The Universe 02
Galaxies
Before us is the cosmos on the grandest scale we know. We are far from the shores of Earth in the uncharted reaches of the cosmic ocean. Strewn like sea froth on the waves of space are innumerable faint tendrils of light, some of them containing hundreds of billions of suns. These are the galaxies drifting endlessly in the great cosmic dark. In our ship of the imagination we are halfway to the edge of the known universe. In this, the first of our cosmic voyages, we begin to explore the universe revealed by science. Our course will eventually carry us to a far-off and exotic world. But from the depths of space, we cannot detect even the cluster of galaxies in which our Milky Way is embedded, much less the sun or the Earth.
We are in the realm of the galaxies, 8 billion light years from home. No matter where we travel, the patterns of nature are the same as in the form of this spiral galaxy. The same laws of physics apply everywhere throughout the cosmos. But we have just begun to understand these laws. The universe is rich in mystery. Near the center of a cluster of galaxies, there’s sometimes a rogue, elliptical galaxy made of a trillion suns which devours its neighbors. Perhaps this cyclone of stars is what astronomers on Earth call a quasar.
Our ordinary measures of distance fail us here in the realm of the galaxies. We need a much larger unit: the light year. It measures how far light travels in a year: nearly 10 trillion kilometers. It measures not time, but enormous distances.
In the Hercules cluster, the individual galaxies are about 300,000 light years apart, so light takes about 300,000 years to go from one galaxy to another. Like stars and planets and people, galaxies are born, live, and die. They may all experience a tumultuous adolescence. During their first 100 million years, their cores may explode. Seen in radio light, great jets of energy pour out and echo across the cosmos. Worlds near the core or along the jets would be incinerated. I wonder how many planets and how many civilizations might be destroyed.
In the Pegasus cluster, there’s a ring galaxy the wreckage left from the collision of two galaxies. A splash in the cosmic pond. Individual galaxies may explode and collide, and their constituent stars may blow up as well. In this supernova explosion a single star outshines the rest of its galaxy.
We are approaching what astronomers on Earth call the Local Group. Three million light years across, it contains some 20 galaxies. It’s a sparse and rather typical chain of islands in the immense cosmic ocean. We are now only 2 million light years from home. On the maps of space, this galaxy is called M31: the great galaxy in Andromeda. It’s a vast storm of stars and gas and dust. As we pass over it, we see one of its small satellite galaxies.
Clusters of galaxies and the stars of individual galaxies are all held together by gravity. Surrounding M31 are hundreds of globular star clusters. We’re approaching one of them. Each cluster orbits the massive center of the galaxy. Some contain up to a million separate stars. Every globular cluster is like a swarm of bees bound by gravity; every bee a sun. From Pegasus, our voyage has taken us 200 million light years to the Local Group dominated by two great spiral galaxies.
Beyond M31 is another very similar galaxy. Its spiral arms slowly turning once every quarter billion years. This is our own Milky Way seen from the outside. This is the home galaxy of the human species. In the obscure backwaters of the Carina-Cygnus spiral arm, we humans have evolved to consciousness and some measure of understanding. Concentrated in its brilliant core and strewn along its spiral arms are 400 billion suns. It takes light 100,000 years to travel from one end of the galaxy to the other. Within this galaxy are stars, worlds, and—it may be—an enormous diversity of living things and intelligent beings and space-faring civilizations.
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