Melotte 22, Messier 45, Pleiades, Subaru, Chicken and Hen, Matariki and the Seven Sisters

The Pleiades are a fascinating star cluster that is easily spotted not far from the Hyades cluster in Taurus and is visible in the Southern Hemisphere summer, Northern Hemisphere winter.

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John Franklin-Adams was a British astronomer that lived from 1843 to 1912 and who created a photographic atlas of the night sky that was made up of 15 degree by 15 degrees squares of the entire night sky. He worked in both South Africa and the United Kingdom to cover the whole night sky and spent many years collecting the 206 photographs. After his death the atlas was published in 1914-1915 and another British astronomer Philibert Jacques Melotte (his parents were from Belgium) who lived from 1880 to 1961 studied this atlas to identify clusters of stars, including open clusters and globular clusters. He identified 245 clusters of significance that he published in 1915 as the Melotte Catalog. The catalog is very good for amateur astronomers as most of the objects are say to spot clusters that can be observed with modest equipment. Melotte was one of many people for many thousands of years who noticed the beauty and significance of a cluster of stars he named Mel 22. It is better known in Charles Messier’s “not a comet list” as M45 and still better known as Pleiades. This cluster of stars has been central to many cultures around the world in many stories and customs. But there is way more to this cluster than what people have described over the centuries, this cluster is a mix of some of the most interesting  stars there are.

M45 taken from Cassini around Saturn (Credit: NASA)

The cluster is one of the nearest to the Earth at about 440 light years distant so is ripe for observation. The visible stars of the cluster take up about 2 degree of the sky and are mainly the blue/white class B giants and some sub giants. When viewed through a telescope there are about 100 stars visible but through more powerful instrument the cluster is made up of around 1000 individual stars. The whole cluster is about 8 light years in size with it’s influence extending some 43 light years away. It’s a very young cluster, blue giants don’t tend to live very long, with astronomers estimating that it is about 100 million years old. Dinosaurs got to see the birth of this beautiful cluster – if they happened to be looking, some of our mammalian ancestors would have seen it too. Though it is appearing to travel about 1 degree every 60,000 years in the direction of Taurus so our little furry ancestors would have seen it in a different place. Though the cluster is made up of a heap of objects it is not all that heavy – at around the same mass as 800 of our Suns. This is because many of the objects in the cluster are white dwarfs, brown dwarfs and smaller stars like our own and only a few are the massive stars that we can see easily with the naked eye.

Some of the stars are rotating very rapidly which results in them having a rather squashed shape. The rapid spinning causes all sorts of problems for these stars and one, Pleione has ejected some of it’s out shell because of this rotation. This also offers an explanation for the large number of white dwarves that are in the cluster. Normally white dwarves are the remnants of stars that have gone through their complete stellar evolution after a few billion years – like what our own Sun will do eventually. The age of the cluster at 125 million years is simply not long enough to enable the formation of white dwarves so scientists have been looking for explanations as to how they are in such a young cluster. One explanation is that the rapid spinning and influence of other close by stars in the cluster has caused them to lose their outer shells and remain as cooling white dwarves that have rapidly spent the nuclear fuel in their core, but unable to continue through the stellar evolution cycle because they have lost so much mass. The age of the cluster is determined by what is not there, red giants. Blue giants burn hot and fast and rapidly advance through the stellar cycle so scientists can determine the age of the cluster by how far advanced through the cycle the stars are, and by how tightly packed the cluster is.

The white dwarves are not the only odd members of the cluster. Other objects are the brown dwarves in the cluster. These are objects that didn’t quite make it to be stars and are are about 13 to 90 times more massive than Jupiter and they have a huge range of energy outputs from being quite hot down to about as cool, in temperature, as a human. Because Melotte 22 is so close it has been a fertile hunting ground for these unusual almost stars, especially in the early days as astronomers wer learning more and more about them. As the cluster is so young the brown dwarves are a bit hotter and bigger than those found elsewhere so they have allowed scientists to understand a bit more about these strange objects and predict the evolutionary stages of them.

The other really interesting aspect of the cluster is that it is moving through space and is currently passing through a cloud of dust. This cloud of dust is what gives the distinctive nebulae around some of the bright stars. The patterns are created by the powerful radiation given off by the hot blue stars which sort of sorts the heavier particles of dust from the lighter ones. There’s also a wake behind the direction of travel of the cluster where the stars have cleared away interstellar dust during their journey.

Hubble Space Telescope zoomed in on the nebula in Pleiades (Credit: NASA)

At this time of the year, in New Zealand, there is quite a focus on this cluster of stars as it begins to rise in the middle of June just before sun rise and signifies the beginning of Matariki, also the name given to the cluster at this time of the year. This will hopefully encourage lots of people to look up and try and spot this beautiful cluster, and though not a lot will be visible because it’s not really in a very favourable position to observe, we can imagine the huge variety of the 1000 or more stars and the weirdness of the young white and brown dwarves.