The Night Sky - Episode 4

The Pleiades

Deep in the heart of the constellation Taurus lies a curious little cluster of stars. A little cluster that has been known since antiquity and has taken on many names through the ages. A little cluster known most famously as the seven sisters of the heavens - the Pleiades.

This is episode 4 in my series on our night sky. View episode 3 here.

The Pleiades, also known by its other designation Messier 45 (M45), is located in the constellation of Taurus in our own galaxy, the Milky Way.

Here is an image of the Pleiades.

The Pleiades is an open star cluster containing a number of hot, middle-aged, B-type stars. A star cluster is a group of stars that have formed from the same molecular gas cloud and are all roughly the same age. B-type stars shine blue and are very luminous, making them relatively bright in the night sky. The stars of the Pleiades have all formed within the last 100-million years, making them much younger than our Sun.

The Pleiades is immediately visible to the naked eye and appears like a miniature replica of the big and small dipper constellations. It is at a distance of ~444 light-years from the Earth and is estimated to contain about 1000 stars. However, most of these stars need magnification to be visible. Seven of the brightest stars in the cluster are very bright and this gives the Pleiades its nickname - The Seven Sisters.

To understand where to find the Pleiades, we must first look for the constellation of Taurus.

Taurus (which is Latin for “the bull”) is a large, prominent constellation and is part of the Zodiac. It is mainly visible in the night sky during the winter months in the northern hemisphere. To find Taurus, we start by looking for another famous constellation — Orion. Three relatively bright stars, Alnitak, Alnilam and Mintaka, form a straight line representing the “belt” around Orion’s waist. Once we find the belt, we can quickly find the other stars that form the main outline of Orion. See this post for more information about Orion and the many wonders located within its boundaries. With Orion in our sights, we turn our gaze upwards to find a group of stars that looks like a “fallen over letter V”. This is the core of Taurus, the face of the bull. See if you can identify Orion and Taurus in the image above.

Taurus is an ancient constellation, used by early humans to mark the position of the Sun during the Spring equinox. It is home to two prominent star clusters - the Hyades and the Pleiades. The image below shows the region within Taurus where these star clusters are located.

Zooming in on our region of interest shows us how colourful the stars here actually are. The largest star in this region is the red giant Aldebaran (α Tauri), a star that is about 44 times larger than our Sun and 400 times brighter. It can be seen as the orange star in the southwest corner in the image below. Notice also how the stars Aldebaran (α Tauri), Prima Hyadum (γ Tauri) and Ain (ε Tauri) form a triangle depicting the face of Taurus (the face of the bull). These stars are part of a star cluster known as the Hyades. The horns of the bull extend out towards the south of the image.

The Pleiades is visible as a tight cluster of bright blue stars in the northwest corner of the image above. If we look closely, we can see that between 7-10 stars are distinctly visible. Here is a closer look at the cluster along with the names of the seven stars that gave it its name. These stars are named after the seven divine sisters of Greek mythology.

Surrounding the stars, we also see nebulous gases that are reflecting the light emitted by the stars in the cluster. Since most of the stars in the cluster are hot blue giants, the nebulosity is bluish as well. Spending even more time to capture more light (higher integration time) from this cluster will bring out even fainter details in the nebulosity. The two images below show the difference between using shorter and longer integration times.

The Pleiades is a deep sky object that has been known to ancient humans. Many cultures across the world independently recognized it, named it, and developed their own mythologies around it. Here are some of the other cultures that have named the Pleiades.

• The ancient Indians called it Krittika

• The ancient Jews called it Kimah

• The ancient Persians called it Parvī

• The ancient Japanese called it Mutsuraboshi (and in fact, Subaru, the Japanese car manufacturer is named after this star cluster and uses it as their logo)

• The ancient Chinese called it Mao

• The ancient Hawai’ians called it Makaliʻi

• The ancient Babylonians called it Mul

• The ancient Egyptians called it Ennead

• The ancient Aboriginal Australians called it Kungkarangkalpa

• And it was even known to the ancient Celts, Indonesians, Mayans, Aztecs, and many other cultures

View episode 3 in this series here.

All episodes in this series