PLEIADES: HISTORY, ORIGIN AND COMPOSITION - ARTE - 2025

The Pleiades are an open cluster of stars, visible to the naked eye in the night sky in the constellation Taurus. The members of an open star cluster are linked through the force of gravity and originate from the same molecular cloud.

With the naked eye most observers distinguish six stars, although exceptionally people with very good eyesight can see seven: Alcyone, Electra, Atlas, Pleione, Maia, Taygeta and Merope. But there are many more than the telescope reveals.

Figure 1. The Pleiades. Source: Wikimedia Commons.

With the help of instruments dozens of them are seen. Thus, an astonished Galileo recorded 36 stars in 1610, although some estimates indicate that there are 3,000.

Since prehistoric times the Pleiades managed to attract attention. During the Bronze Age they were represented on the Nebra sky disk, found in Germany. Likewise, the Pleiades are mentioned in many ancient texts of civilizations around the world, always linked to local mythology.

For the Hindus they were six nymphs, for the Greeks they were the seven daughters of Atlas, the mythological titan who sustains the world, while the ancient inhabitants of Tahiti knew them as Pipirima.

New World astronomers also recorded its appearance, for example in sacred books such as the Popol Vuh of the Mayans.

The Incas considered their first annual appearance as the beginning of their new year and an indicator of what the harvests would be like during that year. And it is that the Incas, along with other ancient peoples, believed that their appearance at dawn, next to the nearby Hyades cluster, was a harbinger of rain.

Colloquially they are still called in many ways: the Seven Sisters, the Goats, the Seven Goats or simply the Seven.

Origin of the Pleiades

The Pleiades are estimated to be 100 million years old, and their stars were formed in the same way that they all do in the Milky Way and other galaxies.

It was from a large cloud of interstellar gas and dust, which at certain moments concentrated a very small part of matter at a point in space.

Where gravity was barely stronger, more matter began to agglomerate, narrowing the distance between the particles further and further. But by no means do they remain static. Every material particle has kinetic energy and if they get very close to each other, they begin to exert pressure to loosen and expand.

These two opposing forces, gravity that compresses, and pressure that expands, are those that end up giving life to the stars and activating the nuclear reactor at their center, which mainly transforms the element hydrogen, the simplest and most abundant in the universe, in other more complex elements.

Once the star's central reactor is operating, hydrostatic pressure and gravity find their equilibrium and the star shines, emitting energy in the form of radiation. How much? That will depend on the initial mass of the star.

Composition

Ancient peoples were not wrong in stating that the Pleiades are sisters, since they all come from the same region rich in interstellar matter: hydrogen, helium and traces of all other known elements on Earth.

Astronomers know this by analyzing the light of the stars, since the information of the elements that compose it is contained there.

The stars of the Pleiades all formed more or less at the same time and have the same composition, although their subsequent evolution will surely be different. The life of a star depends largely on its initial mass, the mass it has when it enters the main sequence.

The higher the mass, the shorter the life of the star, since it has to use up its nuclear fuel much faster than another with a lower mass. And the Pleiades are more massive than our Sun, which is considered a medium or rather small star.

Open star clusters such as the Pleiades are frequent in the Milky Way, where about 1000 of them have been identified. They are also present in other galaxies and are very interesting because in them astronomers can see the beginnings of stellar evolution.

Cumulus physical characteristics

The Pleiades open star cluster has the following characteristics, which it shares with other open clusters:

-Irregular shape.

-Thousands of relatively young or middle-aged stars.

-Composition similar to that of the Sun: hydrogen and helium mostly.

-Your stars are in the so-called main sequence of stars.

-They are located in the plane of the galaxy, near the spiral arms.

For this last quality, they are also known as galactic clusters, but do not confuse the term with galaxy clusters, which is another class of grouping, much larger.

As mentioned before, the Pleiades Cluster emerged about 100 million years ago or so, when the dinosaurs were not yet thought to be extinct. It is about 430 light years from Earth, although there is still some uncertainty about the value.

In reference to its size, the cluster spans approximately 12 light years and in image 1 they appear to be surrounded by a blue nebulosity, the result of light passing through the cosmic gas and dust around the stars.

It is not about remnant material from the formation of the Pleiades, but rather what they are finding in their path, since these stars are moving at a rate of 40 km / s and at this moment they are in a region full of dust. In 250 million years they will have drifted away and will be scattered through space.

The stars of the Pleiades

There are more types of stars present in the Pleiades cluster than we see shining on a clear night:

-There are young and middle-aged stars, blue, very bright and hot, much more massive than our Sun. They are the ones we see with the naked eye and others with telescopes.

-Brown enanas, which do not become stars, since their mass is very low and does not reach the critical value necessary to ignite the central nuclear reactor.

-White dwarfs, which are usually the remnant of stars that are very advanced in their evolution.

How to find the Pleiades in the night sky

It is very easy, since it is a very characteristic object. It is a good idea to have star charts on hand, which can be downloaded from the internet or through phone applications.

The Pleiades often appear on maps under the name of the Messier M45 catalog, an ancient catalog of celestial objects compiled in the 18th century by the French astronomer Charles Messier, still in use today.

Figure 2. To locate the Pleiades it is convenient to use the constellation of Orion as a reference. Source: Wikimedia Commons.

The best time to see the Pleiades is during the months of October, November and December. To locate them easily, the constellation Orion is looked for, which is very easy to identify, since it has the three bright stars as a belt.

Then an imaginary arrow is drawn on the belt that points to the red star on the head of the bull (Taurus) called Aldebaran. Next, in a straight line, are the Pleiades, a beautiful sight in the night sky.

References

1. EarthSky. Pleiades star cluster, aka Seven Sisters. Recovered from: earthsky.org.
2. Very interesting. How are stars named? Recovered from: muyinteresante.com.mx.
3. Pasachoff, J. 2007. The Cosmos: Astronomy in the New Millennium. Third Edition. Thomson-Brooks / Cole.
4. Seeds, M. 2011. Foundations of Astronomy. Seventh Edition. Cengage Learning.
5. Wikipedia. The Pleiades. Recovered from: es.wikipedia.org.