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Dsos not on the map

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That's a very good question. Scientifically a cluster would contain stars of a similar age that are gravitationally bound, siblings so to speak. But how you identify that from a random group of stars I don't know. I saw M39 for the first time tonight and it looked nothing out of the ordinary to me, maybe I need a bigger scope or darker skies ....

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An Open Cluster is a group of stars but more widely spaced apart with no core, but it does contain many stars, but I do see your point, one thing you can do is consult a very good star map such as Sky Atlas 2000, I have Uranometria 2000 which is excellent, but a bigger scope and dark skies perhaps ? :)

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gravitational bonding/interaction/chemical composition, parts of Ursa Major are classed as a group.

from wiki

All stars in the Ursa Major Moving Group are moving in roughly the same direction at roughly the same speed, contain roughly the same mix of metals, and, based on stellar theory, appear to be roughly the same age. This evidence suggests to astronomers that the stars in the group share a common origin. The Cataloge number is Cr285.

Ursa Major Moving Group - Wikipedia, the free encyclopedia

Edited by Si W
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Reason I am asking I was trying out an app that connects my scope to my iPod and just below the star I was looking at (57 herc that's what the app calls it) there was a group of stars that reminded me of the beehive cluster, all stars are listed as yellow but not listed as a cluster. I saw about 6 stars, most around 7 mag, I dont have great seeing conditions.

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Hello Manok,

Your question is a very valid one. Originally (historically) astronomers using telescopes noted closely spaced groups of stars and noted them as "clusters". However, since the 20th century, many scientist have contributed to our knowledge of stars. For example, the work of Slipher and Hubble using doppler shifts to establish velocity and proper motion of stars allows us to define the motion of a star through space. The work of Hertzsprung and Russell (of H-R diagram fame) using spectral class to identify stellar similarities in mass and age. Later work by Russell & Vogt using spectral class led to the Russel-Vogt theorem that determine the age of a star from its mass and composition. The development of techniques like speckle interferometry and luminance/brightness relationship are now used to determine a star's distance.

With all these tools, the meaning of "star cluster" has changed. In the 21st century, we demand that an astronomer can prove that all stars in a cluster are:

1. Of similar age and composition, indicating a likely common origin from a singular molecular cloud of H/He gas and dust. Each cloud has a slightly different composition which, like a DNA sample, indicates that the stars are "related" in this way.

2. Gravitationally bound together (similar overall motion through the galactic environment) and show gravitational interaction, ie: they affect each others motion about a common center of mass for the cluster.

3. Are at a common location in space - ie: that they lie at a similar distance from Earth and from the galactic center.

Obviously, the Ursa Major group, which shows all these characteristics, is non-obvious because it is so close to us. Something like the Hercules cluster M-13 is very obvious to us because it lies far enough away and is bound very tightly together. Proving that a group of stars belongs to a common cluster is often a great deal of work, and may be subject to dispute, requiring further research and evidence. This is the stuff that astronomy PhD's are made of! :)

You can look up the work of these folks I've mentioned. If you like, latch onto a copy of Astronomy: Journey to the Cosmic Frontier, used 4th and 5th editions are available very inexpensively on Amazon. I use this text in class and highly recommend it.

I hope that helps a bit!


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