Open Clusters age into "Moving Groups"

[Widdekind]

Work-Night Rough Draft

 

 

Stars are born from Giant Molecular Clouds (GMCs), in big "batches" that become -- upon the condensation & dispersal of the parent GMC -- Open Clusters. Now, "stars inside an Open Cluster are at first tighly packed, moving at the same speed around the center of the Galaxy"*. Such Open Clusters typically persist for several hundred million years, before becoming disrupted**.

*

Open Cluster - Reference Library - redOrbit

**

Sun's siblings may have seeded Earth life - space - 26 September 2008 - New Scientist

;

cf

.

http://www.astronomyforum.net/planetary-forums/84613-how-find-suns-sibling-stars-exo-planets.html

Now, the Ursa Major Cluster is an aged (~500 million years old*), and partially disrupted Open Cluster.

After

half a billion years or so

, a classic

Open Cluster

such as the

Pleiades

or the

Hyades

(both in

Taurus

)

tends to be disturbed by external factors

(such as

Molecular Clouds

passing by)

, setting its stars moving at slightly different speeds, and so causing them to drift apart,

exactly like the one in

Ursa Major

has done

.

 

When this happens,

the

[Open] Cluster

becomes a stream of stars

, not close enough to be a cluster but all related and moving in very similar directions at similar speeds...

 

Sirius is a former member of this

[Open] Cluster

and our sun is in the outskirts of what is called

The Ursa Major Stream

, a group of stars that are all ex-members of the

Ursa Major Cluster

spanning over

a thousand light years

in space

**

.

These partially disrupted post-Open Clusters are called Moving Groups, and can be up to 2 billion years old***. But, typically,

After a billion or so years

, the

Cluster

is totally lost. Some stars will be on the far side of the galaxy, some on the near

. The sun's original cluster is like this, there is no way to tell which are former members and which just happened to have formed at the same time but somewhere else.

Indeed, "extremely dense Open Clusters can stay together for longer, but no Open Cluster could stay together for the age of the Sun, five billion years**". However, the HR 1614 Moving Group could be about 5 to 6 billion years old#.

*

Ursa Major Moving Group - Wikipedia, the free encyclopedia

**

Open Cluster - Reference Library - redOrbit

***

Moving group - Wikipedia, the free encyclopedia

#

http://adsabs.harvard.edu/abs/1992AJ....104.1906E