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STELLAR SKY

 

I. In moonless cloudless night in the sky the uncountable quantity of stars sparkles, drawing to itself attention of the person. Really, from any point of the Earth surface it is possible to see in the sky with the naked eye about 3000 stars. To distinguish and remember them, in the remote past people in different parts of the Earth united stars in groups by their relative position and assimilated the received figures to animals or various subjects and gave them names. So the first names of constellations were created. Really, the combination of the bright stars making Hercules constellation is similar to a figure of the person who has gone down on his knee; Taurus is similar to horns of a bull; Swan and Eagle are similar to a bird floating with spread wings. It is undoubted that names of the Scorpion, the Snake and the Hydra constellations appeared because of resemblance of their outlines to these reptiles. At the same time the names of some constellations have the astronomical meaning. For example, the name of the Scales constellation (Libra) was given because the Sun on the day of an autumnal equinox was in this constellation (more than 2000 years ago). Some constellations are called in honor of known characters of ancient myths, for example, the Perseus (the Hero), the Sagittarius (the Archer), the Dragon, the Centaur, etc.

The names of some constellations reflect the way of life, philosophical and religious ideas of the people which have thought them up. The Russian scientist D. O. Svyatsky who studied the national astronomy, wrote in his works that the steppe Kazakhs represented the constellation of the Great Bear (the Ursa Major) in the form of celestial «racer» and the constellation of Little Bear (the Ursa Minor) in the form of «lasso» by means of which the racer is tethered to «a celestial stake» — the Pole star or Polaris (fig. 122). The Turkic people name these seven stars due to their outline reminding a scoop the Zhetikarakshi (Seven horse thieves) or the Zhetigen (Seven), Mongols name them the Darhan Burkhan. (Seven gods or Seven aged men), Bashkirs name them the Seven girls, Estonians and Russians name them the Cart, inhabitants of Siberia name them the Deer and the name the Great Bear came from ancient Greeks. Each people have the legends explaining the origin of these names. According to the Kazakh legend these seven stars are the «seven thieves» hiding in the sky from pursuit of people, while the Ancient Greek legend tells that these seven stars form «the bear» that the god Zeus rescued from hunters and lifted on the sky.

48 names of the constellations known since Ancient Egypt, Babylon and Ancient Greece come down from a remote past to the present days. Ancient Greek astronomers Gipparkh (apprx. 180 or 190 - 125 BC) and Claudius Ptolemaios (apprx. 90 - apprx. 160 BC) classified the name of constellations, made the first lists of stars and gave their description. But those lists contained constellations visible only to the people living in the Northern hemisphere of the Earth. Other stars were grouped and marked on celestial maps by the travelers, who visited the Southern hemisphere, only in the XVI - XVIII centuries.



Constellation is the part of the celestial sphere with definite borders.

There are 88 constellations on the celestial sphere. The borders between constellations are defined by the special decision of the International astronomical union (IAU), but they have no physical sense. In the northern hemisphere of the sky there are 31 constellations, in the southern - 48, on the equator - 9. The names of constellations at the international level are used in Latin; their names in national languages are given in translation from Latin. For example, the Great Bear is the Ursa Major (in abbreviated form — UMa), Orion (Ori), Lion (Leo), Cassiopeia (Cas) *.

II. Majority of bright stars have their own names originating from the remote past and reflect the feature of the star (the Sirius — «the brightest», the Aldebaran — «subsequent», «following») or relation to constellations to which they belong. For example, in the known treatise «Canon of Mas'uda» of the great Khorezm astronomer Abu Reykhan Mohammed ibn Ahmed al-Biruni (973 - apprx. 1050) the four stars out of the seven in the constellation of the Great Bear were named after

their location on the pattern of the bear: Merak – «the loins of the bear», Megrez – «the root of the tail», Phecda — «thigh», Mizar – « girdle ». Biruni translated the names of Ptolemaios system of stars into the Arabic language and slightly changed them.

In 300 astronomical names of stars 15% are of the Greek, 5% of the Latin, 80% of the Arab origin.

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* The list of names of stars and constellations is available in a manual to this textbook: Bekbasar N. Practical astronomy. Almaty: Mektep, 2005.

The people of Central Asia including Kazakhs from old times had their own names of stars. There are several names the Kazakhs use for the double star in the constellation of Great Bear (the second star on its «tail» - Mizar and Alcor): Dzhigit and the Girl, Kyran-karakshy (the rapacious robber) or Zheke-batyr (the lonely warrior) with his wife by the name of Ulpildek (Fuzzy, Fluffy). In the Arab national astronomy Mizar and Alcor together are called the «Horse and Rider». In the Indian astronomy they are known as the «old woman and child». The Pole star for the Turkic and Mongolian people is known as Temirkazyk (an iron stake) and Altynkazyk (a gold stake). The name of this star came from its visible immovability that made it to be the "world axes" round which all other stars «tethered» to it rotate. According to a widespread Kazakh legend two horses Akbozat (Kochab) and Kokbozat (Phecda) are tethered to Temirkazyk (the Pole star). They «graze» all night long round this stake. The « masters » of these «horses» are in the sky in the form of two bright stars located on both sides of the Great Bear with names Aydason (Capella) and Aglason (Vega). A celestial guard Kuzetshi (Thuban) watches two celestial racers giving no opportunity to Seven horse thieves to steal them. If robbers steal these horses, they will attack the constellation of Urker (Pleiades). Then a doomsday will come...

In 1603 Johann Bayer (1572 - 1625) started designating bright stars of each constellation by letters of the Greek alphabet in decreasing order of their brightness. These designations are still used nowadays.

III. Having looked at the night sky we will at once notice that brightness of stars is various –the brightest stars in the sky are Sirius, Arcturus and Vega whereas Alcor has a weak light. The brightness of stars numerically determined by a quantity called apparent stellar magnitude.

This quantity was for the first time introduced by the Ancient Greek astronomer Gipparkh. He divided all visible stars into six groups, the brightest stars are the ones of the first apparent stellar magnitude and the stars hardly distinguishable with the naked eye belong to the group of the sixth apparent stellar magnitude.

In the middle of the XIX century the English astronomer Norman Pogson offered a modern scale of stellar quantity. In this scale the difference in five stellar quantities corresponds to change of brightness of a star by 100 times. So the one stellar magnitude difference makes the brightness to differ times.

Apparent stellar magnitudes are designated by m (from Latin magnitudo meaning «size»).

In astronomy another quantity is used, that is stellar brightness which means the illuminance produced by a star on the plane that is perpendicular to beams of light at the point of observation.

Measurements of brightness of celestial bodies in stellar magnitudes, done by using precise methods and special devices – photometers, showed that it has various values. Therefore the brightness of the majority of stars is expressed in fractional and negative values of the stellar magnitude.

The apparent stellar magnitude of the brightest star in the sky - Sirius is m =-1.58, of the Sun is m =-26.6, of the full Moon is m =-12.7.

The stellar brightness being bigger for one stars and smaller for the others doesn't give an objective information about a star that can be at a near distance and therefore have the big apparent stellar magnitude while the same star can be at far distance and have small apparent stellar magnitude. Therefore the true brightness of stars and other celestial bodies is determined by another astronomical quantity called absolute stellar magnitude.

Absolute stellar magnitude M is the apparent stellar magnitude of a star at the standard distance of 10 parsec or 32.6 light years (to know about distance units: parsec and light year see § 46). At this distance the brightest star in the sky Sirius would have the stellar magnitude M =+1.4, the Sun +4.79, whereas the Betelgeuse and Rigel - 6.0 and -7.0 respectively.

 

Table 9

Astronomical characteristics of some stars

Names Short (Bayer) designation Right ascension, a Declination, d Apparent stellar magnitude, m Absolute stellar magnitude, M
Sirius a CMa 6h 45m -16o43¢ -1.58 +1.41
Arcturus a Boo 14h 16m +19o11¢ -0.05 -0.2
Vega a Lyr 18h 37m +38o47¢ 0.03 +0.5
Capella a Aur 5h 17m +46o00¢ 0.08 -0.6
Rigel b Ori 5h 15m -8o12¢ 0.12 -7.0
Procyon a CMi 7h 39m +5o14¢ 0.37 +2.65
Betelgeuse a Ori 5h 52m +07o24¢ 0.42 -6.0
Altair a Agl 19h 51m +08o52¢ 0.77 +2.3
Aldebaran a Tau 4h 36m +16o31¢ 0.85 -0.7
Algol b Per 3h 08m +40o57¢ 1.12 -0.3
Regulus a Leo 10h 08m +11o58¢ 1.35 -0.6
Castor a Gem 7h 35m -26o19¢ 1.58 +0.85
Polaris a Umi 1h 49m +89o02¢ 2.02 -4.6

 

 

Self-testing questions

1. What is the constellation?

2. How many constellations are in the sky?

3. How are the stars in constellations designated?

4. What does the stellar magnitude define?

5. What star (tab. 9) is the brightest and what is the weakest?

6. What star from the list in the tab. 9 would shine brighter and what star would shine weaker than the others if to place them at identical distance from the Earth?

 

 

Exercise 30

 

1. How many times the star of the zero stellar magnitude is brighter than stars of the first magnitude?

2. Stellar magnitude of the Polaris is m = 2.02, of the Vega is m = 0.03. What of the stars is brighter?

 

Experimental assignment

 

In clear night find in the sky the constellations: a) the Ursa Major, the Ursa Minor and the Cassiopeia. Pay attention to their location relative to the Polaris, draw the pattern; b) find the stars from the tab. 9 that are visible in the sky at the time of observation and define to what constellations they belong.

 


Date: 2015-01-12; view: 219


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