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Skywatching

Dots in the sky

With modern astronomical instruments showing us collisions between black holes, and the traces of the birth of the universe, it is easy to think these were the most challenging problems in astronomy. However, the most challenging thing has been to understand stars: those dots in the sky.

We have known about the existence of stars ever since our remote ancestors looked up at night. They saw those little bright dots, some brighter than others, some coloured, and arranged in more or less unchanging patterns. Our forebears grouped the stars into constellations, named after mythical beings and objects. Few of the constellations look anything like what they are supposed to represent, which indicates that a powerful imagination has always been with us.

Telescopes revealed the lunar landscape and the planets as discs with surface and cloud features. However, the stars remained coloured dots. The telescope just made the stars look brighter and made fainter stars visible. The situation changed dramatically when we learned how to analyze starlight.

Before Isaac Newton, sunlight was just yellowish-white light. His experiments with a prism showed that so-called white light is made up of a rainbow of colours, ranging from red to violet. We now know that what we perceive as colour is actually the wavelength of the light. The wavelength – the distance between two successive peaks of waves – of blue light is about half the distance for red light.

Newton's prism experiment led to the development of the spectrograph, an instrument that can analyze the light in far more detail. When sunlight was first passed through one of these, the familiar rainbow of colours appeared, but crossing that rainbow were lots of dark lines, some narrow, some broader and some in groups. These lines were the signatures of the atoms of elements present in the sun, and also contained information of the environment in which those atoms found themselves. We could analyze the Sun from the surface of our planet. When the spectrograph was turned on the stars, what was detected strongly resembled what we observed in the sun. Star colours ranged through blue, white, yellow, orange to red. The colours indicate their temperatures, with blue stars being the hottest and red ones the coolest. The relative abundances of the elements causing those dark lines also vary from star to star. The spectroscope made it clear that our sun is just one of billions of stars in our galaxy.

Some stars look brighter than others because they are intrinsically brighter: a 100w rather than a 25w light bulb. Others look brighter because they lie close to us. What we see in the sky is a combination of both effects. To better understand stars we needed to know how far away they are.

To measure the distances of stars we use parallax, the apparent change in position of nearby objects against a more distant background when observed from two different locations. This is why we have two eyes. Our brains process the views from each eye to estimate how far away things are and to give our view of the world depth. The problem with stars turned out to be that even telescopes separated by the diameter of the Earth could see no parallax when observing stars. It took taking pictures of the stars of interest against a background of distant stars and then taking the same picture again, six months later, when the Earth was on other side of its orbit, about 300 million kilometres from where the first picture was taken. This is how we found out that the nearest star to us lies 4.3 light years away. If we know the observed brightness of a star, its temperature, what it's made of and how far away it is, we are a long way towards understanding it.

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  • Venus lies low in the sunset glow.
  • Jupiter and Saturn rise soon after dark.
  • The moon will reach first quarter on the 15th.


More Skywatching articles

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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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