The Pleiades, the Seven Sisters, or more officially M45 and Melotte 22, is a group of stars in the Taurus constellation. It is an open star cluster that we find at 03h 47m 24s right ascension and +24° 07′ 00″ declination. It is only about 445 light-years away from us.
The Pleiades is the jewel of the winter sky for the northern hemisphere and mid-Southern-latitude dwellers. With an apparent magnitude of 1.6, it is an easy target for the naked eye and binoculars. We can even observe it under a light-polluted city sky. It appears like a fuzzy blue haze with luminous stars. Under clear skies, we can easily distinguish seven stars. But we would not be the first, not at all. Many civilizations have already named and counted the brightest stars of the Pleiades. They even associated legends and stories to them, as much as their imagination allowed them to.
Although the ancients counted around 6-7 stars, with the advances of observation instruments, we can count more than just 7. The brightest stars (known as the Seven Sisters) and their apparent magnitudes are listed below, going from the brightest to the dimmest:
- Alcyone (2.86)
- Electra (3.7)
- Maia (3.86)
- Merope (4.17)
- Taygeta (4.29)
- Celaeno (5.44)
- Sterope / Astrope (5.64 / 6.41) a double star system.
To this list of brightest stars, astronomers added Atlas (apparent magnitude 3.62) and Pleione (a variable star with fluctuating brightness ranging between 4.8 and 5.5). These stars are more difficult to observe with the naked eye than the other seven.
Since the Pleiades have been observed by most ancient civilizations, it does not have a discoverer. However, the first person who observed it through a telescope was Galileo Galilei in 1610. He counted then 36 stars.
But, again, this is not all. In reality, the cluster contains about a thousand stars that are gravitationally bound. We can already see dozens of them in this image, taken by a ZWO Kamera ASI 1600 MM-Cool V3 Mono
mounted on a Celestron Rasa.
These stars formed about 100 million years ago. Most of them are B-type stars. They are blue, very hot, and bright. B-type are so energetic that their life is relatively short. The stars in the Pleiades open cluster are surrounded by reflection nebulae, which gives them their hazy appearance. These nebulae, as their name suggests, reflect the light that the cluster’s stars continually emit, but do not emit light on their own. These clouds are composed of gas and dust.
One of the most spectacular nebulae in the Pleiades cluster is that near Merope. The constant bombardment from the star slowly erodes the cloud. A composite image of visible and infrared observations show some wispy features due to the light of Merope. This radiation tends to slow down the smallest dust particles leaving them behind the heaviest ones bravely facing the flux of energetic photons.
But these are not the only mysteries that infrared images revealed about the Pleiades. In fact, deep infrared observations of Spitze Space Telescope and Gemini North Telescope detected a significant concentration of hot dust particles in the circumstellar disc around the star HD 23514, a star from M45 that is almost the size of the Sun but slightly hotter. It is the same age as the other stars of the cluster. Astronomers think that this dust could reveal the presence of planetesimals, which could be a sign of the current formation of an exoplanet, especially that at 100 million years old, the star is mature enough to start forming planets.
Indeed, B-type stars that compose the beautiful Pleiades have relatively short, a few million to a few hundred million years typically. But, fortunately for stargazers, they will still adorn our skies for a long time.