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unit -- the light year. 

Hercules Cluster 

tumultuous 

cosmos

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 ten 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 hundred 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.

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