What's That Star Called?

The Pleiades

Actually, this time it’s a bunch of stars. You’ve probably noticed them before and you may know their name – the Pleiades or seven sisters. It’s a tight grouping of stars visible in the winter sky. We can currently see them rising in the east after sunset. They aren’t spectacularly bright, but striking enough to have made their mark in many cultures’ skylore.

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The Pleiades are up by sunset, but won’t be easily spotted until after 7:00 or so at which time they’ll be more than the length of one fist held at the end of your arm above the horizon. On the nights of the 16th, 17th and 18th of November, the Moon passes the Pleiades, helping guide your eye towards them.

Check Skynews.ca for a detailed chart.

  The November full Moon is found in Taurus, near 1st-magnitude star Aldebaran. Courtesy Skynews.ca

The November full Moon is found in Taurus, near 1st-magnitude star Aldebaran. Courtesy Skynews.ca

Look to see at least 5 or 6 stars. Under really darks skies and with good eyesight, you can see many more stars there. A simple pair of binoculars, even small ones, will help reveal that there are hundreds of stars associated with the Pleiades.

Why? Well, these stars are connected together by gravity. Literally “sisters,” the stars formed together out of the same cloud of gas and dust. In fact, in long exposure photographs, the dust is still visible, reflecting light from the nearby stars tinting the entire cluster blue.

We explored how stars die looking at nearby Aldebaran, almost directly underneath the Pleiades in the evening sky. The Seven Sisters show us the opposite end of stellar life. These are young stars. Stars form inside giant clouds of gas and dust called nebulae.

Over time nearby stellar activity can perturb the gas in the cloud. Gravity then causes it to collapse and pull itself into a round ball. The gas at the centre of the ball becomes very compressed and hot, so much so that eventually, the atoms in the gas are stripped of their electrons, leaving only the atomic nuclei behind.

Having a positive charge, the nuclei naturally repel each other. But get them moving fast enough and they can overcome this repulsion and slam into one another forming heavier nuclei and releasing energy that we see as light. These nuclear reactions are what power a star, giving it the energy to fight against the gravitational collapse.

As all of this is going on, you can imagine that it creates even more disturbances in the nebula, triggering more collapsing gas and forming more stars. In other words, stars tend to form in groups, families if you like.

Long before this was determined by telescoping observation, people around the world admired this star cluster. The Seven Sisters were the seven daughters of Atlas the Titan who held up the sky and Pleione, a sea-nymph. As Atlas was otherwise occupied, many suitors pursued the sisters, including Orion the Hunter.

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The Inuit tell a story of pack of dogs chasing a bear. There are many stories from North American Indigenous groups about the stars being a group of playmates that wandered either wandered off, did not do their chores or were abandoned by the tribe. In China, the cluster represents the hairy head of the White Tiger of the West, one of the cardinal “four symbols” of the Chinese sky.

 But perhaps most well-known, at least to car drivers, is the Japanese name for this stellar group -- Subaru, meaning “gathering together.” This name was chosen to mark the 5 companies that merged to form it.

Astronomy Picture of the Day gorgeous photo of the Pleiades, also known as M45:

For a comprehensive site about the Pleiades, check out this page from the Arecibo Telescope’s website.

To see the Pleiades in many different wavelengths of light, go to Cool Cosmos