Annotations of elementary astronomical observation


The Pleiades open cluster

Posted by: bitacoradegalileo on: July 21, 2011


Open clusters are groups of tens, hundreds or thousands of stars originated in the same cloud of gas. Often the gas is gone, in whole or in part, because the radiation emitted by the brightest stars and his appearance is that of an irregular splash of bright spots, with no general structure and asymmetric.

The hosted stars are generally young (a few hundred million years; compare with the 4,000 million of the Sun), massive and very hot.
They are scattered on sizes in the order of thirty light-years, and are slowly dispersed by the tidal effect produced by the center of the Milky Way,so that over time, -millions of years- the stars that form it will be mixing with the rest of the galaxy. Open clusters are also known by the name of galaxy clusters.
We bring here, as prominent examples, from top to bottom in this order, the Ptolemy’s Cluster in the constellation Scorpio, the spectacular double cluster in Perseus and the Manger (Praesepe in Latin), or the Beehive Cluster in Cancer.
But today we will focus on the beautiful Pleiades star cluster in the constellation of Taurus, which is undoubtedly the most famous, brilliant and beautiful of them all.


TaurusTaurus is the Zodiac constellation invented by the ancient Babylonians, whose Latin name (Taurus) means bull. Stands out in thewinter sky, between Aries to the west and Gemini to the east, its predecessor and successor in the Zodiac, respectively. To the north are Perseus and AurigaOrion‘s southeast and to the southwest the constellation Eridanus river and Cetus, the Whale.

In this picture we see the night sky in the city of Buenos Aires (34 º S latitude, the same as Cape Town, South Africa), in the morning twilight of June 30th. All the stars that appear are in the ortho, ie, emerging from the eastern horizon, and their way to their transition to the north. Hamal, the main star of Aries, is the highest, while Gemini is still below the horizon. The inhabitants of the Northern Hemisphere will notice the position of Orion, just the opposite of how they normaly see it in their usual latitudes, as the stars representing the feet of the giant hunter, Rigel and Saiph are those which are higher, while Sirius and Aldebaran are at opposite positions as they are seen by the boreals. The Moon is in conjunction with the Pleiades, no strange thing, as will see.
The principal star of this constellation is Aldebaran (Alpha Tauri),a red-orange giant of magnitude +0.87, which is the thirteenth brightest from around the night sky, El Nath (Beta Tauri) or Al Nathis a blue-white giant of magnitude +1.68. In this constellation are famous, as well as the Pleiades, the Crab Nebula (M1), that we see on the right, and the Hyades, another open cluster in the same line of sight of Aldebaran, but this star is not part of the cluster,because it is located halfway.

The open cluster of the Pleiades (M45)

The Pleiades (Pigeons in Greek), is an object visible without optical aid, located at the side of the constellation Taurus. Receivesalso the name of Messier 45 or M45, as Charles Messier included it correspondingly numbered in his catalogue of disturbing objects in 1769. Other names that have been popular tradition are The Seven Sisters or The seven goats.

This is a group of stars, a total of approximately 500, very young (about 100 million years), which is located approximately 400 light-years away, this figure oscillates depending on the sources consulted.The cluster is about 12 light years in diameter, and occupies an angle of about 110 minutes of arc in the sky, equivalent to nearly four times the Full Moonstarring frequent conjunctions with it, as shown on the left, as well as with the other planets in the Solar System, because it is located at is only 5 ° from the ecliptic (the line describing the moon and all planets in its orbit). This circumstance produces beautiful effects of rapprochement between the stars, which are used by enthusiasts ofastrophotography. To the right the Pleaides are in conjunction with Venus.
The brightest stars in the cluster are blue-white color and have a size equivalent to five times the Sun.

The principal stars are Alcyone (magnitude 2.87), Atlas (3.63), Electra (3.7), Maia (3.87), Merope (4.18), Taygeta (4.3), Pleione (5.09), Celaeno (5.46), Tau 18 (5.64), Asterope I (5.76) and Asterope II (6.43).

The cluster is surrounded by a nebula, which becomes more evident in the proximity of the brightest stars, and reaches its greatest prominence wrapping Merope, a fact which has important consequences in the mythology around this beautiful cluster, as we will see. The cluster stars split gradually, and it is estimated that within 250 millions of years will not exist as such, separating and becoming individual stars (or multiple if applicable).


The Pleiades are mentioned in the Mahabharata, the Iliad and the Odyssey of Homer, and three times in the Bible. They form the basis of the Mayan calendar, to whose tradition they are 400 warriors whose souls were put into the sky after being killed by the arrogant Zipacna.

For the classical Hellenistic tradition, the Pleiades are the seven daughters of Atlas and Pleione, sisters among others of the Hyades which, after being persecuted for years by the giant Orion, appealed for help to Zeus, who turned them into pigeons, after which they rested on the back of the Bull (Taurus constellation), which thus constituted the guardian of the sisters. Merope, embarrassed after being raped by the giant, is hidden behind its own nebula.

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