Pleiades

The Pleiades, also known as the Seven Sisters and Messier 45 (M45), is an asterism of an open star cluster containing young B-type stars in the northwest of the constellation Taurus. Located about 444 light-years from Earth, it is one of the nearest star clusters to our planet and the closest Messier object that can be seen with the naked eye in the night sky. This cluster includes bright stars and beautiful reflection nebulae such as NGC 1432 and NGC 1435, also called the Merope Nebula.

The Pleiades is made up mostly of hot blue luminous stars that formed within the last 100 million years. These stars shine brightly and create a stunning view in the night sky. Even in places with lots of city lights, called light pollution, the Pleiades can still be seen.

Scientists believe the Pleiades started out much closer together, similar to the Orion Nebula. Over time, the stars have spread out a bit, and they are expected to continue moving apart. In about 250 million years, the Pleiades will no longer look like a cluster because the stars will be too far apart to appear close together in the sky.

Along with another famous group of stars called the Hyades, the Pleiades forms what is known as the Golden Gate of the Ecliptic. People often compare the shape of the Pleiades to a tiny dipper, but it is not the same as the "Little Dipper" in the constellation Ursa Minor.

Origin of name

The name "Pleiades" comes from Ancient Greek. It likely started because the cluster helped people know when it was safe to sail in the Mediterranean Sea. In Greek stories, the Pleiades were seven sisters. Over time, people said they were the "daughters of Pleione," a mythical mother, but the name actually began with how the star cluster helped with sailing.

Astronomical role of M45 in antiquity

The Pleiades, also known as M45, played an important role in ancient times for creating calendars. Its special shape in the night sky near the ecliptic made it easy to recognize. Long ago, around 2330 BC, this group of stars marked a key point in the year.

The Pleiades appeared on ancient objects like the Nebra sky disc from around 1600 BC. Many ancient cultures, including India, Mesopotamia, Greece, and Arabia, used the Pleiades to mark the start of their calendars. Even though its position in the sky changed over time, it remained important for people in many places.

Nomenclature and mythology

The Pleiades have been recognized for thousands of years by many cultures around the world. People from places like Celts, Filipinos, Hawaiians, Māori, Indigenous Australians, Persians, Arabs, Chinese, Japanese, and many others have given the Pleiades special names and meanings. In Hinduism, they are called Kṛttikā and linked to stories about gods and goddesses.

The Pleiades appear in ancient writings and artworks, like the Nebra sky disk from around 1600 BC. Ancient Greeks wrote about them in poems, and they are mentioned in the Bible. In Japan, the cluster is known as Subaru, which means "to cluster together." This name is used for the Subaru Telescope in Hawaii and also appears in the logo of Subaru cars. In the stories of J. R. R. Tolkien, the Pleiades are called Remmirath, the netted stars.

Observational history

Galileo Galilei was the first astronomer to view the Pleiades through a telescope. He discovered that the cluster contains many stars too dim to see without a telescope and published his observations in 1610.

The Pleiades have always been recognized as a group of stars closely related to each other. Studies of how the stars move across the sky showed they all move together, confirming they belong together. Charles Messier included the Pleiades as "M45" in his catalogue of special objects in 1771.

Distance

The distance to the Pleiades helps astronomers measure distances to faraway objects in space. Because the Pleiades is relatively close to Earth, scientists have tried many ways to find its exact distance. Different tools and methods have given slightly different answers, but most recent measurements suggest it is about 135 parsecs away.

Knowing this distance helps astronomers understand how far other star clusters and even galaxies are from us, which is important for learning about the universe. The Pleiades remains a key example for testing these distance-measuring tools.

Composition

The Pleiades cluster has more than 1,000 stars, with bright blue stars that can sometimes be seen with the naked eye. These stars form shapes similar to the constellations Ursa Major and Ursa Minor.

The cluster also includes many brown dwarfs, such as Teide 1. Brown dwarfs are smaller than stars and do not shine brightly, but they are easier to study when they are young like those in the Pleiades.

Members

The brightest stars in the Pleiades cluster are named after the Seven Sisters from Greek mythology: Asterope, Merope, Electra, Maia, Taygeta, Celaeno, and Alcyone. These stars are also linked to their parents, Pleione and Atlas. The ancient Greeks did not give these stars individual names, but later astronomers did. These names were first used in a book from 1665 and became official in the 19th century.

Age and future evolution

Scientists try to figure out how old star clusters are by comparing them to models of how stars change over time. For the Pleiades, they think it is between 75 and 150 million years old. One way to estimate its age is by studying the smallest stars, called brown dwarfs, which can keep a special material called lithium. By looking at these brown dwarfs in the Pleiades, scientists believe it is about 115 million years old.

The Pleiades is slowly moving toward the feet of the constellation Orion. Eventually, the cluster will not stay together because of gravity pulling its stars apart. It is expected to spread out in about 250 million years due to interactions with big clouds of gas and the spinning arms of our galaxy.

Reflection nebulosity

With larger telescopes, you can see a faint glow around some of the stars in the Pleiades, especially in long-exposure photographs. This glow is called a reflection nebula, caused by dust reflecting the blue light from the hot, young stars.

Scientists used to think the dust was left over from when the cluster was formed, but now they believe the cluster is moving through a dusty area of space. The dust isn’t spread out evenly; it’s mostly found in two layers along our view of the cluster.

Possible planets

Astronomers using the Spitzer Space Telescope and Gemini North telescope found that a star in the Pleiades cluster, called HD 23514, is surrounded by many hot dust particles. This dust might be the early stages of planets forming around the star.