Groups of Galaxies

Galaxies are grouped into clusters, typically tens of millions of light years across, with hundreds or thousands of members. They are held together by their mutual gravitation, and each galaxy moves around the centre of gravity of the cluster. Their speed of movement shows that they experience a greater gravitational pull than can be accounted for by the observed galaxies. Some of this missing mass consists of dim galaxies too faint to be detectable and some in dark intergalactic gas which emits X ray that can be detected. But there must be much more “dark matter” than has so far been accounted for. Furthermore, current theories of the origin of the universe suggest that there must be a hundred times as much matter in the universe as astronomers have so far detected.

Interacting galaxies

 A typical galaxy will have had a collision or a near miss with another galaxy half a dozen times in its life so far. Pair of galaxies that are in the process of near collision can be observed now. One pair is called the Mice because of the streamers of stars, resembling mouse tails that they are pulling from each other; similar features in another pair have led to the galaxies being dubbed the Antennae.

One galaxy can score a direct hit on another and pass right through. The individual stars are much more widely spaced in relation to their diameters than galaxies are, and they do not collide. But gas clouds in the two galaxies can collide and generate intense radiation, thus spawning millions of new stars.  Such objects are called “starburst’ galaxies. When the intruding galaxy has departed,

Structure of the galaxy

The globular clusters contain the galaxy’s oldest stars and consequently look red dish, because many of their stars have expanded to become giants or super giants. The clusters orbit the galaxy’s centre, and often pass through the disc, but there are no collisions among the stars, which are enormous distances apart, even within the relatively closely packed globular clusters. There is almost certainly a vast amount of undetected”dark mater” spread through the halo of every galaxy. It might consist of huge numbers of brown dwarfs – too dim to be seen – but many astronomers think this matter consists of undiscovered types of subatomic particles.

The stars near the centre of the galaxy are also old and reddish, and relatively little gas were left over here from the process of star formation. But there are gas clouds in a central region of ten light years or so in diameter that can be probed only with radio telescopes. The clouds are violently agitated by some object or objects at the centre with a mass of a few million Suns. That object may be a black hole, swallowing mass of the equivalent of ten suns a year, or the remains of super massive stars that may have exploded there within the last 100 million years or so.

The disc of the galaxy is 100,000 light years across. The sun lies about two thirds of the way out from the centre, and takes about 230 million years to orbit the galactic centre. Two or perhaps four tightly coiled spirals arms are marked out by bright bluish stars. The arms are regions where gravitational effects are believed to cause ripples of slightly increased density though this is poorly understood. Stars and gas clouds pass through the arms but their passage is slowed. Star birth is triggered here. Though all kinds of stars are born, the minority of short lived; fast burning blue hot massive stars are conspicuous, marking out the arms.

Evolution of Galaxy

 The solar system is located in a spiral galaxy known as the Milky Way. The galaxy formed from an ill defined globe of hydrogen and helium gas approximately 12 to 14 billion years ago. This proto galaxy gradually shrank under its own gravitational attraction. As it did so, smaller concentrations of gas condensed within it. These were the basis of globular clusters of stars, each orbiting the centre of what was to become the galaxy. Each in turn broke onto hundreds of thousands of smaller knots of gas, which evolved into stars.

 The bulk of the proto galaxy collapsed into the centre without forming part of a globular cluster. As the gas collapsed, it whirled faster round the centre, as water whirls down a plughole. Most of the matter formed a huge flattened globe – the galaxy’s central bulge. Most of this gas broke up into stars, but some was spun out into a great rotating disc, 100,000 light years across. The rotation of the disc slowed the condensing of gas into stars, which continues today.