Galactic Groups: The Evolution and Death of Galaxies
The Conveyor Belt Continues
As the galactic conveyor belt continued to bring galaxies together, they continued to collide, grow, and form larger galaxies. At some point one galaxy became so large its gravitational field dominated the smaller galaxies around it. They began to orbit around the large galaxy, to form a ‘local group.’
Local groups typically consist of 30 to 50 galaxies, most of them smaller irregular or elliptical galaxies orbiting a large spiral. Our Milky Way local group is somewhat unique in that it has two large spirals, our Milky Way and the Andromeda galaxy, orbiting each other. Some others also have two or more giant spirals, but that is not the norm.
Some local groups however contain only a few galaxies. The Seyfert Sextet contains only four galaxies. It is misnamed because in early telescopes it appeared to have six galaxies, but it turns out two of the galaxies thought to be in the group are well beyond the actual four.
As several local groups formed in astronomically neighboring space (they still were hundreds of millions of light years apart), mutual gravitational attraction began pulling the groups together. These neighboring local groups gradually formed ‘clusters.’ Our local group belongs to the Virgo cluster, so called because all the galaxies appear in the constellation Virgo. There are approximately 2000 galaxies in the cluster, and it spreads over 15 million light years.
As clusters grew, they, like groups, began to pull together to form ‘super clusters.’ Our Virgo Super Cluster is 33 megaparsecs in diameter, and contains at least 100 clusters. It is estimated there are 10 million super clusters in the universe.
Super clusters are the largest structures in the universe. This makes the universe ‘clumpy.’ This means that matter is not distributed evenly throughout the universe. It is clumped together in the super clusters. Between them there is true empty space.
Will the super clusters eventually clump together to form hyperclusters? That is a question cosmologists are not yet ready to tackle.
The Death of Galaxies
Within the super clusters there are all types of galaxies. The spirals continue to create new stars in their spiral arms. With the Hubble Space Telescope (HST) we have been able to prove this by measuring the color of the light from spirals. It is blue, meaning they are full of young, new stars.
Ellipticals, on the other hand, are red. In stellar evolution, red stars are old, dying stars. Remember, ellipticals used all their star making material up in their first few million years of existence. Ellipticals are dying galaxies.
But not all spirals are blue. HST has shown that the bulge in most spirals shines with red light. All the stars there are old and dying.
And HST has also found some red spirals. Old galaxies that have used up all their star making materials and are slowly dying. As star formation ceases, the dead spirals become lenticular galaxies, losing their spiral arms.
If our current view of the universe is correct—that it is flat and expanding at an ever increasing rate–eventually all galaxies will meet that fate. Gigabillions of years from now, the last new star will be born in a galaxy far, far away, and gradually the stars will become white or brown dwarfs, neutron stars, or black holes.
As these dead and dying stars pass near each other, one is accelerated and the other slowed. The accelerated star speeds out of the galaxy, and the slowed one spirals into the massive black hole at the center of the galaxy.
After another many gigabillions of years, the universe is populated only with free dead stars and massive black holes.
After 1032 years, the protons that make up all matter and the black holes begin to decay into leptons (a basic subatomic particle like a quark—the electron is a lepton). Eventually, after 10100 years, even these decay into photons and neutrinos (particles with virtually no mass). The universe has reached a state of maximum entropy. It is dead.
Not all cosmologists share this theory. A very few still feel the universe is an oscillating universe. That is, at some time in the far distant future, the expansion will cease and all matter will begin falling back in on itself in what has been called ‘the big crunch.’ Proponents of this theory point to what is happening with our Virgo super cluster as evidence this may already be happening. The tens of thousands of galaxies in the supercluster are all falling in on themselves.
The end point of this theory is all the matter in the universe will crunch itself into a singular point, and explode out again in another big bang to create a new universe.
And in fact, evidence from the HST indicates the current expansion only began some eight billion years ago. Before that, gravity was slowing the expansion.
Still, most cosmologists consider the Virgo ‘collapse’ a ‘local’ phenomenon, and subscribe to the expansion theory. They do not know, however, what is causing the galaxies to rush away from each other at an increasing rate. It appears that at some demarcation time in the universe, gravity reversed itself, and instead of pulling matter together, began pushing it away.
One theory suggests the culprit is dark matter. But if gravity reversed itself in the past, might it do so again?
Perhaps when the Webb space telescope goes into orbit, we will find out.
All photos: NASA Hubble Site
This post is part of the series: All About Galaxies
Specific details about the formation, development, evolution, and death of galaxies. The types of galaxies, how they group together, form clusters and super clusters.