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Star Spy: 'Groups' above

FREIDL HALE - TEKAPO STARLIGHT
Last updated 05:00 30/01/2013

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Horsehead Nebula

We can use the constellation Orion to locate an interesting and well-known celestial object that is not visible to our unaided eyes; the Horsehead Nebula is a popular example of the opaque clouds of dust and gas in our galaxy.

An enormous region of sky around Orion’s belt and sword, is filled with various types of dark and glowing nebulosity. The objects in this image are just a small part.

The eastern-most star in Orion’s Belt, the right-hand most star in the base of the Pot, is called Alnitak.  The Horsehead Nebula is just south of (above) this star. Left of the Horsehead in the photo is the glowing Flame Nebula.

Fraser used different filters to photograph the Horsehead with his Takahashi FSQ85 ED telescope.

He found that the monochrome image above, taken using a hydrogen alpha filter, contains sharper detail than he gets by combining the images produced using other filters. 

What is a Constellation?

Several different dictionaries are telling me that a constellation is a group, especially a group of stars. 

And a group, I read, is primarily “a number of things close together”.

A group of stars then, a constellation, should be a number of stars close together.  But this is not generally the case. 

Objects we have looked at such as the Matariki/Pleiades Cluster and 47 Tucanae qualify as groups of stars close together. They are not constellations.

Matariki is an open cluster of hundreds of stars born together from the same cloud of dust and gas as the cloud collapsed. 47 Tucanae is a globular cluster of millions of stars held together in a ball by the their mutual gravity.

Constellations, on the other hand, are something quite different. Yes, they consist of a number of stars, but those stars are usually not especially close together.

Take a closer look at the stars in the Southern Cross as an example. What we see is a pattern of four stars in the sky, and the word 'pattern' is the key. 

The four stars make the shape of a cross or kite in the sky. But are they a ‘group’ of stars close together?

The star at the top of the cross, called Gamma Crucis or Gacrux, is just 87 light years away from us. The other three are near 300 light years away. 

The pattern we observe only exists in our line of sight with the four stars. They are not together in the sense of a cross-shaped ‘group’. 

This is an important distinction for us when looking up at the night sky; the realisation that the sky has tremendous, almost unimaginable, depth. 

We hand down our interesting stories about the animals, hunters, rivers, gods and other shapes we find in the sky. In fact, sometimes it seems to be not at all about the stars, rather all about the story. 

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For example, the small dog, Canis Minor. It consists of just two stars! The larger dog, Canis Major, at least bears some passing resemblance to a dog and in the stories is Orion’s large hunting dog. Canis Minor could be a pencil or a fence post.

Finding patterns in the shape of familiar objects among the myriad twinkling lights in the night sky is a human pastime, ancient and modern.

The constellations have entertained us and provided an easily remembered map to find our way around the sky, as well as upon the oceans of Earth.

They are shapes, creations of our imaginations, and they link us to ancient people who also looked up, fascinated, and wanting to understand the lights in the sky at night.

- Stuff

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