In Why Is the Orion Nebula Red I introduced emission and absorption spectra. In summary, when we spilt light from an astronomical object using a prism or diffraction grating, we see a series of lines in the resultant spectrum. A prism or diffraction grating bends light of different wavelengths by different amounts. Each line in a spectrum occurs at very specific positions and tells us which chemical element or molecule caused that line.
Here, for example, are laboratory emission and absorption spectra for a cloud of hydrogen gas:
In 1802 the English scientist William Hyde Wollaston observed dark lines in the spectrum of the sun. It took another 50 years to work out what was going on: in 1859, the German physicist Gustav Robert Kirchoff and chemist Robert Wilhelm Eberhard von Bunsen realized that that the lines seen in spectra were unique to each element. (See A Timeline of Atomic Spectroscopy for more information).
Spectroscopy has been incredibly important in astronomy.
A tool had been discovered that allowed us to split the light from stars billions and billions of kilometres away and work out which chemical elements the light had travelled through. Closer to Earth, light reflected by the Solar System’s planets could be analysed to determine the composition of their atmospheres. You can even use spectroscopy to discover planets.
So, how do we do this?
First you attach a diffraction grating to a telescope so that it splits the received light into a spectrum which is focused onto a CCD (essentially, a scientific digital camera).
You could now just point your telescope at a star or planet and get a spectrum. You would see lines in the spectrum. But, at the moment, you would not know the wavelength of each line and hence could not say what each line represented. So, before you start looking at stars you need to work out what each line “means”: you need to calibrate your telescope/diffraction grating/CCD set-up.
You do this by capturing the spectrum of a known light source, for example, a helium-argon cathode lamp:
The data captured by the CCD is essentially number of photons of light that have hit each pixel. We can plot photons vs. pixel number on a graph:
The spikes seen in this graph can then be compared against a reference He-Ar graph:
We can compare the first graph with the reference/standard graph and assign wavelengths to each spike. We can work out, for example, that the three spikes at around the 700 pixel mark in the photon count vs. pixel graph correspond to the wavelengths 667.73 nm, 675.28 nm and 687.13 nm respectively.
By assigning each spike to a wavelength, we end up with a linear relationship between pixel number and wavelength. We are able to say that our particular telescope/diffraction grating/CCD set-up will bend light of wavelength, say, 500 nm to pixel 200 and 600 nm to pixel 480.
In fact, we end up with a formula for a straight line graph. For the graph above I ended up with the formula:
Wavelength = 0.3545 x Pixel Number + 433.3
Now we can capture the light from a star or planet, look for a spike or dip at a particular pixel and calculate the wavelength of that pixel. Once we have the wavelength we can work out which element or molecule caused that spike or dip.
From billions of kilometres away we can determine what an atmosphere of a planet is made of. Or we can see what is in the outer layers of a distant star. That’s awesome.
Let’s examine the spectrum of a star:
Wow. That looks a bit disappointing! The absorption lines are barely visible. But that’s not important – there is an incredible amount of data in the the images captured by the CCD. Remember, we’re just after the photon count at each pixel. So, let’s do that – extract the photon count at each pixel and apply the formula above to convert the pixel number to a wavelength:
You can now easily see some absorption features. There are five distinct drops in photon counts at particular wavelengths. And we know from laboratory experiments which elements would cause each absorption – we basically have a table of wavelengths to atomic elements. For example, in the above graph the absorption feature at around 650 nm is caused by hydrogen – specifically it is the Hydrogen Alpha Balmer line. The dip at around 490 nm is also caused by hydrogen – this is the Hydrogen Beta Balmer line. In fact, we can see the whole Balmer series in this graph.
Altair is 17.73 light years away (1.5827464 × 1014 kilometres) and we’ve confirmed that the element hydrogen exists in its outer layers. Spectroscopy rocks!
Now let’s look at a planet in our Solar System – let’s look at the spectrum for Saturn:
From a distance of over 1 billion kilometres away we’ve discovered that the atmosphere of Saturn contains methane!
I love spectroscopy. Yes, I am a geek.Follow @kashfarooq