Pleiades: history, origin and composition

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 bound together by the force of gravity and originate from the same molecular cloud.

At first glance, 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 is much more that the telescope reveals.

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 have managed to attract attention. During the Bronze Age, they were depicted on the Nebra celestial disk found in Germany. The Pleiades are also mentioned in many ancient texts from civilizations around the world, always linked to local mythology.

For the Hindus, there were six nymphs; to 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 have also recorded its appearance, for example, in sacred books such as the Popol Vuh of the Mayans.

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

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

Origin of the Pleiades

The Pleiades are estimated to be 100 million years old and their stars formed the same way as the Milky Way and other galaxies.

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

Where gravity was hardly more intense, more matter began to clump together, narrowing the distance between the particles ever further. But by no means do they remain static. Every material particle has kinetic energy, and if they get too close, they begin to exert pressure to let go and expand. 

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

Once the star’s central reactor is in operation, hydrostatic pressure and gravity find their balance and the star glows emitting energy in the form of radiation. How much This will depend on the star’s initial mass.

Composition

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

Astronomers know this by analyzing starlight, as the information on the elements that make it up is contained there.

All the stars in the Pleiades formed at about the same time and have the same composition, although their subsequent evolution is certainly different. The life of a star depends largely on its initial mass, which it has when it enters the main sequence.

The greater the mass, the shorter the life of the star, since it has to use up its nuclear fuel much faster than a less massive one. And the Pleiades are more massive than our Sun, which is a star considered to be medium or quite small.

Open star clusters such as the Pleiades are common in the Milky Way, where they have identified about 1,000. They are also present in other galaxies and are very interesting because astronomers can see the beginnings of stellar evolution there.

Physical characteristics of the cluster

The Pleiades open star cluster has the following characteristics, which are shared with other open clusters:

-Irregular shape.

Thousands of relatively young or middle-aged stars.

-Composition similar to the Sun: hydrogen and helium mainly.

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

-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 don’t confuse the term with galaxy clusters, which is another type of much larger cluster.

As stated earlier, the Pleiades cluster emerged about 100 million years ago, when dinosaurs were not yet thinking about extinction. It is about 430 light-years from Earth, although there is still some degree of uncertainty in value.

In reference to their size, the cluster spans approximately 12 light-years, and in Image 1 they appear to be surrounded by a bluish haze, a result of light passing through dust and cosmic gas around the stars.

This is not material left over from the formation of the Pleiades, but what they are encountering in their path, as these stars are moving at a speed of 40 km/s and right now they are in a dusty region. In 250 million years, they will separate and spread out into space.

The stars of the Pleiades

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

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

-White dwarfs, which are usually remnants of very advanced stars in their evolution.

Finding the Pleiades in the Night Sky

It is very easy as it is a very characteristic object. It is convenient to have star charts available, which can be downloaded from the Internet or via phone applications. 

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

The best time to see the Pleiades is in the months of October, November and December. To locate them easily, the constellation of Orion is searched, which is very easy to identify as it has the three bright stars like 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. Then, in a straight line, are the Pleiades, a beautiful sight in the night sky.

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