Greetings! This post is going to be a continuation on part 1 of my Doppler Shift post. Here’s the link if you happened to have missed it : https://therealdegree.wordpress.com/2012/07/29/under-the-rader-the-doppler-effect/
Last time I conversed with you all about the weird phenomenon that is the Doppler effect (Or shift, whichever one floats your boat). I explained it’s influence on sound and on water and how we witness it regularly. This time however, I’m going to be talking about the Doppler shift of light. I’m going to be talking about ‘Red-Shift’, the spectroscope (awesome name) and why the Doppler shift for light is immensely important to astronomy.
First of all we need a little background information. As I’ve mentioned in my previous posts, Newton figured out that white light is actually composed of lots of different colours. (All the colours of the rainbow). The white light he was talking about was just normal sunlight, coming from just a single star in our galaxy. The brilliant thing is that today’s astronomers have taken Newton’s discovery to a whole new level.
They’ve invented what’s called a spectroscope which is simply put- a rainbow machine. It’s attached to a telescope and it detects the light from one particular star or galaxy from the universe, It spreads out the light from said celestial object into the colours of the spectrum, just as Newton’s glass prism first did. However, it’s a bit more advanced than Newton’s prism. It’s because not only does it spread the light from a star into a spectrum, but it also let’s you take exact measurements along the spread-out spectrum of starlight. What does that mean? What could you possibly measure?
Well it turns out that light from different stars produce rainbows in different ways and this can tell us a lot about stars and other celestial objects across the universe. Although, don’t get your hopes up like I did, this doesn’t mean that that there are new mysterious colours that we have never witnessed before. The colours we see is all there is, all that human beings are capable of seeing. All the colours that are available are just different variations of the seven colours of the spectrum, they’re just different shades, tints or mixes.
What I mean by ‘different rainbows’ is that when starlight is detected by a spectroscope strange patterns of thin black lines appear in very specific places along the spectrum. The pattern looks a lot like barcodes you see in supermarkets to identify products. The ‘celestial barcode’ for stars and galaxies is really similar to the supermarket one in the sense that it tells us a lot about the star by telling us what it’s made up of.
Keep in mind that it isn’t only starlight that gives us these barcode lines, light on earth does this too. So what are these barcodes? and what do they tell us?
It turns out that what makes these barcodes is different elements. Sodium for example has prominent lines in the yellow and blue part of the spectrum. So, if you point your spectroscope onto a star and the spectrum has loads of black lines of the yellow/blue part, it means that there is vast amounts of sodium on it. Pretty clever right?
But why does sodium have this affect on light? It’s because of reasons connected with the orbits of the electrons. Every element has a different amount of protons, neutrons and electrons within their atoms and it turns out that an element’s electrons has it’s own unique effect on light. You can tell which of the naturally occurring elements found in the periodic table are present in a star by spreading the star’s light out in a spectroscope and looking at the barcode lines in the spectrum. Here are some examples:
The number next to the name of the element is it’s atomic number and the vertical numbers on top is the wavelength.
Another thing to keep in mind is that there is another type of barcode. Instead of black lines on a coloured spectrum you can also have coloured lines on a black background. Coloured lines on a black background is called the emission spectrum and the absorption spectrum is when you have black lines on a a coloured background. What you get depends on whether the element is glowing with light or getting in the way of light.
Just to recap, ever element has a different barcode pattern, we can look at the light from any star and see which elements are present in that star. Admittedly, it is quite tricky because the barcodes of several different elements are likely to get mixed up, but there are ways to sort them out. So now you can see for yourself how brilliant a spectroscope is.
The main aspect of these spectrums is the prominent, thick lines. These are the ones you should pay the most attention to. These pictures show all the lines, including the really faint ones so try not to pay attention to them too much. Also, the sodium picture isn’t the best at showing the prominent yellow lines so just imagine the sodium spectrum with two prominent emmission spectrum lines. One blue and one yellow.
That sodium spectrum is what you get from sodium light here on Earth or from a nearby star. But, if you look at sodium light from a nearby galaxy you would realise that the two prominent lines (blue and yellow) shift towards the red end of the spectrum ever so slightly. This means that the yellow line I asked you to imagine turns more orange and the blue line becomes more greener. Next, if you look at sodium light from a very distant galaxy, the prominent lines shift quite a lot. The yellow lines become red and the blue lines become green.
Sodium is just a single example, any other element shows the same shift along the spectrum in the red direction. The more distant the galaxy, the greater the shift towards red. This is called the red shift or the Hubble shift as it was discover by the great american astronomer Edwin Hubble. The above picture of the universe was taken by the Hubble space telescope, also named after him. As you’ve probably guessed the hubble shift is just the Doppler effect acting on light. The red end of the spectrum has a longer wavelength than than the blue end. In comparison with the sound Doppler effect , the red end of the spectrum is like the low note you would hear.
I hope now you understand how important the Doppler effect is to astronomy and science. It tells us what stars and galaxies are made out of (which could help towards finding extra-terrestrial life) and it also tells us that the universe is expanding and at an alarming rate. Another thing is that I hope you now see how interconnected everything in science is. It really is a marvellous thing.