An open cluster is a group of close stars, not gravitationally bound together. It consists of a few, up to thousands of stars. The most known open cluster near us is M45 or Pleiades.
This article uses the Pleiades as a model, with a few adaptations for other open clusters.
An open cluster is theorized to be formed from a cloud of gas and dust inside a galaxy. This automatically implies that most of the stars have the same age. Therefore, in a young cluster you should find many active B - type stars and a lot of active Brown Dwarfs. An average-age cluster should contain no or almost no B - type stars, but Red giants and an increasing population of White dwarfs. Old clusters are thought to be ripped apart from gravitational interferences from other cosmic structures.
The stellar population of a star cluster is very interesting to study.
- O - type stars, the brightest of all, are absent or very rare. Their lifetime is too short to survive until the nebula is cleaned-up and stars become visible.
- B - type stars are seen in young clusters, like the Pleiades. Interesting is the fact that there are only 8 B stars and about 500 other types of stars inside the Pleiades, but they produce all the visible light seen from Earth. Given the size of their cluster, all 8 are visible from anywhere within in the day sky. During night, they provide some light, so that you can see where you are going.
- Neutron stars, remnants of B - type stars, are found in many clusters.
- A - type stars are the second source of light of a cluster, but they are the second rare type of stars.
- F - type stars are the third source of light, but also the third most rare type of star.
- G - type stars are detected in all star clusters. They are the closest relatives to our Sun.
- White dwarfs are frequent in older clusters, where they form a significant part of stellar population.
- K - type stars are detected too, in large numbers, even if their light is faint.
- M - type stars are the most abundant of all stars, forming in all clusters over 70% of population. Despite being so many, their dim light makes observations very difficult. From the 1000 stars found in the Pleiades, only 200 are not M - type stars.
- Brown Dwarfs are detected in young clusters, where they are hot enough. Older clusters should contain only cold T - type and Y - type dwarfs.
Stellar density Edit
The Pleiades consist of about 1000 stars and have a radius of 8 light years (LY) (2150 cubic LY). This means there is one star in each 2 cubic LY. Compared, around the Sun, we have 6 stars within 8 LY, which implies a density of one star per 200 cubic LY.
A density of stars 100 times higher means automatically that the sky will be full of visible stars. You will see the 8 bright stars both in day and night, together with many more. The dim M - type stars will be visible only if they are very close, less then 0.5 AU. Still, the multitude of other stars will create a sky like never seen.
The presence of many stars and mainly of B - type stars has another implication. Their solar winds are strong and will interact with winds of nearby stars. This phenomena will affect objects inside Kuiper Belts. Also, a solar system that is less then 1 AU from one of the brightest stars will receive significant light so that lightest elements from Oort Clouds could start to sublimate. If a dim M - type star is at 0.5 AU from a B - type star, planets in its Kuiper Belt will receive more light and heat from the B - type one. This creates confusing situations.
Stellar density is 100 times higher then around the Sun, but still not too much. Stars should be at 1 to 1.5 LY away between each other. This means orbits around stars are safe, at the same time for inner, outer planets and Kuiper Belts. Not the same can be said about Oort Clouds, which don't have space to fit.
There is a high chance for all types of planet to exist in an open cluster, with no exception. The major difference is that, since the Pleiades host 100 times more stars, there should also be 100 times more planets.
Each solar system can develop on its own, each system will have its own planets that are almost undisturbed by gravity and radiations from other systems.
Terraforming and colonization Edit
Inside an open cluster, the same terraforming methods used elsewhere should work. Colonies could be formed anywhere and many planets should be suitable for terraforming.
The major difference comes when we talk about interstellar travel. with many nearby stars, many of them offering planets that are suitable for terraforming, settlers will quickly spread through an open cluster. There will be strong economic and commercial ties between systems. In time, this will unite all states and colonies within a cluster, into a large federation, in a natural way.