Target Overview
The Pleiades (M45), also known as the Seven Sisters, is one of the most famous open star clusters in the night sky. It is located in the constellation Taurus and is easily visible to the naked eye, appearing as a small, bright patch of stars. The Pleiades is one of the closest star clusters to Earth, lying at a distance of about 440 light-years, and it is notable for its bright, blue stars, which are relatively young and hot. The cluster is surrounded by faint nebulosity, known as reflection nebulas, which add to its allure in photographs.
Basic Characteristics:
Type: Open star cluster
Designation: M45 (Messier 45), also known as the Seven Sisters or Subaru (in Japan)
Distance: Approximately 440 light-years from Earth
Apparent Magnitude: 1.6, making it one of the brightest open clusters visible from Earth
Age: The Pleiades is a relatively young star cluster, with an estimated age of around 100 million years.
Number of stars: The cluster contains more than 1,000 stars, although only a few are visible to the naked eye. Typically, six to nine stars can be seen without optical aid, depending on observing conditions.
Mythology and Cultural Significance:
The Pleiades cluster has been known since antiquity and holds significant cultural and mythological importance across many civilizations. In Greek mythology, the Pleiades are associated with the Seven Sisters, the daughters of the titan Atlas and the sea-nymph Pleione. The seven stars are named after the sisters: Alcyone, Maia, Electra, Taygete, Celaeno, Sterope, and Merope. They were placed in the sky by Zeus to protect them from the hunter Orion, who is represented by the nearby constellation.
In other cultures, the Pleiades have been referenced in folklore and celestial traditions. In Japan, the cluster is known as Subaru, which means "unite" or "come together," and is the inspiration for the name and logo of the car brand. Native American tribes, such as the Lakota and the Cherokee, also have legends tied to the cluster, often linking it to creation myths or stories about the stars.
Star Composition and Properties:
The Pleiades is made up of hot, blue, B-type stars, many of which are much brighter and hotter than our Sun. These stars are relatively young, having formed about 100 million years ago, and they are part of a loose grouping of stars that are slowly moving through space together.
Prominent Stars in the Pleiades:
Alcyone: The brightest star in the Pleiades, with a magnitude of 2.86. It is about 10 times more massive than the Sun and much hotter, with a surface temperature of about 12,000 K.
Maia: Another prominent star in the cluster, Maia is a B-type giant star with a magnitude of 3.87.
Electra, Taygete, Celaeno, Sterope, and Merope: These stars are slightly dimmer than Alcyone and Maia but still visible to the naked eye under good conditions.
There are several other stars in the cluster that are fainter and not visible without optical aid. In total, the Pleiades contains hundreds of stars, many of which are low-mass M-type red dwarfs and white dwarfs that are remnants of the initial star-forming event.
Nebulosity and Reflection Nebula:
One of the most striking features of the Pleiades is the faint reflection nebula that surrounds many of its stars. This nebulosity is caused by interstellar dust that reflects the light from the bright, young stars in the cluster. The blue color of the nebulosity comes from the scattering of light, which is more efficient at shorter (blue) wavelengths, similar to how Earth's atmosphere scatters blue light.
The nebulosity was once thought to be the remnants of the gas and dust cloud from which the Pleiades stars formed, but more recent observations suggest that it is actually unrelated to the stars themselves. The cluster is currently passing through a region of interstellar dust, and the dust is reflecting the light of the bright stars, creating the characteristic blue glow. This dust is likely part of the Taurus Molecular Cloud, a star-forming region in the same general area of the sky as the Pleiades.
Merope Nebula:
One of the most noticeable parts of the reflection nebula is the Merope Nebula, surrounding the star Merope. The nebulosity here is denser, creating a more visible glow, especially in long-exposure photographs. This portion of the nebula has been extensively studied because of its proximity to a bright star, allowing astronomers to explore the nature of reflection nebulae and the properties of interstellar dust.
Age and Evolution of the Cluster:
The Pleiades is a relatively young open cluster, with an estimated age of about 100 million years. In astronomical terms, this is young, especially compared to the Sun, which is about 4.6 billion years old. The stars in the cluster are still in the early stages of their life cycles, burning hydrogen in their cores. Eventually, the most massive stars in the cluster will evolve into red giants and later become supernovae or collapse into white dwarfs.
Open clusters like the Pleiades do not remain gravitationally bound for long periods. Over time, the stars in the cluster will drift apart due to gravitational interactions with each other and external forces such as the gravitational pull of the Milky Way galaxy. The cluster is expected to disperse over the next few hundred million years, with its stars eventually becoming part of the general stellar population of the galaxy.
Kinematics and Proper Motion:
The stars in the Pleiades are moving through space together as a gravitationally bound group, though the cluster is not as tightly bound as globular clusters, which have much higher star densities. The Pleiades stars share a common proper motion, meaning they move in the same direction through space at roughly the same velocity. This motion has been measured using both optical telescopes and space-based observatories like the Hubble Space Telescope and the Gaia spacecraft, which track stellar positions and movements with great precision.
Scientific Importance and Observations:
The Pleiades has been a subject of study for astronomers for centuries. Its proximity to Earth and the presence of bright, young stars make it an excellent laboratory for studying stellar evolution, star formation, and the dynamics of open star clusters. The cluster's reflection nebulosity is also of particular interest, as it allows astronomers to study the properties of interstellar dust and how it scatters light.
Infrared and Ultraviolet Observations:
Infrared observations, particularly with the Spitzer Space Telescope, have revealed more about the dust and gas in the Pleiades, particularly in regions where the reflection nebula is prominent. Observations in the ultraviolet, on the other hand, have provided insights into the young, hot stars of the cluster, revealing the high-energy processes occurring in their atmospheres and the surrounding regions.
Future Research:
With ongoing missions like Gaia, which is mapping the positions and motions of stars across the galaxy, astronomers are gaining even more precise data about the stars in the Pleiades. This will allow for more detailed studies of the cluster’s internal dynamics, its relationship with nearby molecular clouds, and its eventual dispersion over the coming millions of years.
Visibility and Observing Tips:
The Pleiades is one of the most easily recognizable objects in the sky. It is best seen during the northern hemisphere winter months, particularly from November to April. The cluster is visible to the naked eye, appearing as a small cluster of blue-white stars in the constellation Taurus. Under dark skies, the six or seven brightest stars can be seen without optical aid, though binoculars or a small telescope will reveal dozens of additional stars and the faint reflection nebulosity surrounding them.
In terms of size, the Pleiades covers an area of the sky roughly equivalent to the full Moon, making it a large and prominent feature. Because the stars are relatively bright, the cluster is an ideal target for beginner astronomers and astrophotographers.
Conclusion:
The Pleiades (M45) is a beautiful and iconic open star cluster, easily visible in the night sky and known for its bright blue stars and faint reflection nebula. With a history steeped in mythology and cultural significance, the cluster continues to captivate both amateur and professional astronomers alike. Its young stars, proximity to Earth, and intriguing nebulosity make it an important object for studying stellar evolution, star formation, and the behavior of interstellar dust. Over time, the Pleiades will disperse, but for now, it remains one of the most beloved celestial sights.
Geeky Details:
Captured November 7th 2021 from my Backyard in Erin, Ontario.
- 43 exposures at 5 minutes for a total integration time of 3 hours and 45 minutes.
- 25 dark frames, 25 flat frames, and 25 flat darks.
- ZWO ASI2600MC Pro camera
- Sky-Watcher Esprit 120 Telescope (120mm aperture, 840mm focal length, f/7)
- iOptron CEM70 NUC Mount