Clusters
are large groups of galaxies, often
several hundreds. Our own galaxy the Milkyway
is part of a cluster, that is called the Local Group. This cluster
comprises of about 30 galaxies of which the Milkyway and Andromeda are the
largest.
The quantities and distances in the Universe are unmeasurable large.
A galaxy comprises on the average, hundreds of billions of stars. A cluster
often holds more than five hundred galaxies and so, in total is formed out
of some hundred billion stars.
Clusters are of a gigantic size, if imagined as a sphere they would have a diameter
of approximately ten million lightyears.
The Local Group spans about 8,000,000 lightyears.
Sometimes the clusters are grouped in even larger structures called
super cluster, which are most probably the largest structures in
the Universe.
Empty space.
The Universe is not only comprised of clusters, a largest part of it
are area's where there are no clusters, sometimes called 'the free space'.
These free spaces are almost empty, while in a cluster millions of stars
may be present, a free space of the same size sometimes holds only one
single galaxy.
It is remarkable that the shape of the 'free' galaxies often deviates
from those inside the clusters. In clusters most galaxies are elliptical
and only a few are of the spiral
type. Also these spiral galaxies are often situated more at the outside
edges of the clusters.In the free space it is mostly the other way around, here we find more
spiral shaped galaxies.
Large collisions.
The galaxies roam around in the clusters with tremendous speeds. All
galaxies influence the others with their mutual gravitational
forces. Galaxies sometimes approach each other and get so close that they collide.
These collisions are coupled with enormous nuclear explosions, from which
new stars are formed.
From computer simulations it appears that out of a collision between
two spiral galaxies often an elliptical galaxy is formed. This explains
why most of the galaxies in the center of the cluster are elliptical, while
spiral galaxies are situated more at the edges.
This cluster in the Peacock formation is situated about 300 million lightyears away from us. The center is formed by larger elliptical galaxies. More to the edges we see many spiral shaped galaxies. Image: Royal Observatory, Edinburgh. Not clear enough? Mouse-over enlarges 2x. |
A large central
galaxy.
Many times the center of a cluster is formed by a gigantic elliptical galaxy. These large central galaxies often are hundreds of times more massive than normal galaxies. They are only found in clusters. It is assumed that these large galaxies are attracting and 'eating' other galaxies, due to there central position and enormous gravitational forces. In proportion to the growth of the central galaxy, it pulls more and more on the other galaxies. Clusters not only contain galaxies, but also very large quantities of gases, at temperatures of several millions degrees kelvin. These hot gases are very concentrated at the center. The total mass of gasses sometimes even exceeds the total mass of all galaxies in the cluster, often hundreds of billions sun masses. Besides the galaxies and gasses the clusters also contain a lot of dark matter. It is estimated that the quantity of dark matter exceeds that of the visible matter some ten times in some clusters. |
Brakes put on the expansion of the Universe.
By their internal gravitational forces the clusters released themselves
from the expansion
of the Universe. The Universe is continuously expanding as a result
of the Big Bang, the force
of the explosion of the Big Bang however is neutralized by these clusters.
While in general all galaxies and cluster are moving away from each other,
the galaxies inside clusters are moving towards each other, which is caused
by their mutual gravtitions. The larger the cluster, the stronger they
pull the brakes on the expanding motions of the galaxies. This is one of
the main reasons why astronomers do a lot of research on clusters, perhaps
they are the key to the solution of the question of the Big
Crunch.