When people learn the immense distances of the stars – and how long it takes their light to reach us on Earth – they're often puzzled. They wonder if astronomers are bothered by the fact that stars and galaxies do not appear as they are now but only as they were long ago.
The answer is a simple “no.” In fact, the greater the distances – and, therefore, the further back into time – we can peer, the happier we are because this enables us to see how our galaxy and universe appeared in the distant past. If all we could see was light from today, we would never learn how things have evolved over time.
Geologists and paleontologists use this technique frequently. The deeper into a canyon they go, for example, the older the sediment layers are. From these they can learn the history of our planet and its diverse and ever-changing life forms. So, in a sense, the sky is a great backyard time machine, and traveling to distant eras is as easy as going outdoors and looking up.
Take, for example, the three stars that outline the large asterism known as the Summer Triangle, low in the eastern sky after dark at this time of year. The southeasternmost star of the great triangle – Altair – marks the constellation of Aquila, the Eagle. Altair appears as bright as it does not only because it is nine times more luminous than the sun, but also because it lies “only” 17 light-years away. This means the photons of light from Altair that enter your eye tonight have been journeying through space since 1991.
The brightest star of the triangle – Vega – is also the brightest in the constellation Lyra, the Harp, at a distance of about 150 trillion miles. Its light takes 25 years to reach the Earth. Finally, the northernmost star of the triangle is Deneb, the chief star in the constellation Cygnus, the Swan. From its great distance of about 1,500 light-years, its light has been traveling across the galaxy since the sixth century. If you want to see the light Deneb emits tonight, you will have to wait 1,500 years for it to arrive.
The next time you gaze skyward on a clear night, think about all the amazing events that have taken place on our planet during the long journeys of starlight.
© Copley News Service
