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Skywatching

How far away is that?

Our telescopes can spot galaxies that formed soon after the birth of the universe. We can measure with great precision whether they are moving towards or away from us. Apart from the nearest galaxies, they are moving away from us, carried by the expansion of the universe. The biggest problem is measuring how far away they are.

To measure the distances of stars in our neighbourhood of our galaxy we use parallax, which is basically a form of the surveyor's technique of triangulation. We take a picture of an area of sky on a certain date. Six months later, when the Earth is on the other side of its orbit and about 300 million kilometres from where the first picture was taken, we take another picture of that same area of sky. Comparing the images will show nearby stars to be in different positions compared with stars in the far background. These position changes tell us how far away those stars are. This procedure works to distances of about 10,000 light years. Since our galaxy, the Milky Way, has a diameter of 100,000 light years and the nearest galaxy like ours is over two million light years away, this technique works only in our cosmic backyard.

In the 19th Century, astronomer Henrietta Leavitt discovered a very useful class of variable star. Because the first one was in the constellation of Cepheus, they became known as Cepheids. These stars cycle in brightness over periods of a few days. This cycle time is easily measured, and Leavitt discovered that the cycle time could be used to calculate the star's actual brightness – its luminosity. So we can measure how bright these stars look, measure the brightness cycle time to get the luminosity, and then calculate how far away that star is. That means if we can spot Cepheids in a distant galaxy, we can work out how far away that galaxy lies. Leavitt gave us a ruler to measure the universe. However, what happens when the galaxy is so far away it is hard to spot individual stars? Then we have to resort to something else – exploding stars (supernovae).

Many stars are double – two stars orbiting one another. In some cases one of them has run out of fuel and has become a white dwarf star. Then, as its more long-lived companion starts to age, and swells into a red giant, the white dwarf starts to pull in material from the other star. When the accumulation reaches a critical amount, there is a nuclear fusion explosion that is visible many millions of light years away. Because these explosions happen when a critical amount of material has accumulated, they are all of more or less the same size. We calibrate nearby events of this type (which are known as Type 1a supernovae) using Cepheids, then use supernovae to measure further out into space, taking us close to beginning of the universe.

Now we have another, even more powerful option – the gamma ray burst. These happen when a really high-energy event happens, like the collapse of a giant star to form a black hole. In a few seconds, more energy is emitted than the sun will produce over its entire lifetime. These bursts of gamma rays have to be observed using satellites because the Earth's atmosphere protects us from them. Once again, we can estimate the energy released, so from measuring what we received here at Earth, we can calculate how far away the event happened. However, the difficulty at the moment is that not all gamma ray bursts are identical. Therefore, this measurement technique still to be calibrated. What we really want is a reliable cosmic ruler that takes us out into space and back in time to about 380,000 years after the Big Bang, where the universe became transparent, and the first stars and galaxies started to form.

  • Before dawn, Jupiter and Saturn are close together in the south.
  • Mars lies low in the southeast.
  • Venus lies low in the sunrise glow.
  • The moon will reach first quarter on the 28th. 

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.



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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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