Ken Tapping, April 23, 2004

Today I was looking at a photograph taken using the Hubble Space Telescope. It showed a galaxy 13 billion light years away, so the light from that galaxy took 13 billion years to get here. For us to see that galaxy, the universe has to be at least 13 billion years old. Actually, it is 13.7 billion years old, so that galaxy existed less than a billion years after the beginning.

It used to be thought that the Earth was less than ten thousand years old, but early geologists soon realized that the thicknesses of sedimentary rocks we see around us must have taken many millions of years to accumulate. To check these results we have also the technique of radioactive dating. When the volcanic rocks such as those in parts of the Rockies and in the Canadian Shield were molten, some radioactive elements got mixed into the hot liquid, and were trapped when the rock hardened.

Radioactivity arises when elements are unstable, so that their atoms break up, or decay, into other elements. If we know how rapidly a radioactive element decays, we can estimate the age of a rock by seeing how much radioactive element there is compared with the decay products. We find some rocks are billions of years old. Allow some time for the rocks to cool off, and we get an age of 4.5 billion years for the Earth. Our current idea is that the Earth was formed along with the rest of the Solar System, so the Sun and other planets must also have existed for 4.5 billion years.

Distances in space can be so large that expressing them in kilometers is meaningless. One of the units we use to measure the distances of stars and galaxies is the light year. This is the distance light travels in a year, which is almost 10,000,000,000,000 kilometers. This provides us with a "time machine." If we look at a star that is a thousand light years away, we are seeing it as it was 1,000 years ago.

We can measure distances out to a few thousand light years by direct triangulation, the technique we use for surveying. Within that range we have identified special stars that vary in brightness in a manner related to their actual energy output. By comparing this with how bright the star looks, we can calculate how far away it is. To measure the distance of a distant star cluster or galaxy, we look for some of these stars. That gets us out to tens or hundreds of millions of light years, and that far back in time.

Stars that are several times more massive than the Sun end their lives in colossal explosions, called supernovae. It turns out that certain classes of supernovae have predictable brightnesses, so we can measure how bright they look and calculate how far away they are. We get distances of billions of light years. Beyond there we use standard galaxies as distance measurers. For comparison we use Hubble's Law for the expanding universe to estimate these distances. Despite the huge times and distances we are finding, there has been nothing particularly magical in determining them. It is simply the result of centuries of meticulous, hard work by many people.

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Ken Tapping is an astronomer at the National Research Council Herzberg Institute of Astrophysics (NRC-HIA), and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9 Tel (250) 493-2277, Fax (250) 493-7767,
E-mail: ken.tapping@nrc-cnrc.gc.ca