Wonders of the Universe - Brian Cox [25]
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These colours hidden in white light are not only revealed in rainbows; wherever sunlight strikes an object the different colours are bounced around or absorbed in different ways. The sky is blue because the blue components of sunlight are more likely to be scattered off air molecules than the other colours. As the Sun drops towards the horizon, and the sunlight has to pass through more of the atmosphere, the chance of scattering rays of yellow and red light increases, turning the evening skies redder. Leaves and grass are green because they absorb blue and red light from the Sun, which they use in photosynthesis, but reflect back the green light.
But what is the difference between the colours that makes them behave so differently? The answer goes back to our understanding of light as an electromagnetic wave. Waves have a wavelength – which is the distance between two peaks (or troughs) of the wave. Blue light has a shorter wavelength than green light, which has a shorter wavelength than red light. Our eye has evolved to discern about ten million different colours, which is to say that it can differentiate between ten million subtle variations in the wavelength of electromagnetic waves. This simple idea is all you need to read the story of the Hubble Ultra Deep Field image
HUBBLE EXPANSION
So, how do we know that the irregular, messy galaxies in the Hubble image are billions of light years away? The picture below shows some of the most distant galaxies we have observed. The most obvious thing about them is that they are all red. Why is this so? To answer this question correctly, we need our friend Edwin Hubble, the astronomer, again.
During the 1920s, Edwin Hubble was using what was then the world’s most powerful telescope at the Mount Wilson Observatory in Pasadena, California, to observe stars called Cepheid variables. These Cepheid variables are stars whose brightness varies regularly over a period of days or months, and they are astonishingly useful to astronomers because the period of their brightening and dimming is directly related to their intrinsic brightness. In other words, it is a simple matter to work out exactly how bright a Cepheid variable star actually is just by watching it brighten and dim for a few months. If you know how bright something really is, then measure how bright it looks to you, you can work out how far away it is. Edwin Hubble’s research project was simply to search for Cepheid variables in the sky and measure their distance from Earth. During his observations, he discovered two remarkable things: firstly, he quickly determined that the Cepheid variables he found in the so-called spiral nebulae (which at the time were thought to be clouds of glowing gas within the Milky Way) were in fact well outside our galaxy. For the first time, Hubble showed that there are other galaxies in the Universe, millions of light years away.
This image shows some of the most distant galaxies that we have observed – and all appear in a bright, sharp, red colour.
Hubble’s second observation was of even greater scientific importance. While he and others were also busy measuring the spectrum of the light from the stars in the spiral nebulae, which thanks to Hubble were now understood to be other galaxies beyond the Milky Way, they quickly observed that many of the galaxies appeared to be emitting light that was redder than it should be. Hubble quantified the amount of reddening in each galaxy as a number called redshift. Remember that red light has a longer wavelength than blue light, so seeing light redshifted simply means the wavelength is longer than expected. Hubble made his second great discovery by plotting a graph of the redshift of the light from the distant galaxies against their distance, which he had calculated from his observations of the Cepheid variables.
To his great surprise, Hubble noticed that his graph was approximately a straight line. This is because the further away a galaxy is, the greater its redshift – i.e. the