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. It is one of the nearest star clusters to Earth, located about 444 light-years away, and is the nearest Messier object to our planet. This cluster is so bright and easy to see that it is the most obvious star cluster visible to the naked eye in the night sky.

The Pleiades contains over 1,000 stars, with the brightest six or seven easily visible without a telescope. It also includes special objects like the reflection nebulae NGC 1432 and NGC 1435, known as the Merope Nebula, as well as an HII region. These bright stars make the Pleiades visible from most places on Earth, even in areas with lots of light pollution.

Scientists believe the stars in the Pleiades are very young, having formed within the last 100 million years. They think the cluster will continue to exist for about another 250 million years before the stars drift apart due to gravitational forces from the rest of the galaxy. The Pleiades, together with another cluster called the Hyades, forms what is known as the Golden Gate of the Ecliptic.

Origin of name

The name Pleiades comes from Ancient Greek. It likely comes from a word meaning "to sail" because this group of stars helped people know when it was safe to start sailing in the Mediterranean Sea. In Greek stories, the Pleiades were seven sisters. Over time, people said the sisters were the daughters of a mother named Pleione. The star cluster’s name was used first, and the stories about the sisters and their mother came later to explain the name.

Astronomical role of M45 in antiquity

The M45 group, also known as the Pleiades, was very important in ancient times for creating calendars. Its special shape made it easy to spot in the night sky near the ecliptic. 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, found in Germany. Many ancient cultures used the Pleiades to start their calendars. In India, it was called Kṛttikā, meaning "Those that mark the break of the year." In Mesopotamia, early astronomy texts listed it as important. The Greeks called them the Pleiádes, a name that suggests a group or crowd. Arabs also began their old calendars with M45, naming it al-Thurayyā. Even though its position changed over time, it remained significant in many cultures.

Nomenclature and mythology

See also: Pleiades in folklore and literature

The Pleiades are easy to see in the winter sky, especially in the Northern Hemisphere. People all around the world have known about them for thousands of years. Different cultures have different names for them. For example, the Celts called them Tŵr Tewdws in Welsh and Streoillín in Irish. In the Philippines, they were known as Mapúlon, Mulo‑pulo, or Muró‑púro. The Hawaiians call them Makaliʻi, and the Māori call them Matariki. In Hinduism, they are known as Kṛttikā and are linked to stories about gods and festivals.

The Pleiades have appeared in stories and art for a very long time. One of the oldest known pictures of them is on the Nebra sky disk, made around 1600 BC. Ancient Babylon called them ^MULMUL, meaning 'stars'. In Greek stories, they are mentioned in famous poems like Homer's Iliad and Odyssey. In Japan, they are called Subaru, which means "to cluster together". The famous Subaru Telescope and even a car brand are named after them. In the stories created by J. R. R. Tolkien, they are called Remmirath, the netted stars.

Observational history

Galileo Galilei was the first astronomer to see the Pleiades using a telescope. He discovered many stars in the cluster that are too faint to see without a telescope. He wrote about his findings, including a drawing of 36 stars, in his book Sidereus Nuncius in March 1610.

People have always thought the Pleiades were a real group of stars, not just stars that happened to line up. In 1767, John Michell figured out that the chance of so many bright stars lining up by accident was very small — only 1 in 500,000. This showed that the Pleiades and other clusters are real groups of stars. Later studies showed that all the stars in the Pleiades are moving in the same direction at the same speed, which also proved they are connected.

Charles Messier added the Pleiades to his catalogue of objects that look like comets in 1771, calling it "M45". He included it along with the Orion Nebula and the Praesepe cluster. Some think he added these bright, easy-to-see objects just to have more items in his list than his rival.

In 1782, Edme-Sébastien Jeaurat made a map of 64 stars in the Pleiades based on his observations from 1779, and shared it in 1786.

Distance

The distance to the Pleiades is very important for astronomers because it helps them measure distances to other objects in space. Since the Pleiades is close to Earth, scientists have tried many ways to find out exactly how far away it is. Different tools and methods have given slightly different answers, but most recent measurements show that the Pleiades is about 135 light-years from Earth. Knowing this distance helps scientists understand how far away other stars and galaxies are too.

Main article: cosmic distance ladder
Main articles: Hertzsprung–Russell diagram
Further information: Hipparcos · Hyades · Coma Berenices cluster · Hubble Space Telescope · spectroscopic parallax · optical interferometric · Atlas · weighted mean · photometric · AB Doradus · Tucana-Horologium · Beta Pictoris · very-long-baseline interferometry · Gaia Data Release 3 · parsecs

Composition

The Pleiades cluster has a core about 8 light-years wide and stretches about 43 light-years across. It contains over 1,000 stars, many of which are young and blue. Up to 14 of these stars can sometimes be seen without a telescope, forming a shape similar to the constellations Ursa Major and Ursa Minor.

The cluster also includes many brown dwarfs, such as Teide 1. These are smaller objects that are not quite big enough to shine brightly like proper stars. They make up a large part of the cluster but do not add much to its total weight. Scientists study these brown dwarfs to learn more about how stars form.

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 also have two parent figures named Pleione and Atlas. The ancient Greeks did not give these stars individual names, but later astronomers did. The names we use today were first recorded by Giovanni Battista Riccioli in 1665 and became popular in the 19th century. These names are now officially recognized.

The table below lists details of the brightest stars in the cluster.

Age and future evolution

Scientists figure out how old star groups 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 guess the age is by looking at smaller objects in the cluster that still have a special material called lithium.

The Pleiades is moving slowly toward the feet of the constellation Orion. Eventually, in about 250 million years, the cluster will break apart. This will happen because the stars will move away from each other and because of the strong pull of gravity from big clouds of gas and the spinning arms of our galaxy.

Reflection nebulosity

With larger telescopes, you can see a glowing mist around some of the stars, especially in long-exposure photographs. Even small telescopes or binoculars might show a hint of this mist under good conditions. This mist is called a reflection nebula and happens when dust particles shine by reflecting the blue light from the hot, young stars.

Scientists used to think the dust was left over from when the star cluster was born. But now they believe the cluster is moving through a dusty area of space. The dust isn’t spread out evenly; it’s mainly found in two layers in the path to the cluster. These layers might have formed as the dust moved closer to the stars.

Possible planets

Astronomers using the Spitzer Space Telescope and Gemini North telescope found that one of the stars in the Pleiades cluster, called HD 23514, might be forming planets. This star, which is a bit bigger and brighter than our Sun, has lots of hot dust around it, which could be the early stages of planets being born.