 8.8; PA 269º, 6.7". It's located at 9º SW of nu Tauri, just north of the brighter 10 Tauri.
Variable stars in Taurus

Many of the more notable variable stars in Taurus are of a type not noticed by casual observation, such as alpha Taurus, which is classified as an Lb type variable. These are irregular giants whose variation can only be detected by means of photoelectric photometry. Alpha Taurus only changes in visual magnitude by 0.2, from 0.75 to 0.95, and the period is irregular.
BU Tauri (Pleione) is a gammaCas type variable, from 4.77 to 5.50. GammaCas variables are also characterised by an irregular period, which may sometimes be very rapid. These are B stars, quite young, and rotate very rapidly. This rotation results in the throwing off of material, which then forms a shell around the star. The cause of its variation is still not understood.
Zeta Tauri is also a gammaCas type variable, with a variation from 2.88 down to 3.17 roughly every 133 days.
Lambda Tauri, in the Hyades cluster, is a good example of an eclipsing variable. The variability is caused by the partial eclipse of the primary by its companion, dimming the 3.3 visual magnitude down to 3.8 every 3.95 days.
R Tauri is a Mira-type variable with a 320.9 day period. Usually at 7.6, it drops to a very dim 15.8 once a year. In 2000 the maximum should occur in the first week of May.

Aucun commentaire: